ROBERT E. RICKLE An Analysis of

f Nesting Mortality

in Birds

SMITHSONIAN CONTRIBUTIONS TO ZOOLOGY • 1969 NUMBER 9
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SMITHSONIAN CONTRIBUTIONS TO

ZOOLOGY

NUMBER 9

Robert E. Ricklefs All Analysis of

Nesting Mortality

in Birds

SMITHSONIAN INSTITUTION PRESS

CITY OF WASHINGTON
ABSTRACT

Ricklefs, Robert E. An Analysis of Nesting Mortality in Birds. Smithsonian Contributions to Zoology, 9:1-48. 1969.—This study was initiated to evaluate nesting

mortality of birds as a feature of the environment and as a selective force in the evolution of reproductive strategies. Representative nesting-success data from the literature

for most groups of birds were transformed into daily mortality rates to eliminate differences among species in the length of the nest cycle. These data are presented by

taxonomic groupings and for passerines by geographical region and nest construction

and placement.

The strength and pattern of various mortality factors are described in detail.

Predation, starvation, desertion, hatching failure, and adverse weather are the most

prevalent factors, but nestsite competition, brood parasitism, and arthropod infestation

may be important in some species. It is demonstrated that the various mortality factors

can be identified by characteristic patterns of nesting losses involving differences in

mortality rates between the egg and nestling periods and the within-nest component of

mortality rates.

Among Temperate Zone passerines, field-nesting and marsh-nesting species have

the highest mortality rates while those species nesting in trees, especially in cavities,

enjoy higher success. Starvation is prevalent in marsh and field species but desertion is

more restricted to tree-nesting species. In general, arctic species have lower mortality

rates and tropical species higher rates, although there is a similar gradient from arid to

humid regions within the tropics. The relative abundance of a species is related directly

to its mortality rate in arctic regions, but is not in temperate and tropical regions.

Birds of prey generally have low mortality rates although starvation is often a

major factor. Nesting losses in seabirds are caused primarily by crowded conditions in

colonies and loss of eggs due to inadequate nest construction. Chick deaths come about

primarily through their wandering away from parental care which is most common

in the semiprecocial Charadriiformes. Precocial shorebirds and water birds enjoy higher

egg success than ground-nesting passerines but game birds exhibit similar mortality

rates. Little is known of the survival of precocial chicks after hatching except that

mortality rates may be initially quite high and decrease with age. The fate of altricial

birds after fledging is also poorly documented.

It is postulated that interspecific differences in mortality rates are determined by

evolutionarily acceptable levels of adult risk to lower mortality rates of offspring

through parental care, adult adaptations of morphology and behavior for foraging

which result in limitations on nesting adaptations, environmental unpredictability

which reduces the effectiveness of adaptations, and—most import—the diversity of

predators to which a species must adapt.

Official publication date is handstamped in a limited number of initial copies and is recorded in

the Institution's annual report, Smithsonian Year.

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Contents

Page

Introduction 1

Acknowledgments 1

Materials 2

Methods 2

Mortality Factors 3

Mortality factors as selective forces 6

Identification of mortality factors 8

Mortality Rates 8

Temperate Zone species 8

Tropical species 17

Arctic species 21

Breeding density and nesting mortality 25

Raptorial species 26

Sea birds (Procellariiformes and Pelecaniformes) 27

Seabirds (Charadriiformes) 29

Shorebirds 32

Water birds 33

Game birds 34

Development and survival 35

Interpretation and Discussion 36

Appendix 1—The Strength of Selection on Development Rates 41

Appendix 2—Alphabetical List of Bird Names 42

Bibliography 43

TABLES :

1. Causes of mortality in six passerine species 4

2. Nesting loss patterns of mortality factors 8

3a. Nesting success of small Temperate Zone altricial land birds 9

3b. Mortality rates of small Temperate Zone altricial land birds 12

4. Further analysis of vertical components and within-nest mortality in

small Temperate Zone altricial land birds 13

5. Nesting mortality parameters for small Temperate Zone altricial land

bird species grouped according to nest location 14

6a. Nesting success of small altricial land birds in humid tropical regions. . 18

6b. Mortality rates of small altricial land birds in humid tropical regions. . 19

7a. Nesting success of small altricial land birds in arid tropical regions—all

data are from a four-year study by Marchant (1960) in Ecuador . . 19

7b. Mortality rates of small altricial land birds in arid tropical regions . . 20

8. Comparison of nesting success of humid tropical species with respect to

nest-type and habitat (from Skutch, 1966) 20

9a. Nesting success of small altricial land birds in arctic regions 22

9b. Mortality rates of small altricial land birds in arctic regions 23

10. Geographical variation in nesting success of open-nesting passerines . . 23
SMITHSONIAN CONTRIBUTIONS TO ZOOLOGY

Page

11. Taxonomic summary of nesting success studies 25

12. Breeding density and nesting success at Cape Thompson, Alaska .. . 26

13. Nest abundance and nesting success of some Costa Rican birds ... . 26

14. Nesting success and mortality rates of raptorial birds 27

15. Nesting success and mortality rates of seabirds [Procellariiformes and

Pelecaniformes] 28

16. Nesting success and mortality rates of seabirds [Charadriiformes:

Laridae and Alcidae] 30

17. Egg and nestling mortality rates in seabirds 31

18. Nesting success and mortality rates of shorebirds [Charadriiformes:

Haematopodidae, Charadriidae, Scolopacidae] 32

19. Nesting success and mortality rates of water birds 33

20. Nesting success and mortality rates of five species of Ciconiiformes in a

Georgia salt marsh 34

21. Adult body weight and nest mortality rates of Ciconiiform birds in a

Georgia salt marsh 35

22. Nesting success and mortality rates of game birds 36

23. Survival of marsh hawk nestlings in successful nests 37

24. Survival of some passerine young during the fledgling period 38

25. Nest construction and placement of temperate and tropical passerines . 40

26. Relative strength of selection on development rates in passerine birds. . 42

FIGURES:

1. Postulated vectors for major mortality factors during the nest period . . 8

2. Partial loss during the egg period and vertical components of mortality

rates of temperate passerine birds 15

3. Partial loss during the nestling period and vertical components of

mortality rates of temperate passerine birds 15

4. Vertical components of individual mortality rates plotted against

vertical components of whole nest mortality rates 17

5. Vertical components of individual mortality rates versus partial loss

during the nest period for Temperate Zone passerine birds (and one

dove and one hummingbird) 17

6. Vertical components of individual mortality rates versus partial loss

during the nest period for humid tropical passerines (and one dove) . 21

7. Vertical components of individual mortality rates versus partial loss

during the nest period for arid tropical passerines (and one dove and

one cuckoo) 21

8. Vertical components of individual mortality rates versus partial loss

during the nest period for arctic passerines 24

9. Relationship between age and mortality rates in the young of five species

of precocial and semialtricial birds 37

10. Coefficient of variation versus average rainfall for each of four months

(February, May, August, November) at five Temperate Zone localities: Cape Spartel, Tangiers; Miyako, Japan; Phoenix, Arizona;

Philadelphia, Pennsylvania; and Portland, Oregon 38

11. Mortality rates of nestlings as a function of body weight in Temperate

Zone altricial land birds 39
Robert E. Rkkiefs A n Analysis of

Nesting Mortality

in Birds

Introduction

Nesting success of birds has often been treated in

relation to reproductive rates to determine population parameters of species (Lack, 1954; Nice, 1957).

The purpose of this report is to evaluate mortality as

a feature of the environment and as a selective force

in the evolution of reproductive strategies.

Mortality rates are evolved characteristics of species

just as body size and plumage coloration, and thus they

indicate the limits to which evolution may reduce losses

through adaptation. These limits vary with species

and habitat. Furthermore, that portion of mortality

which is due to predation or parasitism represents the

balance between two adapted systems: those of the

predator and those of the prey. The outcome of this

interaction also varies with the environment and provides an insight into community organization.

The emphasis of this study is placed upon the

strength of environmental mortality factors as selective

forces rather than upon survival as a specific population parameter. It is not possible, however, to completely separate the species from its environment because specific adaptations of the breeding cycle partly

determine the schedule of mortality rates. We may ask

to what extent nesting mortality is controlled by the

external environment and conversely by specific adaptation to modify or restrict this environment. For example, hole-nesting and open-nesting species of birds

in the same forest are confronted with markedly diverse

"environments" because each presents different problems to predators and affords varying protection from

inclement weather. Nest parasites and nest-site competitors play a significant role in the activities of some

Robert E. Ricklefs, Department of Biology, University of

Pennsylvania, Philadelphia, Pennsylvania 19104.

species but not others, depending on nest-type and

the size and behavior of the adults. Adaptations such

as these clearly limit those portions of the environment

which are directly relevant to nesting success whether

they were evolved in direct response to mortality during

the nest period or not. Many aspects of adult morphology and behavior which bear upon nesting success

are adaptations primarily for foraging rather than for

breeding. On the other hand, such diverse habitats as

deserts, arctic tundra, and tropical rain forests differ

greatly in the availability of nesting sites and the kinds

and abundance of predators as well as in climate. We

must, therefore, relate mortality rates to both general

habitat and specific adaptations.

The following analysis is divided into three major

sections. First, mortality factors are identified, characterized, and quantified for Temperate Zone altricial

land birds in detail. Secondly, there is a comparative

study of nesting mortality of passerine birds in three

other geographical areas: arctic North America, humid

tropical areas of Central America and northern South

America, and an arid tropical area in South America.

Finally, nesting mortality in other groups is surveyed

and the relative effects of mortality factors are compared with passerine species. In the discussion, the

results of these analyses are brought together into a

general statement on the factors influencing the outcome of reproductive efforts of birds. Concluding remarks are offered on the limits to which mortality may

be reduced through adaptation.

Acknowledgements

This study, initiated while the author was a graduate student in the Department of Biology at the University of Pennsylvania, was supported by a National

1
SMITHSONIAN CONTRIBUTIONS TO ZOOLOGY

Science Foundation Graduate Fellowship, and was

completed under a National Research Council Visiting

Research Associateship at the Smithsonian Tropical

Research Institute. Numerous people at these institutions were stimulating and helpful to me, but I especially want to thank Dr. W. John Smith, Dr. Neville

Kallenbach, Dr. Neal G. Smith, and Dr. Martin

Moynihan.

Materials

Reviews of nesting success generally have centered

around statistical measures of the outcome of nesting

attempts (Williams and Marshall, 1938; Kalmback,

1939; Kendeigh, 1942; McCabe and Hawkins, 1946;

Lack, 1954; Hickey, 1955; Nice, 1957; Skutch, 1966).

Most data have been compiled as parts of life history

studies of species, although nesting success also has

been analyzed in relation to the structure of the nest

(Lack, 1954; Nice, 1957), time of the year (Laskey,

1940; Snow, 1962; Nolan, 1963; and several European

studies), brood parasitism (Nice, 1937; Norris, 1947;

Smith, 1968), abundance of food (Lack and Lack,

1951; Owen, 1960; Orians, 1966; Wilson, 1966), interspecific competition (Weins, 1965), and nest placement (Goddard and Board, 1967). The quality and

completeness of this material varies widely. In general,

only studies which contain more than fifty nests are

used here, although less desirable data are included

if they are of comparative interest. I have made no

attempt to present a thorough survey of the literature.

European species are by and large ignored, and their

nesting success is similar to North American equivalents (see Lack, 1954).

Because of yearly variation in nesting success, studies

continued over long periods should provide the most

valid estimates of average nesting success. The stage

at which nests are found, biases incurred in locating

nests, the frequency of visits to nests, and the investigator's criteria for nesting failures are additional

sources of variability. Further discussion of sampling

problems may be found in Lack (1954), Mayfield

(1961), and Skutch (1966).

The amount of disturbance to the study area is

also an important consideration because in comparative studies of breeding biology one is most interested

in the ecological conditions under which the breeding

strategy evolved. For this reason, data from highly

developed areas are disregarded. As Lack (1965) has

pointed out, however, it is unlikely that any temperate

area has not been grossly changed through the activities of man. This would apply equally to most tropical

areas as well.

To obtain high numbers of species for comparative

purposes, all data from undeveloped areas and some

from rural and parklike areas are treated identically.

Differences in field methods correlated with groups of

species or habitats will introduce some biases. Most of

the studies however, represent species whose nests are

readily found, which should result in a degree of uniformity of sampling.

Overall nest success and the percent of eggs hatched

and fledged are most commonly presented in literature

accounts. Further breakdown of nest success into the

egg and nestling periods adds significantly to the value

of the data. Peterson and Young (1950) and Young

(1955, 1963) constructed survivorship graphs for the

duration of the nest period which permit more detailed

analysis of changes in mortality rates during the nest

cycle.

Methods

For the analysis of selective forces, mortality is most

meaningfully treated in terms of instantaneous rates

rather than as percent losses over a given period of

time. Rates are independent of the duration of development stages and may be considered as primarily

environmental features rather than as attributes of the

species.

Percentage survival data may be converted to mortality rates by considering survivorship as a decaying

exponential function of time. Exluding "event-related"

losses, the probability of success of a nesting attempt is

determined by mortality rates during the nest period.

If one divides this period into equal segments there

is a probability, sx, that a nest or individual will survive

through any given segment, x, of the nest period. The

expectation, E(S), that a nest or individual will survive

the duration of the nest period is the product of the

survival probabilities for each segment (s^ .. . sn,

where n is the total number of segments). If mortality

rates are independent of age, that is, if they are constant during the nest period, the product (s^ ... sH)

is s". As one makes the segments arbitrarily small (and

their number arbitrarily large) the relationship

E(S)=sn [Equation 1]

may be written

E(S)=e'mt [Equation 2].
NUMBER 9

where m is the instantaneous mortality rate and t is

measured in any suitable units of time (in this study,

days).

Equation 2 may be rearranged for computing average daily mortality rates from field data on nesting

success. Given the proportion, P, of nests or individuals

which survive any given portion of the nest period, the

mortality rate, m, can be calculated by the equation

m = - (logeP) It [Equation 3].

The value m is strictly accurate only if mortality rates

are constant during the nest period. When this is not

the case, however, errors will be quite small and the

calculated value, m, will be very close to the average

daily mortality rate.

A more serious source of error is that, while mortality

rates are calculated for the entire nest period including

egg laying, nests often are not found until the nest

period is partly over (Mayfield, 1961). In many species,

nests are found readily during construction and they

do not present a problem. Also, some authors (e.g.

Skutch, 1966) are aware of this source of error and

present their data accordingly. For the remaining

studies there is no possible way to compensate for such

differences in the data and they are treated as if nests

were found before the initiation of laying. Thus, calculated mortality rates in some species will be lower

than actual rates, especially during the egg period.

Several stages of the breeding cycle are distinguished

in this study for the purpose of calculating mortality

rates. The "egg period" extends from the initiation of

laying until the eggs have hatched, which includes the

"laying period" and the "incubation period." Because

most species form one egg per day, the laying period

is usually one day less than the number of eggs in the

clutch. The incubation period refers to the time between the laying of the last egg and the hatching of

the last young in the nest (Heinroth, 1922; Nice,

1954). Nidicolous young remain in the nest for part

of their postnatal development, the "nestling period."

Nidifugous young are capable of moving about and

gathering food at, or shortly after, hatching and thus

do not exhibit a lengthy nestling period. Several species

of semiprecocial seabirds do not remain in well-defined

nests after hatching but stay in restricted areas and

may be censused for long periods. For convenience,

the term "nestling period" also will be used for these

species.

After leaving the nest, or nest area, the young remain dependent on their parents during varying and

poorly defined periods. Young may be refered to as

"fledglings" until they are self-feeding, as "juveniles"

until they are independent of parental care, and as

"immatures" until all adult characteristics have been

acquired. Unfortunately, few survival data have been

gathered for these stages.

Mortality Factors

Mortality factors may be referred to two broad

classes: (1) those associated with events such as fertilization, egg laying, hatching, and fledging, and (2)

those which may occur at any time and whose expectation increases with time. The term "mortality rate"

is not strictly applicable to the first class because the

duration of critical events is meaningless. The survival

of young during the development period may be

likened to the outcome of a race which is continuously,

but not necessarily equally dangerous along its length,

and has several high hurdles placed along the way. The

hurdles require special skills, other than swiftness, for

their passage.

For species that raise more than one young, we may

distinguish also between mortality factors which cause

the loss of whole broods and those which result in the

death of individual eggs or young within broods. In

nidicolous species certain factors are characterized by

acting more strongly during either the egg or the nestling period. Mortality rates decrease with the increasing self-sufficiency of nidifugous chicks after hatching,

and of altricial young following fledging, until adult

characteristics are attained.

The relative contribution of different factors to

overall mortality during the nest period of nidicolous

birds may be inferred from difference between withinbrood and whole-brood losses, and between egg and

nestling losses.

Mortality factors are difficult to distinguish in field

studies without considerable care and observation time.

In a few studies, summarized in Table 1, causes of

death are listed in detail and these will be considered

in -conjunction with the following discussion of mortality factors. The species include three open-nesting

blackbirds and grackles (Family Icteridae), a holenesting warbler (Parulidae), a hole-nesting thrush

(Turdidae) and an open-nesting finch (Fringillidae).

These are illustrative of small passerine birds but should

not be taken as being widely representative. Raptorial,
SMITHSONIAN CONTRIBUTIONS TO ZOOLOGY

TABLE 1.—Causes of mortality in six passerine species •

Causes of mortality

Eggs laid

Losses due to:

Hatching failure

Gowbird parasitism

Nest-site competition

Adult death *

Desertion

Predation

Weather

Other

Total losses

Young hatched

Losses due to:

Gowbird parasitism

Nest-site competition

Desertion

Starvation

Predation

Weather

Other

Total losses

Young fledged

1

220

24

17

10

1

52

220

25

17

6

48

172

2a

413

34

' 90

24

79

29

2

258

155

« 11

34

3

1

49

106

2b

163

13

5

18

24

3

63

100

0

100

3

369

15

1

16

102

18

<»42

194

175

13

21

<*22

56

119

Species °

4a

563

31

15

8

87

4

145

418

1

71

8

2

82

336

4b

577

27

9

5

120

8

3

172

405

1

57

1

4

63

342

4c

1632

49

5

59

583

50

• 112

858

774

4

23

284

21

• 112

444

330

5

288

18

6

/37

18

79

209

17

'23

20

14

74

135

6

854

49

46

29

156

25

39

344

510

4

26

156

18

204

306

Total

5131

260

52

90

75

146

1188

134

220

2165

2966

4

11

21

64

671

70

179

1020

1946

Percent of

individuals

5.1

1.0

1.8

1.5

2.8

23.3

2.6

4.3

42.2

57.8

0. 1

0.4

0.7

2.2

22.6

2.4

6.0

34.4

«65.6

Percent

of losses

12.0

2.4

4.2

3.5

6.7

54.9

6.2

10.2

100.1

0.4

1. 1

2. 1

6.3

65.8

6.9

17.5

100. 1

'Species and sources are: 1. Eastern bluebird, Sialia

sialis (Thomas, 1946). 2. Prothonotary warbler, Protonotaria

citrea (Walkinshaw, 1953), in Michigan (a) and Tennessee (b).

3. Yellow-headed blackbird, Xanthocephalus xanthocephalus

(Young, 1963). 4. Red-winged blackbird, Agelaius phoeniceus

(Smith, 1943, two colonies, and Young, 1963). 5. Purple

grackle, Quiscalus quiscula (Peterson and Young, 1950). 6. Song

sparrow, Melospka melodia (Nice, 1937).

b

 Losses during the egg and nestling periods are not distinguished.

c

 Nest-site competition from the house wren, Troglodytes

aedon.

d Disappeared from nests.

• Ninety-two eggs and 107 nestlings disappeared from nests;

20 eggs and 5 young lost wing to tipping of nests.

' Mostly by young boys.

' This represents 37.9 percent of eggs laid,

precocial, and seabirds will be considered separately.

Mortality factors are treated in arbitrary order. In

some respects the categories are subjective and could

be further broken down, but hopefully they correspond

to those factors which can be differentiated in the field

and which produce recognizably diverse patterns of

nesting losses.

Hatching failure, due to infertility, death of the

embryo, or death during hatching, may be attributed to

eggs which remain in the nest longer than the incubation period. This factor can be ascertained with high

reliability because eggs usually are not removed from

the nest after others have hatched. Of 5,131 eggs which

were laid in the nests of those species whose losses are

tabulated in Table 1, 260 (5.1 percent) failed to hatch.

Many of the eggs, however, which were taken by

predators and lost to other factors would also not have

hatched had they survived the egg period. Thus 8.1

percent of 3,226 eggs which were remaining in nests

at the end of the incubation period failed to hatch.

Among the three studies involving hole-nesting species,

hatching failure in successful nests was 9.8, 11.2, and

18.0 percent. The high value for the prothonotary

warbler (18.0 percent) was due to disturbance by

house wrens during incubation. Although hatching

failure among the hole-nesting species is higher than
NUMBER 9

among the open-nesting species in Table 1 (6.0-8.8

percent for six studies), other data for open-nesting

species are comparable: 10.9 percent of 129 eggs in the

traill flycatcher (Berger, 1957), 10.3 percent of 175

eggs in the robin (Howell, 1942), 11.2 percent of 135

eggs in the yellow warbler (Schrantz, 1943), 11.1 percent of 353 eggs in the yellow-headed blackbird

(Fautin, 1941), and 5.1 percent of 195 eggs in the

chipping sparrow (Walkinshaw, 1952).

Brood parasitism by cowbirds and cuckoos occurs

during the egg-laying period of the nesting cycle although its effects on nesting success may appear at

anytime during the nest period through loss of eggs,

desertion, or starvation of the young. Among the

species listed in Table 1, brood parasitism accounted

for the loss of only 52 individuals (1.0 percent of the

total, 2.4 percent of losses) but its incidence is often

much higher, depending on the species and locality.

Norris (1947) observed that 73 of 237 nests (30.8 percent) of 14 species were parasitized by brown-headed

cowbirds in central Pennsylvania. Of the 73 nests, 21

were deserted during early stages of the nest cycle (28.8

percent), 27 were destroyed (37.0 percent) and 25

produced fledglings (34.2 percent). Successful parasitized nests raised about one fewer host young than

successful nonparasitized nests. Of 129 nests of 10 host

species found by King (1954) in eastern Washington,

17 (13.2 percent) were parasitized by brown-headed

cowbirds. Of 500 nests of 20 host species found by

Berger (1951) in Michigan, 112 (22.4 percent, contained 204 cowbird eggs. Several species, such as the

song sparrow, yellow warbler, and cardinal were especially susceptable, 40-60 percent of all nests being

parasitized. In tropical regions, host-parasite relations

are often very complex and parasitized nests may produce more host young than nonparasitized broods in

some situations (Smith, 1968).

Nest infestation by arthropod parasites may play a

significant role in swallows and other hole-nesting

species which reuse nests from year to year (Stoner,

1945; W. Moss, personal communication). Evaluation

of the effects of infestation is difficult as desertion or

losses occurring after fledging could be due to the

weakened condition of the young resulting from infestation. None of the losses in Table 1 were attributed to

this factor. The effects of other bird parasites, such as

fly larvae and ticks, are poorly known, but it is not

likely that these constitute a major mortality factor for

many species. It has been shown, however, that bot

flies exert a significant influence on the nesting success

of oropendulas in Panama (Smith, 1968). For a more

detailed discussion of insect parasites, one should consult Lack (1954:78).

Competition for nest sites may cause substantial nest

losses, as in the prothonotary warbler in Michigan

(Walkinshaw, 1953). Mortality due to this factor occured primarily during the early stages of the nesting

cycle. Ninety of 413 eggs (21.8 percent) compared to

only 11 of 155 nestlings (7.1 percent), were lost owing

to the destruction of nests by house wrens. An additional 24 eggs (5.8 percent) were deserted and hatching failure was also high (18.0 percent of eggs surviving

the egg period). Thus, loss of eggs was three to four

times greater than loss of young, which may have resulted either from increasing difficulty of evicting young

from nests as they grow, or increased parental tenacity

and defense of the nests as the cycle progressed.

Adult mortality is difficult to ascertain unless adults

are marked and searched for in the field. The death of

one parent may result in starvation of some of the

nestlings or desertion of the young by the surviving

parent (e.g. Thomas, 1946). In Table 1, 75 losses

(1.5 percent of all individuals, 3.5 percent of all losses)

are attributed to this factor.

Desertion is a heterogeneous category resulting from

many kinds of disturbances which cause a pair to abandon a nesting attempt. In Table 1, the loss of 146 eggs

(3.4 percent of all eggs, 8.0 percent of egg losses) and

21 nestlings (0.8 percent of all young, 2.6 percent of

nestling losses) resulted from desertion. Again, it is evident either that the factors whose disturbance causes

desertion predominately affect the early stages of the

nest period or that adults develop a stronger nest tenacity as the nesting cycle progresses.

Starvation is restricted to the nestling period, especially when the young are fully grown and require large

quantities of energy, and presumably to the period after

fledging until the acquisition of self-feeding capacities.

Survivorship graphs of several species of marsh-nesting

icterids (Peterson and Young, 1950; Young, 1963)

demonstrate that when starvation is a strong factor the

mortality rate increases continuously during the nestling period. Presumably this results from the increasing

difficulty for the parents to meet the energy demands

of the growing young, although Peterson and Young

do not cite starvation as a major factor. They attribute

the increased mortality in older birds to nest crowding

and enhanced attraction to predators. More recent

336-549 O - 69 - 2
SMITHSONIAN CONTRIBUTIONS TO ZOOLOGY

studies by Orians (1966) and Willson (1966) have

shown that this interpretation may be largely erroneous

although Horn (1968) observed that predation rates

on Brewer blackbird nests increased with age of the

young.

Starvation appears to be of varying importance in

different species. Of those in Table 1, only the icterids

studied by Young (1963) and the song sparrow (Nice,

1937), exhibited any degree of starvation. Young found

that 23 (3.0 percent) of 774 nestling red-winged blackbirds were dead in the nest, apparently due to starvation, and that 107 nestlings (13.8 percent) disappeared

from nests which were not totally destroyed. Many of

these may have been starved young which were removed from the nest by the parents. Thus, as many as

130 of the 444 losses of nestlings (29.3 percent, 16.8

percent of all young hatched) may have been attributable to starvation. Similarly, as many as 35 of the

56 nestling yellowheaded blackbirds lost (63.5 percent,

20.0 percent of all nestlings) may have starved. Orians

(1966, and personal communication) and Willson

(1966) suggest that starvation may be the greatest

single mortality factor in marsh-nesting icterids. It is

impossible to estimate losses due to starvation among

fledglings, but they may be relatively high if energy

requirements of the young are increased because of

activity and temperature regulation, as suggested by

Royama (1966). Starvation has been cited as a significant cause of death in a variety of species: for example,

the common swift (Lack and Lack, 1951), English

blackbird (Snow, 1958), and curvebilled thrasher

(Ricklefs, 1965).

Predation and inclement weather usually have the

same effeect of destroying whole broods, and are important factors during the entire course of the nesting

cycle. Both factors are identified readily in the field, and

the former constitutes the major cause of mortality. In

Table 1, predation accounted for the loss of 1188 eggs

(23.3 percent of the total, 54.9 percent of losses) and

671 young (23.3 percent of all young, 65.8 percent of

the losses). Lack (1954:77) estimated that threefourths of the losses of open-nesting birds in England

are due to predation. Similarly, inclement weather

accounted for 134 egg losses (2.6 percent of the total,

6.2 percent of losses) and 70 deaths of nestlings (2.9

percent of the young, 8.6 percent of losses), but particularly heavy storms may take a much heavier toll

(e.g. Fautin, 1941). These data indicate that the effects

of predation and inclement weather are distributed

evenly between the egg and nestling periods. There are,

however, several situations which may result in heavier

losses during the egg or nestling period. Skutch (1949)

has suggested that the presence of young in the nest

and increased activity of the parents during the nestling

period may attract the attention of predators, but little

evidence to support this hypothesis could be found in

this study (cf., Horn, 1968). A second possibility is that

there may be marked differences in the location of

nests which would result in differences in mortality

rates. Nests that are easy to find would be destroyed

more rapidly, and toward the end of the nest period

the remaining nests would be predominately more

difficult to locate and thus would suffer less predation.

This appears to have occured in the marsh wren (Kale,

1965). Over a four-year period, 56.0 percent of all eggs

were lost to predators but only 30.7 percent of all

nestlings. During the last year of the study, when rice

rats (Oryzomys) were extremely abundant, 75.8 percent of eggs, but only 25.5 percent of young fell victim

to predators.

Thirdly, the development of the young may progressively restrict the variety of animals which prey

upon them. This could be especially important in

large species whose eggs are relatively small, among

raptorial birds which are endowed with formidable

defense capabilities and, of course, among precocial

species which attain mature physical characteristics at

a relatively early age. Also, increasing size enhances

homeostatic capabilities and enables the young to withstand exposure to a greater degree.

MORTALITY FACTORS AS SELECTIVE FORCES.—All

mortality factors must select for adaptations which reduce mortality rates. In addition, many also favor decreased duration of exposure of the eggs and young

through increased developmental rates. This distinction

is important in evaluating the relationship between

selective forces and the breeding strategy. An empirical

method for calculating that component of mortality

which would be reduced by decreasing the development period is presented in Appendix 1.

Hatching failure is determined primarily at fertilization or at later events such as hatching, and thus probably would not be reduced by decreasing the length

of the egg period. Egg failure due to genetic causes

will act to reduce the frequency of deleterious alleles.

To the extent that hatching failure is the result of poor

incubation, selection may also favor closer sitting.

Brood parasites (cuckoos and cowbirds) generally
NUMBER 9

observe nest-building activities of their prospective

hosts before parasitizing the nest (Hann, 1937; Norris,

1947). This factor should exert selection for more

secretive nest building and increased tenacity during

the egg-laying stage, shortened nest-building periods,

and discrimination and destruction of parasite eggs.

The presence of brood parasites may favor more rapid

development rates to increase the ability of the host

young to compete for food. Early hatching and speeded

growth would seem to provide an advantage in this

situation. The parasite is always in a competitive environment as a nestling and it may be significant that,

for its size, the brownheaded cowbird has the most

rapid growth rate of temperate passerine birds

(Ricklefs, 1968b).

Arthropod parasitism is probably a significant factor

only during the nestling period when the young are

poorly feathered and incapable of grooming themselves. When parasites remain in nests which are used

over each season, as in swallows, time is required for

the population to build up during each nesting cycle

(Stoner, 1936) and thus the level of infestation as

well as the total effects must increase with time. The

effects of parasitic flies and other insects which actively

seek host nests must also increase with the length of

exposure. Thus, nest infestation and insect parasitism

should favor the construction of new nests for each

attempt, increased parental attention to the young,

shortened nestling periods, and perhaps more precocious development of grooming activity. These selection

pressures, however, are probably quite weak in most

species.

Among small land birds, nest-site competition probably is found only among hole-nesting species, but it

can be a strong selective force. This factor must favor

increased nest defense, and reduced nest building and

egg periods. For example, the prothonotary warbler

requires 3.3 days on the average to build its nest in

Michigan where house wrens cause the loss of almost

a quarter of all eggs. In Tennessee, where nest-site

competition is not a factor, nest building occupies an

average of 8.8 days (Walkinshaw, 1953). Losses of

young due to nest-site competition are not as great as

losses of eggs and thus this factor will have a smaller

effect on the length of the nestling period.

Adult mortality will select for early breeding and

decreased periods of dependency of the young. One

could also postulate an advantage to reducing the

breeding effort on the part of the parents by reducing

brood size, but this would be strongly opposed by decreased productivity. Adult death is generally weak

compared to other selective forces (in Table 1, 1.5

percent of all eggs laid were lost to this cause) and

probably assumes little significance in the breeding

behavior of birds.

Desertion is a behavioral response to a variety of

disturbances and thus must itself be classified as an

adaptation rather than as a selective mortality factor,

even though it is treated as a mortality factor in empirical analyses of nesting losses.

Starvation has been shown to be an important

selective factor affecting clutch size and breeding season (Lack, 1954), and other behavioral patterns such

as asynchronous hatching and brood reduction (Ricklefs, 1965). Fluctuations in food availability which

result in occasional suboptimal conditions for feeding

would favor reduced periods of dependence on the

parents for food. If nestlings require less food than

fledglings, however, as suggested by Royama (1966)

and the observations of Morehouse and Brewer (1968),

an effect of starvation may be to favor lengthened

nestling periods. On the other hand, sibling competition during periods of food shortage would favor increased development rates and perhaps shortened

nestling periods.

The overall effects of predation and inclement

weather are decreased considerably by reducing the

length of any stage of the development period, chosing

cryptic or well-protected nest sites and adopting antipredator and protective behavior on the part of the

adults. Predation is the most important mortality factor during the nesting cycles of small land birds, although its selective strength varies considerably from

species to species (Table 1).

These conclusions on the action of mortality factors

are based on accounts of nesting success in passerines

only, but should apply more widely. In some groups,

however, the relative stength of the factors may vary

greatly. Large raptorial birds are subject to little predation, but starvation may assume considerable importance (Lack, 1954). In colonially nesting seabirds,

losses may be brought about by crowding (Stonehouse,

1962), and adults often peck unattended checks to

death (Nelson, 1966). Waterfowl nesting success may

frequently be reduced by extensive flooding of breeding areas. In very hot and very cold regions, exposure

may be quite important. These factors will be discussed in further detail where they are relevant.
SMITHSONIAN CONTRIBUTIONS TO ZOOLOGY

IDENTIFICATION OF MORTALITY FACTORS.—It is possible to predict how each mortality factor will affect

the relationship between mortality during the egg and

nestling periods and between individual and nest

mortality (Table 2). When the difference between

mortality rates during the egg and nestling periods

(vertical component) is graphed against within-nest,

or partial mortality (the difference between individual

and whole nest mortality), the resultant vector indicates the predominate mortality factors in the sample

(Figure 1). Losses due to adult death, predation, and

weather are well balanced between the egg and nestling

periods (Table 1) and produce negligible within-nest

loss. Therefore, the vectors for these factors are quite

small. Cowbird parasitism is typically characterized

by high mortality during the egg period caused by loss

of eggs from within clutches and desertion, and high

within-brood losses caused by partial loss of eggs and

possibly starvation. Hatching failure results in a small,

but consistently present vector in the same direction.

If egg mortality is high but within-brood losses are

low, desertion and nest-site competition are probably

significant factors, although it has been shown above

that under some circumstances, predation may produce

this result.

Starvation differs from these factors in that its vector

includes a large within-brood component and is limited to the nestling period. Analysis of nesting losses

presented below indicates that starvation also may result in a large number of whole brood losses, perhaps

through desertion or increased susceptibility to preTABLE 2.—Nesting loss patterns of mortality factors

Relationship between

mortality during the

egg and nestling

periods

Greater during the

egg period

Equally weighted

Greater during the

nestling period

Loss of individuals within nests

Low High

Nest-site Hatching failure <

competition

Desertion Brood parasitism '

Adult death

Predation

Weather

Infestation

Starvation

1—

LU

O

o

o

_J

<

o

1—

Q:

LJJ

>

tio

n seri

Q

/

/

/

PARTIAL LOSS

\

\

• These factors are event related but are included in calculated mortality rates.

FIGURE 1.—Postulated vectors for major mortality factors

during the nest period. The vertical component is the difference between mortality rates during the egg and nestling

periods. Partial losses represent the within-nest component

of mortality rates. Brood parasitism resembles hatching failure

and infestation resembles starvation in mortality pattern.

dation and death from exposure or inclement weather.

The action of several mortality factors together will

produce intermediate vectors which are more difficult

to identify.

Mortality Rates

TEMPERATE ZONE SPECIES.—Fifty studies of nesting success in small altricial Temperate Zone land
NUMBER 9

birds are summarized in Table 3a. Many of these have

been discussed by Lack (1954) and Nice (1957) and

the reader will find their comments valuable. Daily

mortality rates, calculated from Equation 3 for nests

and individuals, and egg and nestling periods where

possible are presented in Table 3b for each of these

studies. Differences between individual and nest mortality rates (m — M) are related to the magnitude of

within-nest loss. Similarly, the difference between individual mortality rates during the egg and nestling

periods (me — mn) also are presented. Positive values

indicate that rates of egg loss were greater than nestling

TABLE 3a.—Nesting success of small Temperate Zone altricial land birds

Species and study designation

number

Costa hummingbird

(Calypte costa)

1

Mourning dove

(Zenaidura marcoura)

2

3

4

5

6

TraiU flycatcher

(Empidonax traillii)

7

Eastern and Say phoebes

(Sayornis phoebe, S. sayus)

8

Rough-winged Swallow

(Stelgidopteryx ruficollis)

9

9a. Six or fewer eggs

9b. More than six eggs

Tree swallow

(Iridoprocne bicolor)

10

11

12

13

14

15 Average

Horned lark

(EremophUa alpestris)

16

Black-capped chickadee

(Parus atricapillus)

17

Long-billed marsh wren

( Telmatodytes palustris)

18

18a Best year

18b Worst year

Length

of

study

(years)

6

8

3

9

4

2

2

4

4

4

9

3

3

2

8

25

2

1

4

1

1

Nests

(number)

29

4273

249

592

235

204

47

99

66

39

23

219

352

80

37

60

30

10

269

46

62

Eggs

(number)

58

8018

500

398

403

222

165

1123

1759

430

184

363

102

74

1111

178

235

Nest success (percent)

Egg

65.6

78.8

79.3

91.3

Nestling

63.2

82.7

74.2

95.3

Total

41.4

47.7

52.2

52.3

51.9

69.7

76.6

77.5

65.2

59.0

87.0

60.0

Egg success (percent)

Egg

58.7

54.6

77.9

81.1

79.3

91.4

83.4

81.0

72. 1

88.6

98.6

84.7

77.4

29.3

51.7

11. 1

Nestling

55.9

85.2

88.4

84.2

74.4

95.3

73.2

60.2

97.7

75.5

95.0

80.3

58.2

66.6

80.5

61.6

Total

32.8

46.6

42.6

68.9

68.3

59.0

87.3

61.0

48.7

70.5

66.8

93.7

68. 1

45. 1

71.6

19.5

41.6

6.8

Source and locality

Woods, 1923, California

McClure, 1946a, Iowa

Nice, 1931, Oklahoma

Pearson and Moore, 1939,

Alabama

Monk, 1949, Tennessee

Cowan, 1952, California

Berger, 1957, Michigan

McClure, 1946b,

Nebraska

Lunk, 1962, Michigan

Do.

Do.

Chapman, 1939

Low, 1934

Kuerzi, 1941

Shelley, 1937

Weydemeyer, 1935,

Montana

Pickwell, 1931

Odum, 1941, New York

Kale, 1965, Georgia

Do.

Do.

See footnotes at end of table.
10 SMITHSONIAN CONTRIBUTIONS TO ZOOLOGY

TABLE 3a.—Nesting success of small Temperate Zone altricial land birds—Continued

Species and study designation

number

House wren

(Troglodytes aedon)

19

20

21

Cactus wren

(Campylorhynchus brunneicapillus)

22

American robin

(Turdus migratorius)

23

24

Eastern bluebird

{Sialia sialis)

25

26

27

Cedar waxwing

(Bombycilla cedrorum)

28

Starling

(Sturnus vulgaris)

29

Prothonotary warbler

(Protonotaria citrea)

30 Michigan

31 Tennessee

Yellow warbler

(Dendroica aestiva)

32

Ovenbird

(Seiurus aurocapillus)

33

Orchard oriole

(Icterus spur ins)

34

Yellow-headed blackbird

(Xanthocephalus xanthocephalus)

35

Red-winged blackbird

(Agelaius phoeniceus)

36

37a

37b

Combined

Length

of

study

(years)

19

21

12

2

3

11

2

9

6

3

4

2

2

2

1

2

1

1

1

<•!

Wests

(number)

1056

64

104

55

136

176

1401

86

67

60

121

36

41

24

50

116

91

Eggs

(number)

6773

333

581

259

548

6260

377

272

245

801

413

163

168

161

157

369

325

563

577

1140

Nest success (percent)

Egg Nestling

79. 9 85. 0

64. 7 83. 7

80. 6 87. 0

80. 1 95. 8

85. 7 73. 1

Total

82.8

67.8

57.4

48.7

70.2

76.7

23. 1

69.5

41.0

62.6

Egg success (percent)

Egg

82.3

59.7

73.1

60.7

57.7

63.0

80. 1

78.3

77. 1

91.2

38.5

61.3

70.8

63.3

83.5

45.2

79.4

74.3

70.3

72.3

Nestling

94.0

80.9

91.8

83.5

77.9

70.6

90. 7

80.6

90.4

84.2

66.7

100.0

76.5

68.7

96.2

67.0

65.9

80.4

84.5

82.5

Total

79.0

48.3

67.2

50. 7

44.8

44. 5

72.7

63.3

69.8

76.8

25.7

61.3

54.2

43.5

80.3

30.0

52.3

59.7

59.4

59.5

Source and locality

Kendeigh, 1942, Illinois

Walkinshaw, 1941,

Michigan

Balwin and Bowen, 1928,

Ohio

Anderson and Anderson,

1960, Arizona

Howell, 1942, New York

Young, 1955

Laskey, 1943, Tennessee

Low, 1933

Thomas, 1946, Arkansas

Putnam, 1949, Ohio

Dunnet, 1955, Scotland

Walkinshaw, 1953

Do.

Schrantz, 1943

Hann, 1937, Michigan

Dennis, 1948, Louisiana

Young, 1963, Wisconsin

Beer and Tibbitts, 1950,

Wisconsin

Smith, 1943, Illinois

Do.

Do.

See footnotes at end of table.
NUMBER 9 11

TABLE 3a.—Nesting success of small Temperate gone altricial land birds—Continued

Species and study designation

number

Red-winged blackbird—Con.

38a

38b

Combined

39a

39b

Combined

40

41

Brewer blackbird

(Euphagus cyanocephalus)

42

Common grackle

(Quiscalus quiscula)

43

44

Common goldfinch

(Spinus tristis)

45a

45b

45c

Combined

Chipping sparrow

(Spizella passerina)

46

Field sparrow

(Spizella pusilla)

47

Song sparrow

[Melospiza melodia)

48a

48b

Combined

House finch

(Carpodacus mexicanus)

49

McCown longspur

(Rhynchophanes mccowni)

50

Length

of

study

(years)

1

1

2

1

1

2

1

1

3

2

1

1

1

3

25

10

"3

6

5

3

Nests

(number)

62

76

138

238

280

518

67

243

107

62

33

56

81

102

239

50

593

147

76

211

45

Eggs

(number)

221

255

476

730

902

1632

214

821

521

288

146

170

206

320

696

152

1738

585

321

854

no

153

Nest success (percent)

Egg

82.3

68.3

74.7

81.8

66.0

69.6

Nestling

74.5

69.2

71.8

74. 1

93.9

68.0

Total

61.3

47.4

53.7

34.9

23.9

29.0

26.7

49.5

54.8

60.7

62.0

38. 1

47.4

60.0

Egg success (percent)

Egg

76.0

63.6

69.3

53.8

44.6

48.7

77.6

40.2

62.7

72.6

80. 1

63.5

57.8

71.3

65.4

68.5

51. 1

66.5

45.8

59.7

72.8

60.2

Nestling

64.9

56.8

60.9

51.9

40.3

45.5

63.3

59.6

62.7

64.7

65.8

58.3

77.3

80.3

74.3

89.4

69.8

62.5

54.4

60.0

71.3

77.7

Total

49.4

36. 1

42.3

28.0

18.0

22.5

49. 1

23.9

39.3

46.9

52.8

37. 1

44.7

57.2

48.6

61.2

35.7

41.5

24.9

35.8

51.8

46.4

Source and locality

Weins, 1965, Wisconsin

Do.

Do.

Young, 1963, Wisconsin

Do.

Do.

Williams, 1940, Ohio

Goddard and Board,

1967, Oklahoma

La Rivers, 1944, Nevada

Peterson and Young,

1950, Wisconsin

Weins, 1965, Wisconsin

Stokes, 1950, Wisconsin

Do.

Do.

Do.

Walkinshaw, 1952, Michigan

Walkinshaw, 1952, Michigan

Nice, 1937, Ohio

Do.

Do.

Evenden, 1957, California

Mickey, 1943, Wyoming.

o Combined data represent two colonies.

» First three years when the environment was favorable.

« Last three years when the environment was badly disturbed.
12

SMITHSONIAN CONTRIBUTIONS TO ZOOLOGY

TABLE 3b.—Mortality rates of small Temperate Zone altricial land birds

Species

study

number »

1

2

3

4

5

6

7

8

9

15

16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

31

32

33

34

35

36

37

38

39

40

41

42

43

44

45

46

47

48

49

50

Laying

1

1

2%

3

5

4

3

6

3

5

3

2K

3

3 ^

4

3K

3

3%

2

2

2%

3M

3

3>2

2%

2X

3

3H

2}i

Length of nesting cycle

(days)

Incubation

16

15

12

16

15

15

11

13

13

13

16

13

14

14

i2y2

n

12

14

13

12

13

n y2

13*

10

10

12*

13

12

Nestling

22

13*

14

16

20

19

10

18

14

16

20

13

17*

15*

21

11

10

8

13

13

i o *

13

12

12*

11

11

10

15

10

Total

39

2 9 *

2 8 *

35

40

38

24

37

30

3 4 *

39

2 8 *

3 4 *

3 0 *

39

27

24

2 3 *

29

28

25

2 9 *

2 6 *

29%

2 3 *

2 3 *

2 5 *

3 1 *

2 4 *

Daily mortality rate of nests

Egg

M,

2.48

1.19

1.25

2.81

1.27

1.48

1.07

2.01

1.39

3.33

2.34

(percent)

Nestling

Mn

2.09

0.94

0.81

0.93

0.80

0.28

2.99

3.16

2.50

0.57

3.86

Total

M

2.26

2.51

2.20

2.20

2.23

1.22

0.94

0.73

1.07

2.13

0.55

1.00

1.89

2.52

1.03

0.87

5.43

1.35

3. 19

1.87

2.49

4.96

5.28

2.39

2.27

1.88

2.04

4.11

2.93

2.09

Daily mortality rate of eggs

Egg

m.

3.14

3.79

1.56

1.05

0.88

1.83

7.68

1.08

2.87

1.74

3.22

3.55

2.72

1.31

1.44

1.74

0.51

5.97

3.06

2.47

2.95

1. 13

5.30

1.59

2.24

2.53

4.97

1.81

6.28

2.83

2.21

1.53

2.50

3.03

5.38

3.33

1.93

3.50

(percent)

Nestling

"»n

2.65

1.18

0.92

0.85

1.16

5.28

2.87

0.37

1.28

0.52

1.38

1.77

2.03

0.56

1.23

0.65

0.82

3.68

0.00

2.68

4.70

0.30

3.08

3.98

1.84

4.72

7.50

4.36

4.94

3.59

3.63

3.49

2.38

1.02

3.27

5.12

2.26

2.53

Total

m

2.86

2.59

2.90

1.26

0.96

1.01

3.32

0.90

5. 46

0.68

2. 11

1. 15

2.38

2.82

2.35

0.92

1.33

1. 18

0.68

5.04

1.82

2.56

3.54

0.76

4.31

2.59

2.08

3.44

5.97

2.85

5.73

3. 16

2.86

2.41

2.45

2.09

4.38

4.03

2.09

3. 14

Partial

loss

(percent)

m-M

0.60

0.08

0.70

0.04

-0.12

1.19

0.60

0.49

0.30

0.30

0.31

-0.39

0.47

1.12

0.72

0.95

1.01

0.45

0.77

0.59

0.53

0.05

0.27

1.10

1.05

Vertical

component

(percent)

"»«-»!„

0.49

2.61

0.64

0.20

-0.28

-3.49

4.81

0.71

1.59

1.22

1.78

1.78

0.69

0.75

0.21

1.09

-0.31

2.29

3.06

-0.21

-1.75

0.83

2.22

—2.39

0.40

-2.19

-2.53

-2.55

1.34

-0.76

-1.42

-1.96

0.12

2.01

2.11

-1.79

-0.33

0.97

See Table 3a for species designation in relation to study number.
NUMBER 9 13

loss. The data for a few species permitted analysis of

within-brood mortality separately for the egg and

nestling periods (Table 4).

Daily mortality rates range from about 0.5 percent

to more than 5 percent. It is evident that mortality

rates may vary considerably from year to year (e.g.

marsh wren and common goldfinch) and between localities (e.g. red-winged blackbirds), but no attempt

has been made to analyze this aspect of nesting loss.

One must always keep in mind that most studies are

small and limited samples and thus are only crude

estimates of nesting success for the entire population.

For this reason, the data for any individual species will

not be dwelled upon at length. The conclusions of this

paper are derived largely from comparisons among

species groups.

The studies are grouped according to nest location

and habitat in Table 5. Hole-nesting and niche-nesting

species suffer the least mortality during the nest period

and marsh-nesting species are subject to the greatest

losses. Species which nest on or near the ground have

substantially lower nest survival than those which build

nests in large bushes or trees above the reach of grounddwelling animals. The orchard oriole builds a hanging

nest in trees and enjoys higher nesting success than

open-nesting species (Dennis, 1948). Another study

which is difficult to place is that of the cactus wren

(Anderson and Anderson, 1960), the only desert species

represented in the sample. Its nest is an enclosed

structure usually placed in treelike cholla cacti which

evidently provide excellent protection in view of the

cactus wren's low mortality rate (m = 1.00).

Brewer blackbirds and common grackles feed in wet

or marshy habitats, but usually nest in trees bordering

their foraging habitats (species studies 42, 43). Overall

mortality rates of both species are most similar to those

of open-nesting birds above the ground, but the vertical

components (—0.76, —1.42) and partial losses (0.77,

0.59) are closest to some marsh-nesting species (e.g. the

red-winged blackbird). Grackles nesting in marshes

have similar mortality rates to those nesting in bordering trees in one study (species study 44).

Most of the ground-nesting species in the sample

inhabit open areas, but the ovenbird, which builds its

nest on the forest floor, exhibits high nesting mortality

(m = 3.35) as do field species, and suffers greater losses

than forest and second-growth species nesting above

the ground. Thus, the location of the nest rather than

the habitat would appear to be the major factor influencing nesting success. Unfortunately, little is known

of the nesting success of other ground-nesting warblers

and finches in forests. When comparing mortality rates

of marsh wrens and ovenbirds with hole-nesting

species, it is evident that enclosed nests do not provide

the protection against predators of hole nests which

are built within such substantial materials as tree

trunks, mud banks, and rock outcroppings. The value

of an enclosed or covered nest may lie in concealment

for the overbird, or protection from rain and cold

for the marsh wren. The enclosed nest of the cactus

TABLE 4.—Further analysis of vertical components and within-nest mortality in small Temperate

Zone altricial land birds

Species and study designation number

1. Costa hummingbird

9. Rough-winged swallow

23. American robin

27. Eastern bluebird

28. Cedar waxwing

36. Red-winged blackbird

38. Red-winged blackbird

44. Common grackle

46. Chipping sparrow

48. Song sparrow

Vertical component

(percent)

Nests

M,~MD

0.39

0.25

1.88

0.57

1.20

-1.92

- 1 . 15

- 1 . 11

2.76

-1.52

Eggs

0.49

0.20

1.84

0.21

1.09

-2.39

- 2 . 19

-1. %

2.01

-1.79

Partial loss (percent)

Egg

0.66

- 0 . 14

0.41

0. 17

0.26

0.52

0.52

0. 14

-0.30

0.99

Nestling

ma-Mn

0.56

-0.09

0.45

0.53

0.37

0.99

1.56

0.99

0.45

1.26

Total

m—M

0.60

- 0 . 11

0.43

0.30

0.31

0.72

0.95

0.53

0.05

1. 10

336-549 O - 69 - 3
14 SMITHSONIAN CONTRIBUTIONS TO ZOOLOGY

TABLE 5.—Nesting mortality parameters for small Temperate gone altridal land bird species

grouped according to nest location »

Nesting mortality parameters

{percent per day)

Overall individual mortality rate, m

Number of species

Average

Range

Vertical component, me—mn

Number of species

Average

Range

Partial loss, m—M

Number of species

Average

Range

In marshes

3

4.52

3. 78,5. 46

3

1.90

-1.32,4.81

2

0.95

0. 78, 1. 12

Open nests

On ground

5

3.68

3. 14,4. 38

5

-0.7 9

-3.49,2 . 11

4

0.90

0. 27, 1. 19

Above ground

9

2.12

0. 94, 2. 86

8

0.82

- 0 . 33, 2.01

5

0.33

0. 05, 0. 60

Hole and

niche nests

7

1.02

0. 68,1. 53

3

0.29

- 0 . 28, 1. 13

5

0.26

- 0 . 12, 0. 60

° Excluding prothonotary warbler, cactus wren, orchard oriole, Brewer blackbird and common

grackle (see text).

wren has been shown to aid temperature regulation of

the young by providing a moderated environment

(Ricklefs and Hainsworth, 1968).

The largest negative vertical components of mortality occur among ground-nesting species and some

marsh birds. Within-nest losses are also very high. The

starvation components of these species suggest that

fields and marshes are characterized by more variable

food supplies than forests and second-growth habitats.

Hole-nesting and niche-nesting birds exhibit small

vectors. Species nesting above the ground in secondgrowth and forested areas generally have small withinnest components and positive vertical components, suggesting either that desertion is a major factor or that

predation is heavier on eggs than nestlings because of

substantial variability in the accessability of nests to

predators (see above). The latter mechanism would

be expected to be more prevalent in marshes, which are

simple habitats, and where high breeding densities of

some colonial species may limit the number of optimum

nest sites. No matter how unlikely, predation cannot

be ruled out as a factor causing positive vertical components in forest-nesting species.

Values for the rate of loss of individuals within nests

during the egg period ranged from — 0.30 to 0.99 percent per day with an average for the 10 studies of 0.32

(Table 4). Negative values indicate that the average

number of young hatched per successful nest was

greater than the average number of eggs laid. This can

be explained either by the loss of clutches before their

completion or by greater losses of small clutches. For

example, the within-nest component in the roughwinged swallow was —0.14 percent per day during the

egg period. (Lunk, 1962). Clutches of six or fewer

eggs had an observed survival of 79.3 percent, whereas

larger clutches had an observed survival of 91.3 percent through the egg period.

The average value for within-nest loss during the

incubation period is similar to that which would be

expected owing to hatching failure alone. If 5.0 percent of all eggs fail to hatch (Table 1) and the egg

period is 16 days (Table 3b) the apparent mortality

rate would be 0.32 percent per day. The highest value,

0.99 percent for the song sparrow, would be expected to

be due to cowbird parasitism, which was prevalent in

that study (Nice, 1937).

Within-brood loss during the nestling period varied

from —0.09 to 1.56 percent per day (average, 0.71).

The single negative value—for the rough-winged swallow—can be attributed to the cause discussed above

for this species. The within-brood components nestling mortality indicate that starvation is often a strong

mortality factor. Three of the four highest values were

observed for marsh-nesting blackbirds and grackles
NUMBER 9 15

which are often subject to heavy starvation (Young,

1963; Orians, 1966; Willson, 1966; Table 1). The song

sparrow (Nice, 1937), whose environment was badly

disturbed during four years of a seven-year study and

which was the victim of heavy cowbird parasitism, also

exhibited large within-brood loss. Nice, however, attributed, perhaps erroneously, the heavy loss of young to

increased predation (Table 1) rather than to starvation.

The studies may be analyzed graphically by plotting

the difference between mortality rates during the egg

and nestling periods on one axis and the within-nest.

component of mortality on the other (Figure 1). MorO)

-1

-2

ITL-Me

 m e

FIGURE 2.—Partial Ion during the egg period and vertical

components of mortality rates of temperate passerine birds.

Numbers correspond to species in Table 3a. Abcissa and

ordinates are calibrated in percent per day. Vectors for the

various mortality factors are as in Figure 1.

tality caused by adult death, predation, and weather

produces negligible vectors on this coordinate system

(Table 2). Other factors will produce vectors in the

directions shown in Figure 1. The studies presented in

Table 4 are shown graphically in Figures 2 and 3. In

this sample, high mortality during the egg period relative to the nestling period (species studies 23, 28, 46)

must be caused either by nest-site competition or desertion rather than hatching failure and cowbird parasitism because the later also would produce large within-clutch components. Because these species are open

nesters, desertion, rather than nest-site competition,

<D

-1

-2

mn-Mn

FIGURE 3.—Partial loss during the nestling period and vertical components of mortality rates of temperate passerine

birds (from Table 3a). Scale and vectors are as in Figure 2.

336-549 O - 69 - 4
16

must have been primarily responsible for the observed

vectors. The position on the graph of two hole-nesting

species, the rough-winged swallow (species study 9)

and the eastern bluebird (species study 27) indicate

that predation was probably the only significant mortality factor acting during the egg period. In the redwing blackbird (species studies 36, 38) and the common grackle (species study 44), hatching failure could

account for the within-clutch component. Other factors acting strongly during the nestling period are responsible for the large negative vertical components in

these species.

Interpretation is difficult when more than one factor

(other than adult death, predation, and weather) act

strongly. The high within-clutch component of the

song sparrow (species study 48) may be attributed to

cowbird parasitism, whose vector lies in the upper

right quadrant of this graph (Figure 1). Apparently,

another factor acting during the nestling period completely obscured the positive vertical vector which

should result from cowbird parasitism. It is also possible that parasitism may bring about starvation of

nestlings. Both Nice (1937) and Norris (1947) found

that the presence of a cowbird nestling resulted in

about one fewer host young being raised. This loss,

however, usually is incurred during the egg-laying

period rather than through the starvation of nestlings.

Factors which produce within-brood loss of nestlings

also produce a negative vertical component of the same

magnitude. Within-brood loss of species studies 1, 23,

27, 28, and 46 (Figure 3) are low (0.37-0.56) and

their small negative vertical components are obscured

by other factors acting more strongly during the egg

period. It is interesting to note that large positive

vertical components (desertion) are not present in the

species whose within-brood losses are high owing to

starvation.

The negative horizontal component of the roughwinged swallow (species study 9) has been discussed

above. Within-brood losses of species studies 36, 38,

44, and 48 are large (0.99-1.50) and corresponding

vertical components are large and negative, more so

than would be predicted. Therefore, either predation

and inclement weather produce a negative vertical

vector, or starvation results in the loss of whole broods.

Predation and weather accounted for the loss of 26.7

and 23.8 percent of the total eggs and young in Table

1. The average egg and nestling periods are 16.0 and

12.4 days, respectively, and thus the average daily

SMITHSONIAN CONTRIBUTIONS TO ZOOLOGY

mortality rates are 1.93 and 2.13 percent. This difference would result in a vertical vector of —0.20 percent

which would account for little of the difference between the observed and predicted vertical components

(—0.56 to —1.40 percent). Thus, it seems necessary

to reevaluate the effects of starvation by postulating

considerable loss of whole broods to this agent, presumably by desertion of broods in which one or more

of the young had died, or less likely by the starvation

of entire broods. Infestation probably does not constitute a significant portion of the observed components.

From Figure 3 one may estimate that about one third

of nestling losses due to starvation involve the loss or

desertion of whole broods.

We may also graph the difference between the rate

of loss of individuals during the egg and nestling

periods, me — mn, against the difference between the

rate of loss of nests during those periods, Mc — Mn

(Figure 4). Positive values of both components should

be highly correlated because factors specifically producing high egg loss (desertion and, in some circumstances, predation) also result in high nest loss. In

the chipping sparrow (species study 46), nest loss is

higher than egg loss during the egg period which suggests selective destruction either of small clutches or

of nests during the laying period. This is further implied by the negative within-clutch mortality component of this species. The low within-brood loss indicates

that starvation was probably not a factor.

If starvation does not act on whole broods, the

difference between rates of nest loss during the egg

and nestling periods would remain near zero when the

rate of nestling loss exceeds egg loss. In this sample,

however, the nest loss component of two studies nearly

equals the nestling loss component. This may be due

partly to within-clutch losses from hatching failure and

brood parasitism, especially in the song sparrow (species study 48), which tend to move the points up on

the graph. In addition, Figure 4 indicates that in conjunction with starvation, whole brood losses occur at

a proportionally higher rate among small broods; that

is, it appears that more than one third of nestling losses

in excess of egg losses were from entire broods. Here

again, of course, any increased predation during the

nestling period would have the same effect.

In Figure 5, the within-nest mortality component

during the egg and nestling periods combined is

graphed against the difference in mortality rates between incubation and nestling periods, as before. The
NUMBER 9 17

/ W

_i i_

M,-Mr

FIGURE 4.—Vertical components of individual mortality

rates plotted against vertical components of whole nest mortality rates. Dashed line represents theoretical effects of desertion (or unbalanced predation) for positive values of m.—m»

and of starvation for negative values of m.—nin. Dotted line

represents losses restricted to whole nests for negative values

of m.—mn. Scale as in Figures 2 and 3. Data from Table 4.

vectors which will be produced by within-nest loss during either the egg or nestling period are shifted toward the vertical axis a distance proportional to the

amount which the nestling and egg periods, respectively, are of the total nest period. The predicted lines

in Figure 5 are based on the average egg and nestling

periods of the species shown. These data further indicate that starvation and desertion are not likely to be

major factors in the same study. Apparently, the McCown longspur (species study 50) and perhaps the

Costa hummingbird (species study 1) and Brewer

blackbird (species study 42) are exceptions, yet studies

such as that on the yellow-headed blackbird (species

study 35) emphasize the difficulties in interpretation.

This species has a very large within-nest mortality component with losses weighted heavily toward the egg

period, and, in fact, lies on the vector for brood parasitism and hatching failure. Starvation caused a portion

of the within-nest losses but the disappearance of eggs

from within clutches was equally important (Table 1).

The positive vertical component was caused primarily

by high predation on eggs (27.7 percent compared with

12.0 percent of nestlings), perhaps resulting from the

same circumstances discussed above for the marsh

FIGURE 5.—Vertical components of individual mortality rates

versus partial loss during the nest period for Temperate

Zone passerine birds (and one dove and one hummingbird).

Dashed lines represent vectors for hatching failure and starvation (see text). Scale of daily mortality rates are in percent. Numbers correspond to species in Table 3a.

wren. The fact that both of these are marsh-nesting

species is interesting.

TROPICAL SPECIES.—Relatively few nesting-success

data are available for tropical species and this account

will rely strongly on Skutch (1966) for humid tropical
18 SMITHSONIAN CONTRIBUTIONS TO ZOOLOGY

areas (Tables 6a and 6b) and Marchant (1960) for

an arid tropical area (Tables 7a and 7b). In general,

birds of humid tropical regions are less successful

breeders than temperate species although some have

comparable nesting success. Skutch (1966) found the

survival of forest species to be lower .than those of

clearings and second growth (Table 8). Snow and

Snow (1963) also demonstrated that nesting success

of robins is higher on plantations than in forested areas

(Table 6a). Open habitats, however, are probably

fairly recent in humid tropical areas because of their

association with man, and thus predators may not have

fully adjusted to these new conditions. Hole nests are

more successful than open nests (Table 8), but less

so than hole nests in temperate regions (Tables 3 and

5).

TABLE 6a.—Nesting success of small altridal land birds in humid tropical regions

Species and study designation number

Ruddy ground-dove

(Columbigallina talpacoti)

1

Black-and-whitC manalrin

(Manacus manacus)

2

Gray-capped flycatcher

(Myiozetetes grenadensis)

3

Yellow-bellied elaenia

(Elaenia JUwogaster)

4

Clay-colored robin

(Turdus grayi)

5

Robins (Turdus spp.)

6 Forest »

7 Plantation •

Scarlet-rumped tanager

(Ramphocelus passer inii)

8

Blue-gray tanager

(Thraupis episcopus)

9

Yellow-laced grassquit

(Tiaris olivacea)

10

Blue-black grassquit

(Volatin ia jocarina )

11

House wren

(Troglodytes aedon) d

12

Nests

{number)

21

227

30

20

24

57

56

80

27

44

22

25

1

(m

?ggs

tmber)

40

454

76

36

62

156

50

105

53

90

Nest

success

(percent)

total

period

23.8

« 19.4

43.3

20.0

25.0

21.0

37.5

43.8

44.4

29.6

72.0

Egg success (percent)

Egg

50.0

61.9

36.1

30.7

61.6

68.0

44.8

73.7

75.6

Nestling

40.0

65.9

46.2

52.7

68.7

64.7

57.4

76.9

82.3

Total

20.0

16.3

40.8

16.7

16.1

42.3

44.0

26.7

56.6

62.2

Source and locality

Skutch, 1956, Costa Rica

Snow, 1962, Trinidad

Skutch, 1966, Costa Rica

Skutch, 1966, Costa Rica

Do.

Snow and Snow, 1963,

Trinidad

Do.

Skutch, 1966, Costa Rica

Do.

Do.

Alderton, 1963, Canal

Zone

Skutch, 1966, Costa Rica

•Nest success during the egg period, 40.1 percent, during

the nestling period, 48.4 percent.

» T./umigatus and T. albicollis.

e

 T.fumigatus and T. nudigenis.

d

 Hole-nesting species.
NUMBER 9 19

Species

study

number

1

2

3

4

5

6

7

8

9

10

11

12

TABLE 6b.1—Mortality rates of small altricial land birds in humid tropical regions

Length of nesting cycle (days)

Laying

1

1

2

1

2

2

2

1

1

IK

2

Incubation

12,4

18

16

15H

12

12

12

12

13tf

13

10

16

Nestling

12tf

14

20

17*4

15

15

15

12

18

14

9

18

Total

26

33

38

34

29

29

29

25

32^

28^

21

36H

Daily mortality rate

of nests (percent)

Egg Nestling Total

M. Mn M

5.53

4.81 5. 18 4.98

2.20

4.73

4.78

5.38

3.38

3.30

2.50

4.28

0.90

Daily mortality rate

£

gg

m.

5.14

2.67

6.18

8.45

3.73

2.66

5.54

2.54

1.51

r eggs

 (percent)

Nestling Total

7.33

2.09

4.42

4.27

3.13

2.42

3.97

2.93

1.08

m

6.20

5.50

2.36

5.27

6.31

3.44

2.53

4.63

2.71

1.30

Partial

loss,

m—M

(percent)

0.67

0.52

0.16

0.54

1.53

0.14

0.03

0.35

0.40

Vertical

component,

fflf—171 n

(percent)

- 2 . 19

0.58

1.76

4.18

0.60

0.24

1.57

-0.3 9

0.43

TABLE 7a.-—Nesting success of small altricial land birds in arid tropical regions—all data are from a four-year study by

Marchant {I960) in Ecuador

Species and study designation number

1. D'Orbigny ground-dove (Eupelia cruziana)

2. Groove-billed ani (Crotophaga sulcirostris)

3. Vermillion flycatcher (Pyrocephalus rubinus)

4. Short-tailed ground-tyrant (Muscigralla brevicauda)

5. Fulvous-headed pygmy-tyrant (Euscarthmus molorhyphus)

6. Long-tailed mockingbird (Mimus longicaudatus)

7. White-browed gnatcatcher (Polioptila plumbea)

8. Ecuadorian neorhynchus (Neorhynchus peruvianus)

9. Chestnut-throated seed-eater (Sporophila telasco)

10. Crimson finch (Rhodospingus cruentus)

11. Bonaparte warbling finch (Poospiza hispaniolensis)

Nests

(number)

283

27

188

36

22

184

35

264

254

96

68

Eggs

(number)

477

111

330

103

38

447

84

472

419

241

147

Nest success

(percent)

total period

56.5

51.8

49.4

38.9

59.1

50.6

62.8

50.0

37.4

64.5

47.1

Egg success • (percent)

E

gg

76.7

69.2

79.8

57.3

»68.4

76.3

»72.7

80.2

65.7

76.4

83.0

Nestling

81.4

82.9

57.4

69.5

* 73.1

66.0

»60. 7

74.3

67.7

84.3

54.2

Total

55.2

55.8

38.4

37.3

50.0

40.8

44.1

51.0

36.8

59.1

37.7

Includes only eggs and young whose fates are known.

6

 Fates of all eggs and young are known.

The tropical data, graphed on the coordinate system used in Figures 2, 3, and 5, appear strikingly different from the temperate region sample. The humid

tropical species (Figure 6) almost completely lack the

high within-brood losses and negative vertical components which characterize starvation. The large

within-nest mortality component of the clay-colored

robin (species study 5, m-M= 1.53 percent per day)

is associated with high losses during the egg period

which indicates hatching failure as the primary mortality factor other than predation. Skutch (1966)

makes no mention of brood parasitism, although there

are numerous parasitic cuckoos and cowbirds in the

tropics. Positive vertical components can be accounted

for largely by within-clutch loss of eggs, thus eliminating desertion as a factor. It is also clear that predation

does not act more strongly on either the egg or nestling

periods. Skutch attributes the greatest portion of nesting mortality to predation and his paper should be

consulted for a more detailed account of nesting success
20 SMITHSONIAN CONTRIBUTIONS TO ZOOLOGY

TABLE 7b.—Mortality rates of small altricial land birds in arid tropical regions

Species study

number °

1

2

3

4

5

6

7

8

9

10

11

Length of nesting cycle (days)

Laying Incuba- Nestling Total

tion

1

4

1#

3

1

3

2

IK

IK

2

2K

14

14

13K

14

15

12]4

13

HH

n

n

12

10

9

13}^

13

11

13K

13

10%

10

8

9

25

27

28K

30

27

29

28

23H

22K

21

23*

Daily mortality

rate of nests

(percent)

total

M

2.28

2.44

2.48

3.15

1.95

2.35

1.66

2.95

4.37

2.09

3.21

Daily mortality rate of eggs

(percent)

Egg Nestling Total

m9 mn m

1.77

2.05

1.51

3.28

2.38

1.75

2. 13

1.70

3.37

2.07

1.29

2.06

2.09

4.12

2.80

2.85

3.08

3.85

2.83

3.91

2. 14

6.82

2.38

2.16

3.36

3.29

2.57

3.09

2.93

2.87

4.47

2.51

4. 15

Partial

loss

(percent)

m-M

0. 10

-0.2 8

0.88

0.14

0.62

0.74

1.27

-0.0 8

0. 10

0.42

0.94

Vertical

component

(percent)

m,-mH

-0.2 9

-0.0 4

-2.6 1

0.48

-0.4 7

-1.3 5

-1.7 2

- 1 . 13

-0.5 4

-0.0 7

-5.5 3

See Table 3a for species designation in relation to study number.

TABLE 8.—Comparison of nesting success of humid tropical species with respect to nest-type and

habitat (from Skutch, 1966)

Nest-type and habitat

Open or roofed nests in clearings or

second growth (23 species)

Open or roofed nests in forests (30

species)

Hole nests (16 species, excluding

the house wren Troglodytes

musculus)

Nests

(number)

434

52

43

Eggs

(number)

883

107

» 62

Nest success

(percent)

Total period

34.6

23.1

53.5

Egg success (percent)

Egg Nestling Total

51.1 59.7 30.4

45.8 46.9 21.5

67.8 64.3 43.6

• Clutch size and egg success could be determined in few of the hole nests.

in tropical birds. Snow (1962) also states that 86 percent of nesting losses of the black-and-white manakin in

Trinidad were due to predation.

Marchant's (1960) data indicate that starvation is

an appreciable component of nesting loss in an arid

region of Ecuador (Figure 7). The vertical components are negative in all but one case (average, —1.20

percent), and the average vector is similar to that predicted for starvation. Marchant attributes the extremely

high rates of nestling loss of the Bonaparte warbling

finch (species study 11) to starvation resulting, in one

season, from the rapid dedication of the environment

after a rain suitable for breeding. The large negative

vertical component which represents the loss of whole

broods must have been brought about by desertion because, in nests which were not terminated prematurely,

17 young fledged from 23 eggs. Thus, in these nests,

only about one fourth of the young starved. Starvation

and desertion of eggs are both cited as significant factors in the mortality of the long-tailed mockingbird

(species study 6). No comments are given to help explain the position of the Ecuadorian neorhynchus (spe-
NUMBER 9 21

T) /

FIGURE 6.—Vertical components of individual mortality rates

versus partial loss during the nest period for humid tropical

passerines (and one dove). Numbers correspond to species

in Table 6a. Scale and vectors as in Figure 5.

T) \

FIGURE 7.—Vertical components of individual mortality rates

versus partial loss during the nest period for arid tropical passerines (and one dove and one cuckoo). Numbers correspond

to species in Table 7a. Scale and vectors as in Figure 5.

cies study 8), but it must be assumed that desertion of

nestlings, especially of small broods, was a significant

factor.

Hatching failure is much lower in the Ecuadorian

sample than for temperate North American species,

averaging 4.3 percent (range, 2.7-5.9) of eggs surviving the egg period in six species.

ARCTIC SPECIES.—The few data available on arctic

nesting success (Table 9a) are restricted primarily to

finches (Fringillidae), all of which nest on, or near, the

ground. Daily mortality rates calculated for these

species (Table 9b) indicate that mortality factors are

weaker in the arctic compared with more southerly

latitudes. Mortality rates of ground-nesting species in

temperate areas are almost one half again as great.

Oakeson (1954) found nesting success in a limited

sample of the white-crowned sparrow to increase from

California to the Arctic. Of 30 nests found near

Berkeley, 12 (40 percent) were successful. At Friday

Harbor, Washington, 19 of 32 nests (59 percent)

fledged young, and at Mountain Village, Alaska, 6 of 8

nests were fully successful and 2 were partly successful.
22 SMITHSONIAN CONTRIBUTIONS TO ZOOLOGY

TABLE 9a.—Nesting success of small altricial land birds in arctic regions

Species and study designation number

Lapland longspur

(Calcarius lapponicus)

1

2

3a

3b

Combined

Chestnut-collared longspur

{Calearius ornatus)

4

White-crowned sparrow

(Zonotrichia leucophrys)

5a

5b

Combined

Savannah sparrow

(Passerculus sandwichensis)

6a

6b

Combined

Common redpoll

(Acanthis flammed)

7

8a

8b

Combined

Snow bunting

(Plectrophenax nivalis)

9

10

Yellow wagtail

(Motacilla flava)

11

Length

of

study

(years)

1

1

1

2

1

1

2

1

1

2

1

1

2

1

2

2

Nests

(number)

22

53

38

91

10

15

13

28

8

9

17

9

46

50

%

16

5

4

Eggs

(number)

97

54

262

175

437

44

72

60

132

47

50

97

33

177

182

359

80

24

20

Nest success

(percent)

total period

77.7

45.2

44.7

45. 1

53.3

69.2

60.7

75.0

66.6

70.6

66.7

19.6

16.0

17.7

93.8

100.0

75.0

Egg

77.3

80.5

66.9

75.0

79.6

68. 1

70.0

69.0

74.4

94.0

84.5

72.7

34.5

39.6

37.1

90.0

91.7

95.0

Egg success

(percent)

Nestling

82.8

47.8

54.7

50.3

91.5

71.4

83.3

76.9

80.0

61.7

69.5

54.2

44.3

34.7

39.1

83.3

95.5

79.0

Total

63.9

81.5

38.5

36.6

37.8

72.8

48.6

58.3

53. 1

59.5

58.0

58.8

39.4

15.3

13.7

14.5

75.0

87.5

75.0

Source and locality

Sutton and Par melee, 1955,

Baffin Island

Drury, 1961, Northwest

Territories

Williamson.Thompson, and

Hines, 1966, Cape Thompson,

Alaska

Harris, 1944, Manitoba

Williamson.Thompson, and

Hines, 1966, Cape Thompson,

Alaska

Williamson.Thompson, and

Hines, 1966, Cape Thompson,

Alaska

Grinnell, 1943, Manitoba

Williamson,Thompson, and

Hines, 1966, Cape Thompson,

Alaska

Sutton and Parmelee, 1955,

Baffin Island

Williamson,Thompson, and

Hines, 1966, Cape

Thompson, Alaska

Williamson.Thompson, and

Hines, 1966, Cape

Thompson, Alaska

At Cape Thompson, Alaska, however, Williamson,

Thompson, and Hines (1966) reported only 8 successes

of 15 nests (53 percent) of the white-crowned sparrow.

In fact, all of the species studied by these investigators

exhibited low nesting success (m = 3.89) compared

with other arctic studies (m = 1.21). Thus, arctic nesting success may be higher generally than the averages

presented in Tables 9a and 9b. For example, Irving

and Krog (1956) reported that of 45 eggs (9 nests, 7

species), 84 percent left the nest spontaneously.

In six of seven arctic species mortality rates were

greater during the nestling period than during the egg

period and within-brood losses are high (Figure 8).

Starvation is implicated but it is not unreasonable to

assume that death from exposure also may be important. This factor would probably affect the young to a
NUMBER 9 23

TABLE 9b.—Mortality rates * of small altricial land birds in arctic regions

Species study

number h

1

2

3

4

5

6

8

9

Length of nesting cycle (days)

Laying

4

4

4

4

4

3 *

Incubation

12

12

11

11

io#

12H

Nestling

9

10

9

9

HM

12M

Total

25

26

24

«24

26

28H

Datfy mortality

rate of nests

{percent) total

M

3. 19

2.08

1.45

6.08

0.22

Daily mortality rate of eggs

Egg

m.

1.61

1.80

1.43

2.47

1. 12

6.20

0.64

(percent)

Nestling

2. 10

7.64

0.89

2.93

4.05

8. 17

1.45

Total

m

1.79

0.82

3.90

1.22

2.64

2.22

6.79

0.99

Partial loss

(percent)

m-M

0.71

0.56

0.77

0.71

0.77

Vertical

component

(percent)

m,-mn

-0.4 9

-5.8 4

0.46

-0.4 6

-2.9 3

-1.9 7

-0.8 1

o Mortality rates are calculated only for studies represented » See Table 3a for species designation in relation to study

by more than 10 nests. number.

• The length of the nesting cycle was estimated.

greater degree than eggs. Predation apparently causes

negligible mortality in many arctic species. For example, from 22 nests of the Lapland longspur on Baffin

Island (Sutton and Parmelee, 1955) more than one

third of the eggs failed to produce fledglings, but only

one of these nests was lost to a predator. None of 16

broods of the snow bunting, which nests in crevices,

were destroyed by predators.

Geographical variation in the breeding success of

open-nesting species is summarized in Table 10. Most

of these inhabit open areas (fields, scrub, secondgrowth, and parklike or edge habitats associated with

man) because nests are more readily found and observed in open areas than in deep woods (Skutch,

1966). The average length of the nest period is similar

for all localities and, thus, differences in overall nesting

success may be attributed almost entirely to differences

in mortality rates.

The four areas are not strictly comparable because

different types of habitats are sampled. The arctic

species are typical of treeless tundra and riparian situations, the humid tropical species of wet second-growth

and clearings, and the arid tropical species of desertscrub. It is more realistic to compare humid tropical

species (m=4.09) with temperate species building

open nests above the ground (m=2.12, Table 5)

rather than with the overall temperature zone average

(m=3.06). The arid tropical species more properly

would be compared with birds of the southwestern

deserts of the United States than with eastern species.

TABL E 10.—Geographical variation in nesting success of open-nesting passerines

Nesting parameter

Overall nest success (percent)

Overall egg success (percent)

Average nest period (days)

Mortality rate of nests (percent)

Mortality rate of eggs (percent)

Partial loss (percent)

Vertical component, (percent)

Arctic

•60.9

60.0

28.3

2.60

2.55

0.70

-1.72

(6)

(9)

(6)

(5)

(7)

(5)

(6)

Region

Temperate

55.4 (12)

46.6 (13)

26.5 (14)

2.40 (12)

3.06 (13)

0.69 (11)

0. 10 (13)

Humid

tropical

30.5 (9)

32.4 (8)

30.4 (10)

3.95 (9)

4.09 (9)

0.47 (7)

1.22 (7)

Arid

tropical

51. 1

43.9

25.9

2.69

3.25

0.56

-1.4 4

(9)

(9)

(9)

(9)

(9)

(9)

(9)

• Number of species constituting each average is given in parentheses.
24 SMITHSONIAN CONTRIBUTIONS TO ZOOLOGY

\

FIGURE 8.—Vertical components of individual mortality rates

versus partial loss during the nest period for arctic passerines.

Numbers correspond to species in Table 9a. Scale and vectors

as in Figure 5.

The low mortality rate of the catcus wren (M = 1.00,

Table 3b) and personal observation of other desert species in Arizona suggest that nesting success is generally

higher in arid than in humid habitats. Thus, the mortality rates of dry tropical species may not be so nearly

similar to those of temperate ecological counterparts

as they would seem from Table 10.

Starvation is important to at least some species in

all but humid tropical regions. The large starvation

components of arid tropical birds probably are related

to the sparce and unpredictable rainfall which controls

the abundance of food resources. Marchant (1959)

and Lloyd (1960) have shown that breeding is closely

correlated with the occurrence of irregular rainfall.

The absence of starvation components in the humid

tropical data indicates either that food resources are

highly predictable and clutch size is finely adjusted to

this level, or that the availability of food does not limit

clutch size in the tropics (Skutch, 1949, 1967). But

it should be noted that the humid tropical sample includes only one field or marsh species, the blue-black

grassquit which does exhibit a small negative vertical

component. Temperate species which exhibit starvation vectors include three marsh-nesting icterids. The

availability of food in marshes is known to fluctuate

greatly (Orians, 1966). Several field-nesting species

are characterized also by negative vertical components

suggesting that food is relatively unpredictable in this

habitat as well.

The large positive vertical compenents of three of

the species studied by Skutch can be explained by

within-nest loss. Desertion, therefore, appears to be of

major importance only among temperate species.

It should be kept in mind that desertion is not solely

a feature of the environment, but rather it is the result

of interactions between adult behavior and environmental factors. Thus, geographical differences with respect to desertion may be the result of variation in the

intensity of parental care rather than the amount of

disturbance to nests.

Clearly most of the variation in nest mortality rates

between localities is related to predation which will be

considered in the discussion. Nesting-success studies are

further summarized by genera in Table 11 according

to the length and direction of their vectors to emphasize

geographical differences.
NUMBER 9 25

TABL E 11.—Taxonomic summary of rusting success studies

Region

Arctic

Temperate

Humid tropical

Arid tropical

Vertical component large

and positive (>0.50 )

Zenaidwra (2)

Telmatodytes

Troglodytes (3) *

Turdus (2)

Sialia (2) *

Bombycilla

Protonotaria (2) *

Agelaius

Icterus

Xanthocephalus

Spizella (2)

Rhynchophanes

Myiozetetes

Elaenia

Turdus

Ramphocelus

Tiaris

Vertical component small

Calcarius (2) "

Acanthus

Calypte

StelgidopteryX "

Iridoprocne *

Sialia *

Stumus"

Dendroica

Agelaius

Spinus

Carpodacus

Troglodytes b

Thraupis

Volatinia

Eupelia

Crotophaga

Muscigralla

Euscartkmus

Rhodospingus

Vertical component large

and negative « — 0.50)

Calcarius

Passerculus

Zonotrichia

Plectrophenax

Eremophila

Seiurus

Euphagus

Agelaius (4)

Quiscalus (2)

Melospiza

Columbigallina

Pyrocephalus

Mimus

Polioptila

Neorhynchus

Sporophila

Poospiza

1

 Number of studies is given in parentheses if more than one.

* Hole-nesting species.

BREEDING DENSITY AND NESTING MORTALITY.—We

may postulate that increasing nesting density causes

greater mortality either through competition for food

or more efficient predation. This appears to be the

case at Cape Thompson, Alaska (Williamson et al.,

1966; Table 12). The most abundant species in riparian situations, the common redpoll, exhibits extremely

high nest mortality. Yellow wagtails and white-crowned

sparrows, nesting in the same localities but in fewer

numbers, had much higher nesting success. Mortality

was high in the redpoll during the egg as well as during

the nestling period and, thus, starvation through increased competition for food was not a major factor.

Also, because the three species nest at the same time,

unusually bad weather should have affected all species

to the same extent. We must conclude that predators

specialized on the abundant redpoll nests, which is

surprising because nests of wagtails and white-crowned

sparrows are fairly similar. One would have expected

that increased predator efficiency on the redpolls might

have carried over to the less common species (Fretwell, 1968). That is, one would not expect predators

to distinguish, on a species level, nests of similar construction, although this likely has occurred.

Corresponding situations could not be found for

temperate and tropical localities. Nolan (1963) found

that in a deciduous scrub habitat in Indiana ". . . the

rates of nesting success of each of the 4 species whose

nests were most numerous were not significantly different than the pooled results for the other 10 spe-
26 SMITHSONIAN CONTRIBUTIONS TO ZOOLOGY

TABL E 12.—Breeding density and nesting success at Cape

Thompson, Alaska °

TABL E 13.—Nest abundance and nesting success of some

Costa Rican birds a

Species

Yellow wagtail

Savannah sparrow

White-crowned sparrow

Lapland longspur

Common redpoll

Average

nesting

density,

pairs 1100

acres b • *

9 (2)

13 (7)

28 (2)

35 (10)

80 (2)

Nests

found

(number)

4

17

28

91

96

Overall

nest

mortality

rate

{percent)

* 1. 15

1.45

2.08

3. 19

6.08

o Data from Williamson, Thompson, and Hines (1966).

* Only habitats where the species were present are included

in the sample, i.e., riparian (willow, Salix sp.) for the yellow

wagtail, white-crowned sparrow and common redpoll, and

tundra for the savannah sparrow and Lapland longspur.

* Number of census plots with species present are given in

parentheses.

d

 Calculated from estimated nesting cycle length of 25 days.

cies. . . " Fretwell (1968) found that in North Carolina, where field sparrows were common, their nesting

success as well as that of other uncommon species was

low and conversely where field sparrows did not nest in

numbers the nesting success of all species was high.

The field sparrow is generally the most abundant species in cleared areas and apparently its density controls

the success of other less common species with similar

nests. Thus, within an area nest mortality is independent of breeding density but between areas the overall

density of similar species controls their success. It

should be pointed out that Fretwell used Mayfield's

(1961) method for calculating nesting mortality and

thus avoided possible biases incurred when nests are

found at different stages.

From Skutch's (1966) data on nesting success at his

farm in Costa Rica we again can show that common

species are at least as successful as rarer species, if not

more so (Table 13).

On the Santa Elena Peninsula of Ecuador, the five

commonest species in Marchant's (1961) study, each

being represented by more than 180 nests, had an

average daily nest mortality rate of 2.89 percent,

whereas six species of which 20 to 100 nests were found

had an average mortality rate of 2.42 percent.

Although the differences in both of the tropical

studies between the mortality rates of common and uncommon species are not significant, they are suggestive

Species

Scarlet-rumped tanager

Yellow-faced grassquit

Gray-capped flycatcher

Blue-gray tanager

Clay-colored robin

Combined

18 less common species

Nests

found

{number)

80

44

30

27

24

205

229

Nests

successful

{number)

35

13

13

12

6

79

71

Percent

43.8

29.6

43.3

44.4

25.0

38.5

31.0

» Data from Skutch (1966) includes only nests found before

the last egg was laid.

that in simpler habitats, or perhaps those with low

productivity (i.e., arid and arctic), the uncommon

species have relatively high success, whereas in the

more humid tropical and temperate environments,

nests of uncommon species are not distinguished from

those of common species by predators. This phenomenon merits more detailed investigation.

RAPTORIAL SPECIES.—Nesting success of birds of

prey is generally high, and because of their long nest

periods calculated mortality rates are low (Table 14).

Predation is apparently a negligible factor in raptorial

species because of their ability to defend their nests.

Fitch et al. (1946) reported, however, that jays destroyed eggs in 2 of 26 nests of the red-tailed hawk in

California and blood-sucking flies were responsible for

the deaths of 7 of 15 nestlings in one year. During

the preceding year, none of 11 nestlings were lost.

Mortality rates of nests and eggs are generally less

than 1.0 percent per day. In the African fish eagle

young were lost almost eight times more rapidly than

eggs, suggesting that starvation was a significant factor

(Brown, 1960). Lack (1954) emphasized asynchronous

hatching and selective starvation as a common feature

of raptorial nesting behavior. Ratcliffe (1962) found

that duck hawk nests contained averages of 3.4 eggs

but only 1.9 young at all stages during the nest period.

Nests of the raven, which held averages of 4.6 eggs and

4.2 young one to three days after hatching, contained

an average of 2.55 young when found more than three

days after hatching. Similarly, brood size of the marsh

hawk declined from 4.1 at hatching to 3.2 at thirty-
NUMBER 9 27

five days (Hammond and Henry, 1949). Nest loss in

this species was also quite high during the first half of

the nest period (2.07 percent daily) compared with

the egg period (0.83 percent daily). Conversely, mortality rates of golden and bald eagles were quite low

during the nestling period (Table 14), which suggests

that starvation was not a factor in these studies.

SEABIRDS [PROCELLARIIFORMES AND PELECANIFORMES].—Seabirds nest predominately on inaccessable islands, thus eliminating predation by most terrestrial animals. Most nest losses are "internal" (Pettingill, 1939)—i.e., caused by hatching failure and

crowded conditions resulting in direct intraspecific

strife—although some avian predators such as gulls

and frigate birds may take a heavy toll of unguarded

eggs and chicks.

Some of the data available on nesting success in seabirds is presented in Table 15. Rates of egg loss in eight

studies varied from 0.86 to 3.69 percent per day with

an average of 2.17. Mortality rates of young are only

about one third as great (0.13-1.19 percent in eight

studies, average 0.66). Most losses of eggs in seabirds

TABLE 14.—Nesting success and mortality rates of raptorial birds

Species

African eagles

(10 species)

Bald eagle

(Haliaetus

leucocephalus)

African fish eagle

{Haliaetus vocifer)

Golden eagle

(Aquila

chrysaetos)

Red-tailed hawk

(Buteo jamaicensis)

White-tailed kite

(Elanus leucurus)

Marsh hawk

(Circus hudsonius)

Great-horned owl

(Bubo virginianus)

Length

of

study

(years)

3

3

2

3

2

3

3

Nests

(number)

77

32

12

51

87

<=22

23

60

41

Eggs

(number)

57

25

U02

94

Egg

period

(days)

34

44

40

30

35

36

34

Nestling

period

(days)

80

70

70

45

'40

66

Nest success °

(percent)

Egg Nest- Total

ling

70. 1

73.6

(0.41)

86. 3 73. 7 63. 7

(0.40) (0.68) (0.60)

71.7 «68.6

(0.93) (2.07)

80.5

(0. 22)

Egg success «

(percent)

Egg Nest- Total

ling

64. 9 59. 5 38. 6

(1.27) (0.65) (0.84)

92. 0 52. 2 48.0

(0. 12) (0. 93) (0. 64)

79.4 86. 4 68. 7

(0.58) (0.21) (0.34)

78.7

(0. 32)

Source and locality

Brown, 1955,

Kenya

Hensel and

Trover, 1964,

Alaska

Brown, 1960,

Kenya

McGahan, 1968,

Montana

Orians and Kuhlman, 1956,

Wisconsin

Fitch, Swenson,

and Tillotson,

1946, California

Dixon, Dixon and

Dixon, 1957,

California

Hammond and

Henry, 1949,

North Dakota

Orians and

Kuhlman, 1956,

Wisconsin

° Mortality rates are given in parentheses.

b

 Estimated from average clutch size of two.

cNot including four nests which were deserted because of

the investigators' disturbance.

<* Estimated nestling period.

• Based on the survival of 51 nests through the first 16-20

days of the nestling period.
2 8 SMITHSONIAN CONTRIBUTIONS TO ZOOLOGY

TABLE 15.— Nesting success and mortality rates of seabirds [Procellariiformes and Pelecaniformes]

Species

Madeiran storm petrel

(Oceanodroma castro)

Red-billed tropic bird

(Phaethon aethereus)

White-tailed tropic bird

(Phaethon leptunts)

Brown booby

(Sula leucogaster)

Blue-faced booby

(Sula dactylatra)

Pelagic cormorant

(Phalacrocorax pelqgicus)

Double-crested cormorant

(Phalacrocorax auritus)

Shag

(Phalacrocorax aristotelis)

Length

oj

study

{years)

1

2

2

4

Nests

number

67

328

821

185

150

631

68

37

39

66

294

Eggs

number

67

328

821

370

300

1262

136

141

•101

273

893

Egg

period

(days)

42

43

41

47

47

45

45

35

35

33

28

Nestling

period

Uays)

64

100

80

120

120

120

120

45

45

39

53

Nest

success "

(percent)

total

period

9.7

(1.39)

26.0

(0.81)

8.9

(1.47)

17.7

(1.05)

Egg success " (percent)

Egg Nestling Total

43.3

(0. 79)

69.2 74.4 51.5

(0.86) (0.30) (0.46)

48. 1 63.0 30. 3

(1.77) (0.58) (0.99)

19. 2 25. 4 4. 9

(3.46) (1. 14) (1.80)

13.0

(1.22)

18. 5 23. 9 4.4

(3.69) (1. 19) (1.87)

27. 9 31. 6 8. 8

(2.80) (0.94) (1.47)

49.6

(2.00)

76.2

(0.60)

60. 4 95. 2 57. 4

(1.54) (0. 13) (0.77)

70.8 87. 1 61. 7

(1.23) (0.26) (0.60)

Source and locality

Allan, 1962, Ascension

Island

Stonehouse, 1962, Ascension Island

Stonehouse, 1962, Ascension Island

Dorward, 1962, Ascension Island »

Dorward, 1962, Ascension Island6

Drent, et al. 1964,

Mandarte Island,

British Columbia

Drent, et al, 1964, Mandarte Island, British

Columbia

Snow, 1960, Lundy

Island, English

Channel

« Daily mortality rate is given in parentheses.

» Data from two colonies are presented separately.

• Young hatched.

are due to their rolling out of nests—particularly in

boobies (Dorward, 1962)—becoming overheated in

the sun, and being destroyed by avian predators. Drent

et al. (1964) found that 32 percent of eggs of the

pelagic cormorant were preyed upon and 18 percent

were addled. Nine and 28 percent of eggs of the doublecrested cormorant were lost to predators in two years,

and 24 and 18 percent were found addled.

Starvation appears not to be a significant factor in

mortality of the young of species which lay only one
NUMBER 9 29

egg. It will be shown below that mortality rates of

tropic birds decrease as the chicks grow, whereas these

rates increase when starvation is a major factor, as in

blackbirds and grackles (Peterson and Young, 1950;

Young 1955, 1963). The primary source of chick mortality is crowded conditions in colonies and the resulting

death of unattended young caused by other adults

(Stonehouse, 1962; Nelson, 1966). The tropical

boobies lay two eggs but only one chick is raised, the

other starving if two eggs survive the incubation period

(Dorward, 1962). Thus, nest success is always approximately twice egg success and the former value is of

greater interest from the standpoint of selective forces

because the loss of the second egg is probably a part

of the reproductive strategy.

Starvation may play a role in the mortality of cormorants which raise broods of two to four young. Snow

(1960) found that the survival of young varied between

90.2 and 95.5 percent during three years of a four-year

study on the shag, but that only 66.9 percent of chicks

survived during the fourth year. Clutch size was also

somewhat reduced (2.8 compared to 3.0-3.2 eggs)

which often is observed in birds in response to poor

feeding conditions (Lack, 1954). Egg survival (68.7-

73.6 percent) varied little during the study period.

SEABIRDS [CHARADRIIFORMES].—The gulls and terns

(Table 16) have different mortality patterns than

other seabirds in that nestling mortality rates generally

exceed egg rates. Egg losses are somewhat lower (average 1.18 percent per day, range 0.13-1.91 for eleven

studies, not including Hagar, 1937, where introduced

rats caused high mortality, and Ashmole, 1962, which

does not include data for the nestling period) and, as

in other seabirds, "internal" factors are primarily responsible. Nestling mortality rates averaged 2.34 percent for fourteen studies (range, 0.35-5.74), about

twice that of the egg period. In the Alcidae, the reverse situation occurs. Egg losse are rather high (1.97

percent, range 1.29-2.81) as in the Procellariiformes

and Pelecaniformes and chick losses are substantially

reduced (average 1.06 percent, range 0.63-1.78).

Pettingill (1939) estimated that less than one fourth

of all losses in the arctic tern were caused by external

factors. In the pigeon guillemot, 18 percent of eggs

were found adled, 3.5 percent rolled out of nests, and

13 percent were taken by crows, nearly all within the

first five days after being layed (Drent et al. 1964).

High tides destroyed 30 percent of eggs of the black

guillemot and 18 percent were infertile (Winn, 1950).

Most of the mortality noted by Tuck (1960) in the

thick-billed murre was "internal." Of egg losses, 19 percent were caused by rock falls, 8 percent rolled into

crevices, but only 1 percent were taken by gulls. The

remainder were suspected to have fallen off ledges.

Similarly chicks suffered heavily from falling off ledges

(41 percent of losses), exposure (38 percent), and falling into cracks (12 percent). Thoreson (1964) also

cites several internal factors in the cavity-nesting Cassin's Auklet such as the cave-in of burrows due to

wind erosion. One third of all eggs in that study were

deserted because of human disturbance—a factor not

often mentioned in nest-success studies. Where predators have been introduced onto offshore islands,

nesting losses may be quite high. Rats took 67 percent

of eggs and 50 percent of young of the least terns

studied by Hagar (1937). An additional 19 percent of

eggs were destroyed by storm tides. High tides killed

41 percent of black guillemot young studied by Winn

(1950) and several also fell victims to gulls and crows.

Coulson and White (1958) have demonstrated that

nesting success of the black-legged kittywake increases

with age. Birds in their first and second breeding years

had overall fledging successes of 35.8 and 55.8 percent,

respectively, while those which had bred at least twice

previously had 69.7 percent fledging success.

The Arctic tern is the only species in which starvation is cited as a significant factor (Hawksley, 1957).

Of the 90 chicks hatched, 37 percent starved, mostly

in nests with two young. The fledging success from

broods of one and two was 71 and 35 percent, respectively. In six of eleven studies on Laridae presented

in Table 16, nestling mortality rates exceeded egg

mortality rates by more than 0.50 percent per day. This

does not necessarily indicate starvation, however, as

would be the case in altricial nidicolous birds, because

individual young occasionally wander off on their own

and are killed by predators or other adults. The only

gulls and terns in which mortality rates are much

lower during the nestling period than during the egg

period are the black-legged kittywake (Cullen, 1957)

and the least tern (Hagar, 1937) The former nests

.on ledges on vertical cliffs. Egg mortality in this

species is about average for gulls and terns, but chick

loss is extremely low. Needless to say, kittywake chicks

do not wander far from their narrow nest ledges. In

the last tern study, rats were the major mortality

factor, and it is reasonable to assume that they would

prey more heavily on eggs than on chicks.
30 SMITHSONIAN CONTRIBUTIONS TO ZOOLOGY

TABLE 16.—Nesting success and mortality rates of seabirds [Charadriiformes: Laridae and Alcidae]

Species

LARIDAE:

Black noddy

(Anous tenuirostris)

Arctic tern

(Sterna paradisea)

Least tern

(Sterna albifrons)

Black-legged kittywake

(Rissa tridactyla)

Ring-billed gull

(Lams delawarcnsis)

Glaucous-winged gull

(Lams glaucescens)

Lesser black-backed gull

(LOTUSfuscus)

California gull

(Lams californicus)

Herring gull

(Lams argentatus)

ALCIDAE:

Thick-billed murre

( Uria lomvia)

Length

of

study

(years)

1

4

2

2

2

2

1

Nests

(nutnber)

100

428

100

1136

100

400

Eggs

(number)

178

144

308

820

348

300

2967

293

716

293

324

641

247

632

Egg

Period

(days)

35

24

23

27

29

30

»30

27

29

34

Nestling

period

(days)

27

27

38

»40

40

»40

40

42

19

Egg success "(percent)

Egg

23.6

(4. 13)

63.2

(1.91)

64.0

(1.86

25.9

(5.90)

68.9

(1.38)

72.3

(1.12)

59.8

(1.71)

96.2

(0. 13)

60.9

(1.71)

86.7

(0. 53)

91.8

(0. 29)

70.3

(1.22)

71.3

(1.17)

38.5

(2.81)

Nestling Total

25.3

(5. 10)

54.3

(2. 26)

35.4

(3. 85)

87.5

(0. 35)

30.9

(2.94)

56.9

(1.41)

54.7

(1.51)

10.1

(5. 74)

69.7

(0.90)

41.4

(2. 10)

38.3

(2. 28)

61.9

(1.14)

88.7

(0. 63)

Source and locality

Ashmolc, 1962, Ascension

Island

Pcttingill, 1939, Bay of Fundy,

New Brunswick

Hawksley, 1957, Bay of Fundy,

New Brunswick

Hagar, 1937, New England

Cullen, 1957, Great Britain

Emlen, 1956, Michigan

Drent et al, 1964, British

Columbia

Darling, 1938, Great Britain

Paludan, 1951, Denmark

Bchlc and Goates, 1957,

Utah

Darling, 1938, Great Britain

Paludan, 1951, Denmark

Paynter, 1949, Bay of Fundy,

New Brunswick

Tuck, 1960, Cape Hay,

Canada

See footnotes at end of table.
NUMBER 9 31

TABLE 16.—Nesting success and mortality rates of seabirds [Charadriiformes: Laridae and Alcidae]—Continued

Species

Black guillemot

(Ceppkus grille)

Pigeon guillemot

(Cepphus columba)

Cassin's auklet

{Ptychoramphus aleutica)

Length

study

{years)

4

3

1

1

Nests

{number)

46

109

75

«50

Eggs

{number)

84

200

91

44

Egg

Period

{days)

29

31

37

Nestling

period

{days)

39

35

45

Egg success {percent)

Egg Nestling Total

52. 4 50. 0

(2.23) (1.78)

62.0

(1.54)

90. 1 )

(0. 30) 1 74. 1

40.9 [ (0.86)

(2. 56)J

41. 4 64. 5

(2. 38) (0. 98)

62.0

(1-29)

Source and locality

Winn, 1950, Bay of Fundy,

New Brunswick

Drent et al, 1964, British

Columbia

Thoreson, 1964, Oregon

"Mortality rates are given in parentheses.

6

 Estimated.

-Excluding nests deserted because of human disturbance.

Nest-site dependence may more generally determine

the relationship between egg and nestling loss in seabirds, as summarized in Table 17. The Procellariiformes

and Pelecaniformes, which exhibit altricial or semialtricial development (Nice, 1962) and hence are not

physically capable of leaving the nest until well grown,

and the Alcidae, which are semiprecocial, that is,

capable of walking, but restricted to the nest because

of its location until flight capabilities are attained, suffer relatively little nestling loss. Most of the Laridae,

except for cliff-nesting species such as the kittywakes,

often wander away from the nest site and thus from

parental protection. Of the many dangers to which

unattended chicks are exposed, other adults of the

same species are perhaps the most significant. This explanation for the discrepancy in nestling mortality

rates does not account for the reverse difference in egg

mortality. Cliff nesting, which affords the young some

protection from other adults, results in high egg losses

due to their rolling out of place. But many of the

Pelecaniformes (i.e., cormorants) build substantial

nests which should prevent most egg loss caused by

rolling. In fact, cormorants do have lower egg mortality (2.00, 1.54, 1.23 percent per day) than brown

and bluefaced boobies which do not build nests (3.46,

3.69, 2.80 percent per day). Interesting comparisons

TABLE 17.—Egg and nestling mortality rates in seabirds

Order or family

Procellariiformes and Pelecaniformes

Laridae

Alcidae

Number of

species

7

11

4

Egg mortality rate

{percent)

Average Range

2.17 0.86-3.69

1.18 0.13-1.91

1.97 1.29-2.81

Nestling mortality rate

{percent)

Average Range

0. 66 0. 13-1. 19

2. 34 0. 35-5. 74

1.06 0.63-1.78
32 SMITHSONIAN CONTRIBUTIONS TO ZOOLOGY

could be made with species of boobies which build

nests in trees.

SHOREBIRDS.—Precocial chicks characteristicly leave

the nest shortly after hatching. Most of the data available for shorebirds, therefore, encompasses only the

egg period. Eleven studies from widely different localities and habitats are summarized in Table 18. Mortality rates average 1.44 percent per day during the egg

period (range, 0.27-3.53 percent) which is similar to

that for gulls and terns. Soikkeli (1967) found the

mortality rate of dunlin chicks during the first two

weeks after hatching to be 1.76 percent per day, or

about two thirds again as great as during the egg

period. Most of the losses occured during the first

three days out of the egg. No difference seems to exist

between temperate and arctic species but it is clear

that the rate of egg loss of precocial shorebirds is lower

than that of ground-nesting altricial passerines, espeTABL E 18.—Nesting success and mortality rates of shorebirds [Charadriiformes: Haematopodidae, Charadriidae, Scolopacidae]

Species

Egg success °

Egg {percent)

Nests Eggs period

{number) {number) {days) total period Source and locality

HAEMATOPODIDAE:

Black oystercatcher

{Haematopus bachmani)

CHARADRIIDAE:

Golden plover

{Pluvialis dominica)

Semipalmated plover

{Charadrius semipalmatus)

Piping plover

{Charadrius melodus)

Ringed plover

{Charadrius hiaticula)

SCOLOPACIDAE:

Upland plover

{Bartramia longicauda)

Ruddy turnstone

{Arenaria intrepes)

Dunlin

{Calidris alpina)

Semipalmated sandpiper

(Ereunetes pusillus)

Western sandpiper

{Ereunetes mauri)

16

35

174

47

42

123

38 29 71.1 (1.18) Drent et al, 1964, British Columbia

139 29 70.5 (1.25) Williamson, Thompson, and Hines, 1966,.

Cape Thompson, Alaska

12

28

28

100.0 Williamson, Thompson, and Hines, 1966,

Cape Thompson, Alaska

668 32 91. 7 (0. 27) Wilcox, 1959, New York

» 29 36. 0 (3. 53) Laven, 1940, Germany

25 66. 0 (1. 67) Buss and Hawkins, 1939, Wisconsin

25J/2 72. 0 (1. 29) Bergman, 1946, Finland

25 80. 9 (0. 85) Holmes, 1966, Alaska

479 25 78. 1 (0.99 ) Soikkeli, 1967, Finland

96. 4 Williamson, Thompson, and Hines, 1966,

Cape Thompson, Alaska

96. 4 Williamson, Thompson, and Hines, 1966,

Cape Thompson, Alaska

1

 Mortality rates are given in parentheses.

h

 Estimated.
NUMBER 9 33

cially in temperate regions. But shorebirds also nest in

slightly different habitats than passerines, prefering

either wetter or more barren situations.

WATER BIRDS.—A sample of nesting success in several different groups of water birds (Anseriformes,

Ciconiiformes, and Gruiformes) is presented in Table

19. Except for the Ciconiiformes (herons and ibises)

water birds have precocial young, and reliable data on

posthatching survival were not found. Nest mortality

rates during the egg period in eight studies average

1.03 percent per day (range, 0.17 to 1.98). Lack (1954,

Table 15) has summarized hatching success in ten

species of ducks which averaged 66.5 percent (range,

45 to 90) of eggs laid. Williams and Marshall (1938)

found that hatching success of seven species of ducks

in the United States varied between 26 and 85 percent. In the ruddy duck the difference between nest

and individual egg mortality rates was 0.16 percent

per day, whereas in two rails differences were negative

(—0.23 and —0.31) suggesting the selective loss of

nests before the completion of laying or of small

clutches. Rails generally have large clutches (average

9-11 eggs in the species in Table 19) and the laying

period occupies at least a third of the total egg period.

TABLE 19.—Nesting success and mortality rates of water birds

Species

COLYMBIDAE:

Pied-billed grebe

(Podilymbus podiceps)

ARDEIDAE:

Least bittern

(Ixobrychus exilis)

Little blue heron

{Florida caeruleq)

ANATIDAE:

Ruddy duck

( Oxyurajamaicensis)

Canada goose

(Branta canadensis)

RALLIDAE:

Coot

(Fulica americana)

Sora rail

(Porzana Carolina)

Clapper rail

(Rallus longirostris)

King rail

(Rallus elegans)

Nests

(number)

138

38

30

71

1094

16

36

56

16

Eggs

(number)

124

546

119

266

147

Egg period (days)

Laying Incu- Total

bation

6 23 29

4K 18 22H

4 22 26

8 25 «

5 24 29

7/2 24 31>$

9 17 26

10 20 30

11 22 33

Nest success a

(percent)

total

period

70.3 (1.22)

84. 2 (0. 77)

93. 3 (0. 17)

73.2 (0.94)

56. 1 (1. 98)

61. 1 (1.88)

89.3 (0.38)

75.0 (0.87)

Egg success °

(percent)

total

period

74. 2 (0. 75)

69.4 (1. 10)

47.9 (2.32)

66.5 (1.57)

81. 0 (0. 64)

Partial

loss

0.58

0. 16

-0.31

-0.23

Source and locality

Glover, 1953, Iowa

Weller, 1961, Iowa

Meanley, 1955, Arkansas

Low, 1941, Iowa

Klopman, 1958, various

Gullion, 1954, California

Walkinshaw, 1940,

Michigan

Kozicky and Schmidt,

1949, New Jersey

Meanley, 1953, Arkansas

Mortality rates are given in parentheses.
34 SMITHSONIAN CONTRIBUTIONS TO ZOOLOGY

Predation, adverse weather, and high tides are probably the most important mortality factors in the marsh

habitat.

Survival of water birds nests in marshes is much

higher than for passerines in similar habitats. Apparently, adaptation to aquatic life permits the use of

safer nesting sites in these species. Passerines breeding

in marshes typically build their nests as far off the

water as possible, rendering them quite" conspicuous.

Greater nesting densities and smaller size also may contribute to increased nest mortality of passerines in the

marsh habitat.

Little is known of the survival of water birds after

they hatch and leave the nest. Meanley (1955) found

that 81 percent of 32 little blue herons survived through

the first two weeks after hatching, before leaving the

nest. The calculated mortality rate (1.48 percent) is

much higher than for nests during the egg period

(0.75 percent).

Teal (1965) has compiled data on the nesting success of tree-nesting species of herons, egrets, and an

ibis in the salt marshes of coastal Georgia (Table 20).

Overall nest mortality is comparable to water birds

nesting among marsh grasses (average, 1.46 percent per

day, range 0.0 to 3.13 during the egg period). Predation accounted for most losses (0 to 50 percent of eggs

and 0 to 30 percent of young). Hatching failure was

2-3 percent in the egrets and 8-11 percent for the other

species. Starvation accounted for the loss of 0-10 percent of the young in the five species. Teal attributed

the high success of the black-crowned night heron to its

large size and aggressive disposition. None of the eight

nests of this species were lost to predators. Chick mortality rates were lowest among the larger species (Table

21) owing to their ability to defend nests against predators and the larger size of the chicks. Conversely,

the nests often are left unguarded during the egg period, leaving the nest vulnerable, and the two smaller

species which have less conspicuous nests exhibited

higher egg success than two of the larger species.

GAME BIRDS.—Hickey (1955) has reviewed nesting

success in game birds (Phasianidae and Tetraonidae).

Calculated nest mortality rates for this sample (Table

22) average 2.96 percent per day for 15 studies (range,

TABLE 20.—Nesting

Species

Common egret

(Casmerodius

albus)

Snowy egret

{Leucophoyx

tkula)

Louisiana heron

{Hydranassa

tricolor)

Black-crowned

night-heron

{Nycticorax

nycticorax)

White ibis

(Eudocimus albus)

Nests

{number)

35

30

15

8

14

Eggs

{number)

99

%

47

24

36

success and mortality rates of five species of Ciconiiformes in a

Nest period

{days)

Egg Nestling

28 18

' 2 7 28

27 25

29 21

«29 18

Nest success b

{percent)

E&& Nestling

54. 3(2. 18) 89. 5(0. 62)

73.3(1. 15) 50.0(2.48)

86. 7(0. 53) 53. 9(2. 47)

100.0(0.00) 87.5(0.64)

42.9(2.92) 83.3(1.02)

Egg success h

{percent)

Egg Nestling

52.6(2.30) 84.6(0.93)

64.6(1.62) 38.7(3.39)

72.3(1.20) 44.2(3.27)

91.7(0.30) 77.3(1.23)

38.9(3.26) 71.4(1.87)

Georgia salt

Partial loss

{percent)

Eggs Nestling

0. 12 0. 31

0. 47 0. 91

0. 67 0. 80

0. 30 0. 59

0. 34 0. 85

marsh a

Vertical component

{percent)

Nests Eggs

1. 56 1. 37

-1.3 3 -1.7 7

-1.9 4 -2.0 7

— 0.64 —0.93

1.90 1.39

Data from Teal (1965).

Mortality rates given in parentheses.

Estimated.
NUMBER 9

TABLE 21.—Adult body weight and nest mortality rates of

Ciconiiform birds in a Georgia salt marsh •

Species

Snowy egret

Louisiana heron

Black-crowned night heron

White ibis

Common egret

Adult

weight

(grams)

350

450

800

800

900

Nest mortality

rate (percent)

Mt Mn

1.15 2.48

0. 53 2.47

0. 00 0. 64

2.92 1.02

2. 18 0. 62

a

 Nesting success data from Teal (1965). Source of weight

data: Gross (1923), Hartman (1946, 1955), Hartman and

Brownell (1961), Norris and Johnston (1958), Palmer (1962),

Poole (1938).

1.55 to 4.66 percent) during the egg period. This is

about 0.5 percent less than individual egg mortality

rates for ground-nesting passerines in similar habitats.

Taking into account the discrepancy between egg and

nest survival, the mortality rates must be nearly the

same for the two groups.

Survival of the precocial young of game birds has

been recorded in several studies. In four cases, 54 to

88 percent of the chicks survived from hatching to the

age of eight or nine weeks. Calculated mortality rates

average 0.64 percent (range, 0.23 to 0.98) for this

period, which is much lower than during the egg

period. Of course, mortality rates are not constant

during the period out of the nest. Survival is lowest

just after hatching and increases as the young grow

(see Figure 9). Mortality rate of the greater prairie

chicken during the first four weeks after hatching

was 2.45 percent per day in one study (Lehman,

1941), which is greater than nest mortality during the

egg period.

Predation is probably the greatest mortality factor

on the eggs of game birds. Foxes took 37 percent of

ruffied grouse losses in New York and other predators

accounted for an additional 52 percent of losses (Bump

et al., 1947). Coyotes were responsible for the destruction of 42 percent of bobwhite quail cluches in Texas

(Lehman, 1946). Other factors, including predators,

caused 12 percent additional mortality. Predation,

especially by raptorial birds, is also a significant factor

in the loss of chicks (Bump et al., 1947).

DEVELOPMENT AND SURVIVAL.—During the course

35

of development, physical capabilities of young birds

increase with a resulting decrease in mortality rates.

Daily mortality rates of five species are graphed as a

function of age in Figure 9. Initially, the glaucouswinged gull and the California quail suffer high losses,

but mortality rates decrease rapidly. Although the

curves for the two species are similar, the glaucouswinged gull grows nearly twice as fast as the California quail (Ricklefs, 1968b) and thus, relative to

growth, the survival capabilities of the latter increase

more rapidly. This phenomenon will be discussed in

more detail in a subsequent paper.

Initial mortality rates of the two tropic birds are

much lower than for the above species. Tropic birds

raise only one young per brood and the relative intensity of parental care must be quite high. Because of

their remote nesting localities, tropic birds must also

have few predators.

When starvation plays a prominent role in nestling

survival, mortality rates may increase as the young

grow in size and require energy at a more rapid rate.

Peterson and Young (1950) and Young (1963) have

demonstrated this phenomenon in three species of

marsh-nesting blackbirds and grackles. Mortality

rates of young marsh hawks (Hammond and Henry,

1949) do not decrease with age (Table 23), probably

because of increasing effects of starvation which balance increasing physical capabilities.

Virtually nothing is known of the survival of altricial

birds after they leave the nest. Data for the cactus wren

and ovenbird indicate that fledgling mortality rates are

lower than those of nestlings (Ricklefs, 1968a; Table

24), but there may be a brief period of marked increase

in losses while the young gain experience immediately

after leaving the nest. Mortality rates for fledgling English blackbirds (Snow, 1958) were 4.18 percent per

day for the first five days out of the nest and 1.64 percent during the next ten to fifteen days. Calculated nest

mortality rates were 6.68 percent in Wytham Wood and

2.35 percent in the Botanic Garden at Oxford. Smith

(1967) found that 83 of 95 fledgling black-capped

chickadees survived to the time of family breakup (3-4

weeks). These data indicate a fledgling mortality rate

averaging between 0.5 and 0.6 percent during this

period, which is probably lower than during the nestling

period. Eleven of the young died during a severe storm

less than a week after the birds fledged and thus subsequently only one of 84 young perished.
36 SMITHSONIAN CONTRIBUTIONS TO ZOOLOGY

TABLE 22.—Nesting success and mortality rates of game birds

Species

PHASIANIDAE:

California quail

{Lophortyx californicus)

Bobwhite

(Colinus virginianus)

European partridge

(Perdix perdix)

Ring-necked pheasant

(Phasianus colchicus)

TETRAONIDAE:

Prairie chicken

( Tympanuchus cupido)

Sage grouse

(Centrocercus urophasianus)

Ruffed grouse

(Bonasa umbellus)

Nests

(number)

96

602

189

143

435

445

310

563

149

527

100

161

238

134

Egg period (days)

Laying

14

13

14

11

11

10

11

Incubation

22

24

24

24

23

22

23

Total

36

37

38

35

34

32

«34

Nest success "

(percent)

egg period

18 (4.66)

36 (2.73)

46 (2.08)

32 (2.%)

32 (2.96)

23 (4. 12)

20 (4.50)

58 (1.55)

36 (2.88)

26 (3.78)

50 (2.02)

60 (1.58)

35 (3.23)

34 (3.32)

59 (1.55)

Chick

age

(days)

56

56

56

28

63

Chick

success °

(percent)

71 (0.61)

88 (0. 23)

<70 (0. 64)

50 (2.45)

54 (0. 98)

Source and locality b

Glading, 1938, California

Stoddard, 1931, GeorgiaFlorida

Lehmann, 1946, Texas

Klimstra, 1950, Iowa

Yeatter, 1934, Michigan

McCabe and Hawkins, 1946,

Wisconsin

Hamerstrom, 1936, Iowa

Randall, 1940, Pennsylvania

Leedy and Hicks, 1945, Ohio

Rasmussen and McKean,

1945, Utah.

Baskctt, 1947, Iowa

Hamerstrom, 1939, Wisconsin

Lehmann, 1941

Rasmussen and Griner, 1938,

Utah

Keller, Shepard, and Randall,

1941, Colorado

Patterson, 1949, Wyoming

Bump et al, 1947, New York

° Mortality rates are given in parentheses.

» As compiled in Hickey (1955), except for the ruffed grouse.

0

 Estimated.

Interpretation and Discussion

Developing organisms must sacrifice mature characteristics—and thus survival capabilities—for the sake of

growth and for the attainment of future stages of development. Similarly, adults must accept a reduction

in survival rate for the sake of breeding. Clearly, reproduction involves a compromise between survival

and fecundity which must be adjusted to optimum

levels through adaptation. In species with precocial

chicks a greater part of adaptation to reduce mortality

rates falls upon the young than in species whose development is progressively more altricial. In such birds,

parental care is increased and the adults take a larger

share of the burden of this adaptation. The accomplishment of a given decrease in the mortality rates of

the young must involve risks and also results in benefits, both of which will vary with environmental mortality factors. The evolved level of adaptation is that

beyond which added risks more than balance new

benefits, and the resulting mortality rate reflects the

relationship of costs and benefits to the level of adaptation. We may ask, "Are observed variations in mortality rates the result of differing levels of adaptation of
NUMBER 9 37

<

tr.

tr

O

0 10 20 30 40 SO 60 70 80

DAYS AFTER HATCHING

FIGURE 9.—Relationship between age and mortality rates in

the young of five species of precocial and semialtricial birds:

left-hand scale, white-tailed tropic bird (solid line, open

circles) and red-billed tropic bird (solid line, solid circles)

from Stonehouse (1962); right-hand scale, California quail

(dashed line, open squares), Williams (1959), glaucouswinged gull (dashed line, solid squares), Drent et al. (1964),

ring-necked pheasant (dotted line, open triangles), Errington and Hammerstrom (1937). Mortality rates are expressed

in percent per day.

the species, or does the environment primarily determine mortality rates and all species have achieved similar levels of adaptation?" The question is, of course,

somewhat circular because the purpose of adaptation

is to modify the environment to one's advantage. How

should we classify the mortality rates of raptorial species

which are low partly because of their large size and

defense capabilities? Do we say that their environment

differs from that of smaller birds because of reduced

predator pressure and facilitated homeostasis, or, that

their large size is an adaptation to reduce nesting mortality? Obviously, the body size of raptorial birds is

well suited for the organisms they prey upon and thus

is not strictly an adaptation of the reproductive strategy. Similarly, the hole nests of woodpeckers clearly

alter the nestling environment and reduce mortality

rates but the ability to excavate hole nests depends on

adaptations of adult morphology for gathering food

under bark. Is this adaptation to be included as a part

of the reproductive strategy? Any answer to these

questions will necessarily be arbitrary. A more satisfactory approach, comparing a uniform group, such

as passerines, in different environments, will be employed below.

Some components of mortality rates almost certainly

are determined environmentally. For example, where

feeding conditions fluctuate unpredictably owing to

weather, two extreme strategies are possible. A pair

TABLE 23.—Survival of marsh hawk nestlings in successful nests

Age interval

(days)

0-5

6-10

11-15

16-20

21-25

26-30

31-35

Number of

nests

43

43

39

35

25

15

11

Number of

young

175

172

148

128

83

50

35

Average number

of young

per nest

4.07

4.00

3.79

3.66

3.32

3.33

3. 18

Survival

within nest

(percent)

98.28

94.75

96.57

90.71

100.30

95.50

Mortality rate

within nest

(percent)

0.35

1.08

0.70

1.95

-0.0 6

0.92

Data from Hammond and Henry (1949).
38 SMITHSONIAN CONTRIBUTIONS TO ZOOLOGY

TABLE 24.—Survival of some passerine young during the fledgling period

Species

Black-capped chickadee

Cactus wren

Ovenbird

English blackbird

Mortality rate during nest

period (percent)

mt mn m

1. 52 0. 33 0. 90

1-0.65

1.25 0.81 » 1.00

2. 95 4. 70 3. 54

"• <* 6. 68

<>• • 2 . 3 5

2. 79 1. 75 2. 29

Number of

fledglings

61 (1964)

50 (1964)

34 (1965)

39

70

140

113

Period out of

nest (days)

0to 7

7 to 21-28

0 to 21-28

0to21«

0 to 30-35

0 to 5

5 to 15-20

Mortality rate

(percent)

' 2.82

0.00

0. 11-0. 14

0. 60-0. 67

1. 66-1. 93

4. 18

1. 36-2. 05

Source and locality

Smith, 1967, British Columbia

Ricklefs, 1968a, Arizona

Anderson and Anderson, 1960

Hann, 1937, Michigan

Snow, 1958, England

Lack, 1954, England

° All deaths resulted from a particularly severe storm less

than one week after fledging.

6

 Nest mortality rates (M) rather than m.

'Average (range, 3-40 days).

d

 Wytham Wood, near Oxford.

• The Botanic Garden in Oxford.

can attempt either to raise only as many young as can

be fed in the worst year and not exhibit starvation in

any year, or adjust brood size to the best feeding conditions and have young starve in most years. Clearly

an optimum strategy between these extremes is defined, depending on the cost of raising young which

eventually starve, by the environmental conditions and

cannot be further improved upon by specific adaptations. Thus, the level of starvation is determined

largely by the magnitude and unpredictability of fluctuations in food supply. We have seen that in arid

regions starvation is an important mortality factor. It

can be shown that rainfall, to which breeding in desert

species is closely related (Lloyd, 1960; Marshall, 1961)

is most variable and unpredictable where it is sparsest

(Figure 10). In some cases, yearly fluctuations in food

supply may be predicted by environmental conditions

prior to the breeding season and birds adjust their

clutch sizes accordingly (Lack, 1954; Cody, 1966).

This greater sophistication of clutch-size determination must represent a higher level of adaptation and

certainly reduces mortality levels, but it is also achieved

with little risk. More important, the increased level of

adaptation is permitted by the predictability of fluctuations and thus is largely environmentally determined.

Increased adaptation to reduce mortality caused by

direct environmental factors, such as inclement

weather, is limited by the risks to adults and reduction

of fecundity imposed by strategies such as increased

sturdiness and insulation of nests and intensified parental care such as brooding. In this system, adaptation

is restricted in a different manner than with starvation.

The environment is permissive, allowing the possibility

of reducing mortality rates of individual young essentially to zero. With starvation the nature of the

environment precludes the reduction of mortality rates

below a given level depending on energetic aspects of

reproduction.

Predation is the most important mortality factor for

most species. The outcome of predator-prey interactions reflects the relative effectiveness of predator and

200

> 100

O

50

"'-•-. 0

1 10 100

MONTHLY RAINFALL, mm

FIGURE 10.—Coefficient of variation versus average rainfall

for each of four months (February, May, August, November)

at five Temperate Zone localities: Cape Spartel, Tangiers;

Miyako, Japan; Phoenix, Arizona; Philadelphia, Pennsylvania; and Portland, Oregon. Variability based on 25-year

periods (data from Clayton, 1944).
NUMBER 9 39

prey adaptations. The observed variation in mortality

rates due to predation is large, from less than one

half percent per day in many raptorial and oceanic

species to perhaps more than 5 percent in some small

land birds and precocial species.

A large part of this variation is correlated with body

size of the adults. Teal (1965) has demonstrated a

good negative correlation between body weight and

nestling mortality of herons nesting in the same colony

(Table 20). The relationship between mortality and

body weight is shown graphically in Figure 11 for

temperate passerine and raptorial species. Nesting

mortality decreases as approximately the —0.5 power

of adult body weight, with considerable variation due

to nesting ecology, and, of course, sampling error. Part

of the overall relationship may be due to the sharp

claws and aggressive dispositions of birds of prey rather

than to their large size. But it also seems that small

prey species may be taken by a wider variety of predators than large species simply because they are smaller

than a greater number of predator species. Additionally, the nests of many large birds are preyed upon by

relatively small predators which are capable of climbing to their nest sites and, thus, the physical capao

d .5

<

Q

.25

o •

.01 .1 1

ADULT WEIGHT, kg

10

FIGURE 11.—Mortality rates of nestlings as a function of body

weight in Temperate Zone altricial land birds. Solid circles:

above-ground, open-nesting passerines and raptorial species;

open circles: hole-nesting species; open squares: groundnesting and marsh-nesting species. Adult body weight data

from Baldwin and Kendeigh (1938), Hartman (1946, 1955),

Hartman and Brownell (1961), Imler (1937), Norris and

Johnston (1958), and Wetherbee (1934).

bilities of the young become important in reducing

mortality. Jays and opposums may occasionally steal

hawk eggs when the nests are left unguarded but they

are no match for well-grown young.

Differences in mortality rates among passerines related to habitat and nest construction are striking.

We have seen that species nesting in marshes and fields

have higher mortality rates than those nesting in trees,

and additionally that hole and niche nests are more

secure than open nests. These habitats must present

differences either in predation pressure or the ability

of species to evolve antipredator adaptation. The structural simplicity of field and marsh habitats would seem

to favor predators by reducing the complexity of their

hunting tasks. Furthermore, the structural restrictions

of the habitat must prevent adaptations against specific

groups of predators and limit adaptations to generalized

defenses such as crypsis. The forest habitat provides

nest sites, such as at the tips of small branches, which

completely eliminate a whole size class of predators.

No striking differences exist between most field and

forest species in their levels of adaptation. Nest construction, development rates, brood size, and dependency periods seem to be roughly similar. If it is true

that desertion is more prevalent in forest habitats than

in field habitats, it would seem that adults of fieldnesting species are willing to accept greater risks in

the care of their offspring.

Major environmental differences of widely separated

geographical localities result in a large variation in

nesting mortality owing to predation. At one extreme

we find isolated oceanic islands practically devoid of

life except for the seabirds which use them for nesting,

and at the other extreme, the tropical forests which

exhibit the greatest diversity or terrestrial life. Mortality rates seem to follow the trend in species diversity.

Terrestrial predators are few on offshore islands

because these are difficult to colonize and many are

devoid of alternative food resources during the months

that seabirds do not nest. As we would expect, adaptations against terrestrial predators are poorly developed in insular forms. When rats and domesticated

animals are introduced onto offshore islands they increase mortality rates many fold (e.g., Hagar, 1937).

Other predatory seabirds are potentially strong sources

of mortality, but effective antipredator adaptations

such as parental guarding (e.g., Nelson, 1966) appear

to be evolved with little risk to the adults.

Differences in nest mortality rates of passerine birds
40 SMITHSONIAN CONTRIBUTIONS TO ZOOLOGY

due to predation from arctic to tropical regions reflect

the outcome of opposing adaptations of predators and

prey. Either individual predator species are more efficient in tropical areas, or prey species are beset by a

wider variety of predator species and thus are forced

to adapt to a wider variety of predation strategies.

There is no reason to suppose that individual predator

species are capable of finding prey at a higher rate or

maintaining larger populations in the tropics than in

temperate and arctic regions. The outcome of a limited

two-species predator-prey system would not seem to

vary with changes in latitude: physical structure of the

habitat differs little for birds, overall densities of breeding birds are comparable or perhaps somewhat higher

in tropical than in temperate or arctic areas, but individual species are probably not so abundant on the

average (see Davis and Davis, 1962; Davis and Guion,

1961; Skutch, 1966), and considering the low nesting

success and wider spacing of broods in the tropics

(Ricklefs, 1966), nest densities are probably lower in

the tropics. The occurrence of breeding in the tropics

is nearly as seasonal in some areas as in temperate

regions, although on the average nesting seasons are

longer, especially compared to arctic localities (Ricklefs, 1966). Additionally, tropical predators have comparable or lower reproductive rates (litter and clutch

sizes) than temperate and arctic species (See Lord,

1960, for mammals; Brown, 1952-53,1955, for tropical

eagles; Lack, 1954, and Cody, 1966, for birds in general) . Thus, if individual predators are no more efficient, and if predator species are no more abundant, we

can explain the higher predation rates in the tropics

by postulating a wider variety of predator species for

each prey species. This is precisely the "food web"

pattern of energy flow suggested by many ecologists for

tropical regions. That prey are beset by an increased

number of predator species with different hunting strategies is born out by observers such as Skutch (1949,

1954, 1960, 1967) who indicate that there indeed is a

greater diversity of predator types in the tropics responsible for nesting losses in birds. Numerous arboreal

mammals (including primates), nest-robbing birds

(especially toucans) and snakes, not to mention ants

and parasitic insects, are conspicuous in tropical forests

but virtually absent in similar temperate habitats.

If tropical predators are opportunistic and employ

generalized rather than narrowly directed hunting

techniques, they may be less efficient than their temperate counterparts. We may also ask how prey adaptations differ in the tropics. Do prey species adopt

a generalized antipredator strategy in their nesting

cycles? This would tend to increase predator efficiency

(as it does in field and marsh habitats). Or do prey

species direct their adaptations primarily at one group

of predators? An analysis of nest construction of oscine birds from a temperate and a tropical region

show that the tropical sample builds a greater diversity

of nest-types (Table 25). Apparently, predation pressure largely precludes ground-nesting in the tropics,

whereas many temperate forest species nest on or near

the ground (Taylor, 1965; Preston and Norris, 1947).

In the tropics, numerous species construct domed or

pensile nests, choose special localities, as over water,

and have evolved nesting relationships with termites

and wasps. Thus, nest construction and placement of

some species is more specialized than in temperate

regions. The same may be true of adult behavior.

TABLE 25.—Nest construction and placement of temperate

and tropical passerines °

Type of nest construction

and placement

Nest construction:

Open

Domed or enclosed

Pensile

Niche or natural

cavity

Total number

of species

Height:

Ground

Understory or shrubs

and bushes

Trees

Total number

of species

Special placements:

Over water

Near wasps or bees

New

Number

of

species

63

5

3

18

89

24

20

45

89

2

0

York*

Percentage

71

6

3

20

27

22

51

Canal Zone <

Number Perof centage

species

45

24

15

12

96

6

38

52

96

8

9

47

25

16

12

6

40

54

° Neal G. Smith greatly aided in putting this table together.

» Species list from Bull (1964).

c

 Species list from Eisenmann and Loftin (1967). Only

species whose nests are well known are included.
NUMBER 9 41

Skutch (1949) cites a reduction in the number of feeding trips to the nest and more secretive behavior of

some species to avoid predator detection, although this

has not been properly analyzed. Oropendulas do not

incubate at night during the early stages of the egg

period, presumably to avoid being trapped by predators

in their pensile nests (Schaeffer, 1957; N. G. Smith,

personal communication).

In conclusion, we may recognize four types of factors

which act to restrict the ability of birds to reduce the

mortality rates of their young. First, and perhaps most

important is that adaptations often have multiple effects, some good and some bad, and therefore adapted

strategies are at best only compromises. In general, any

trend toward enhancing the survival of offspring will

involve added risks to the parents. Secondly, unpredictability of environmental resources leads to increased

mortality of the young through starvation. In spite of

these losses, adaptations to reduce mortality from starvation could not be improved upon—except by sacrificing fecundity, which is essentially avoiding the problem rather than adapting to it. Unpredictability of food

imposes which might be called "statistical restrictions"

on survival levels which cannot be exceeded by biological systems. Thirdly, adult adaptations for nonreproductive activities impose restraints on the range of

adaptations which may be exploited in breeding activities. Body size, morphological adaptations for foraging, habitat selection and others fall into this category.

Finally, the survival of every species is challenged by

other adapting systems bent on their own survival and

these are the greatest cause of mortality in most species.

The equilibrium between predator and prey adaptations remains one of the most fascinating and elusive

of biological problems.

Appendix I

The Strength of Selection on Development Rates

The selective strengths of environmental factors in

producing specific adaptations to reduce mortality rates

are directly related to their contribution to nesting

losses. Unfortunately, it would be very difficult to ascertain differences in fitness resulting from changes in

most aspects of reproductive behavior. It is only possible to guess that equivalent adaptation in different

species will most benefit those with the highest mortality rates—i.e., selection for equivalent changes is

proportional to the mortality rate. On the other hand,

changes in fitness brought about by altering the duration of exposure to mortality factors may be treated

mathematically quite easily, as will be shown in subsequent papers. Adjustments in the length of various

phases of the nesting cycle are general adaptive responses to most mortality factors, largely excepting

brood parasitism, hatching failure and starvation. For

many species, the strength of those factors which influence developmental rates is nearly as great as the

overall mortality rate. In others, factors which do not

exert selection on the length of the nesting cycle may

contribute substantially to total mortality. It will be

useful to distinguish these quantitatively for subsequent

analyses of the relationship between mortality and

development. As we have seen, it is possible to recognize

these factors and make suitable corrections from patterns of nesting losses.

The strength of selection on the length of the nesting

cycle is summarized in Table 26 for different groups

of passerine birds using four rules to adjust mortality

rates:

1. If nest loss is known and its rate during the egg

period exceeds that during the nestling period, no

correction is needed. It has been demonstrated that

loss of nests due to starvation is probably negligible

under these conditions.

2. If nest loss during the nestling period exceeds that

of the egg period, it has been shown that desertion is

probably a minor factor and that the excess mortality

during the nestling period is primarily due to starvation. This can be corrected by setting the mortality

rate during the nestling period equal to that of the

egg period, thus eliminating the starvation component.

3. When individual losses are known and mortality

rates during the egg period exceed those of the nestling

period, most of the within-nest loss is due to hatching

failure and brood parasitism and this can be subtracted

from the overall rate of nest loss. If the within-nest loss

is not known, the subtraction of 0.4 percent per day

(Table 5) should improve the estimate of selective

forces in most cases.

4. If individual loss is known and the rate of mortality is greater during the nestling period than during

the egg period, it is sufficient to correct the egg period

rate for hatching failure by subtracting about 0.3

percent per day (Table 1) and to set the nestling period

rate equal to the corrected rate for the egg period.
42 SMITHSONIAN CONTRIBUTIONS TO ZOOLOGY

TABLE 26.—Relative strength of selection on development

rates in passerine birds

Region and nest-type

Temperate

Marsh-nesting

Ground-nesting

Tree- and shrubnesting

Hole- and nichenesting

Tropical

Humid areas

Arid areas

Arctic

Number of

species

3

5

9

7

11

11

8

Average relevant

mortality rate

(percent)

Per

day

3.70

2.55

1.82

0.83

3.86

1.82

1.66

Range

2. 84-5. 06

1.53^. 11

0. 87-2.43

0.21-1.63

2. 20-5. 38

0.99-3. 15

0. 34-5. 90

Residual

nonrelevant

mortality

rate

{percent

per day)

0.82

1. 13

0.30

0. 19

0.23

1.43

0.89

Since nestling losses of precocial birds are caused

by factors similar to those acting on small land birds,

the same corrections may be applied to estimate the

strength of selection acting on development rates.

Average values calculated for six species of shorebirds

are 1.54 percent per day, for nine species of waterbirds, 1.08 percent, and for six species of game birds,

3.02 percent.

The relative strength of factors acting on raptorial

birds and seabirds are more difficult to ascertain. In

large raptorial birds, mortality which is relevant to

development rates must certainly average less than

0.5 percent per day, based on mortality rates presented

in Table 14, and probably is closer to 0.2-0.3 percent

for most species. Seabirds present additional problems.

In the Procellariiformes and Pelecaniformes, most

chick mortality (average, 0.66 percent per day) would

be reduced by increasing the rate of nestling development but it is difficult to estimate how much of the egg

mortality (average, 2.17 percent) is relevant until

more detailed studies of losses have been made. This

is also true of the Charadriiformes (average mortality

rates, 1.49 percent and 2.02 per day during the egg

and nestling periods, respectively). Losses of eggs in

seabirds due to hatching failure and rolling out of nests

is quite high, perhaps one half of total egg mortality

in most species. Thus, egg mortality rates pertinent to

the evolution of development rates should be about onehalf total mortality rates, or about 1.08 and 0.75 percent per day for the two groups of seabirds. These

values are not much lower than for some groups of

land and waterbirds and are not nearly as low as in

raptorial species.

Except for the possibility of starvation in some gulls

and terns, most of the mortality of young seabirds bears

on the evolution of development rates. A conversion

factor of three fourths would probably give a reasonable

and conservative correction for losses and thus we may

estimate that selective forces are approximately 0.5

and 1.5 percent per day during the nestling period in

the two groups of seabirds. Additionally, it must be kept

in mind that mortality rates of seabirds decrease markedly with age.

Appendix 2

Alphabetical List of Bird Names

African fish eagle, Haliaetus vocifer

American robin, Turdus migratorius

Arctic tern, Sterna paradisea

Bald eagle, Haliaetus leucocephalus

Black-and-white manakin, Manacus manacus

Black-capped chickadee, Parus atricapillus

Black-crowned night heron, Nycticorax nycticorax

Black guillemot, Cepphus grylle

Black-legged kittywake, Rissa tndactyla

Blue-black grassquit, Volatinia jocarina

Blue-faced booby, Sula dactylatra

Blue-gray tanager, Tkraupis episcopus

Bobwhite quail, Colinus virginianus

Bonaparte warbling-finch, Poospiza hispaniolensis

booby, Sula spp.

Brewer blackbird, Euphagus cyanocephalus

Brown booby, Sula leucogaster

Brown-headed cowbird, Molothrus ater

Cactus wren, Campylorhynchus brunneicapillus

California quail, Lophortyx californicus

Cardinal, Richmondena cardinalis

Cassin auklet, Ptycoramphus aleutica

Cedar waxwing, Bombycilla cedrorum

Chestnut-collard longspur, Calcarius ornatus

Chestnut-throated seed-eater, Sporophila telasco

Chipping sparrow, Spizella passerina

Clay-colored robin, Turdus grayi

Common goldfinch, Spinus tristh

Common grackle, Quiscalus quiscula

Common redpoll, Acanthis flammea

Common swift, Apus apus

Costa hummingbird, Calypte costa

Crimson finch, Rhodospingus cruentus

Curve-billed thrasher, Toxostoma curvirostre

D'Orbigny ground-dove, Eupelia cruziana
NUMBER 9 43

Double-crested cormorant, Phalacrocorax auritus

Duck hawk, Falco peregrinus

Dunlin, Calidris alpina

Eastern bluebird, Sialia sialis

Eastern phoebe, Sayornis phoebe

Ecuadorian neorhynchus, Neorhynchus peruvianus

English blackbird, Turdus merula

Field sparrow, Spizella pusilla

finches, (Fringillidae)

Fulvous-headed pygmy-tyrant, Euscarthmus molorhyphus

Glaucous-winged gull, Larus glaucescens

Gray-capped flycatcher, Myiozetetes grenadensis

Greater prairie chicken, Tympanuchus cupido

Great-horned owl, Bubo virginianus

Golden eagle, Aquila chrysaetos

Horned lark, Eremophila alpestris

House finch, Carpodacus mexicanus

House wren, Troglodytes aedon

jays, Cyanocitta, Aphelocoma, etc (Corvidae)

Lapland lonspur, Calcarius lapponicus

Least tern, Sterna albifrons

Little blue heron, Florida caerulea

Long-billed marsh wren, Telmatodytes palustris

Long-tailed mockingbird, Mimus longicaudatus

Madeiran storm petrel, Oceanodroma castro

Marsh hawk, Circus hudsonius

Marsh wren, Telmatodytes palustris

McCown longspur, Rhynchophanes mccowni

Mourning dove, ^enaidura macroura

Orchard oriole, Icterus spurius

oropendulas, ^arhynchus wagleri and others (Icteridae)

Ovenbird, Seiurus aurocapillus

Pelagic cormorant, Phalacrocorax pelagicus

Pigeon guillemot, Cepphus columba

Prothonotary warbler, Protonotaria citrea

Purple grackle, Quiscalus quiscula

Raven, Corvus corax

Red-billed tropic bird, Phaethon aethereus

Red-tailed hawk, Buteo jamaicensis

Red-winged blackbird, Agelaius phoeniceus

robins, Turdus spp.

Rough-winged swallow, Stelgidopteryx ruficollis

Ruddy duck, Oxyura jamaicensis

Ruddy ground-dove, Columbigallina talpacoti

Ruffed grouse, Bonasa umbellus

Savannah sparrow, Passerculus sandwichensis

Say phoebe, Sayornis sayus

Scarlet-rumped tanager, Ramphocelus passerinii

Shag, Phalacrocorax aristotelis

Short-tailed ground-tyrant, Muscigralla brevicauda

Snow bunting, Plectrophenax nivalis

Song sparrow, Melospiza melodia

Starling, Sturnus vulgaris

Thick-billed murre, Una lomiia

toucan (Ramphastidae)

Traill flycatcher, Empidonax traillii

Tree swallow, Iridoprocne bicolor

Vermillion flycatcher, Pyrocephalus rubinus

White-browed gnatcatcher, Polioptila plumbea

White-crowned sparrow, ^jonotrichia leucophrys

White-tailed kite, Elanus leucurus

White-tailed tropic bird, Phaethon lepturus

Yellow-bellied elaenia, Elaenia flavogaster

Yellow-faced grassquit, Tiaris olivacea

Yellow-headed blackbird, Xanthocephalus xanthocephalus

Yellow wagtail, Motacilla flava

Yellow warbler, Dendroica aestiva

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1946. Der Steinwalzer, Arenaria i. intrepes (L.), in seiner

Beziehung zur Umwelt. Ada Zoologica Fennica,

47:1-143.

Brown, L. H.

1952-1953. On the Biology of the Large Birds of Prey

of the Embu District, Kenya Colony. Ibis,

94:577-620;95:74-114.

1955. Supplementary Notes on the Biology of the Large

Birds of Prey of Embu District, Kenya Colony.

Ibis, 97:38-64; 183-221.

1960. The African Fish Eagle Haliaetus vocifer Especially

in the Kavirondo Gulf. Ibis, 102:285-297.
44 SMITHSONIAN CONTRIBUTIONS TO ZOOLOGY

Bull, John

1964. Birds of the New York Area. New York: Harper

and Row.

Bump, G., R. W. Darrow, F. C. Edminster, and W. F. Grissey

1947. The Ruffed Grouse. New York State Conservation

Department, Buffalo: Holling Press.

Buss, I. O., and A. S. Hawkins

1939. The Upland Plover at Faville Grove, Wisconsin.

Wilson Bulletin, 51:202-220.

Chapman, L. B.

1939. Studies of a Tree Swallow Colony. Bird-Banding,

10:61-72.

Clayton, H. H.

1944. World Weather Records. Smithsonian Miscellaneous Collections, volume 79.

Cody, M. L.

1966. A General Theory of Clutch Size. Evolution,

20: 174-184.

Coulson, J. C, and E. White.

1958. The Effect of Age on the Breeding Biology of the

Kittywake Rissa tridactyla. Ibis, 100: 40-51.

Cowan, J. B.

1952. Life History and Production of a Population of

Western Mourning Doves in California. California

Fish and Game, 38: 505-521.

Cullen, E.

1957. Adaptations in the Kittiwake to Cliff-nesting.

Ibis, 99: 275-302.

Darling, F. F.

1938. Bird Flocks and the Breeding Cycle. Cambridge.

Davis, L. I., and D. Davis

1962. Breeding Bird Census No. 14: Humid Tropical

Forest; Breeding Bird Census No. 26: Tropical

Savanna. Audubon Field Notes, 16:525-526.

Davis, L. I., and B. Guion

1961. Breeding Bird Censuses Nos. 16: Humid Tropical

Forest; 21: Tropical Savanna; 22: Upland Savanna; 23: Lowland Savanna. Audubon Field

Notes, 15:510,512-513.

Dennis, J. V.

1948. Observations on the Orchard Oriole in Lower

Mississippi Delta. Bird-Banding, 19:12-21.

Dixon, J. B., R. E. Dixon, and J. E. Dixon

1957. Natural History of the White-tailed Kite in San

Diego County, California. Condor, 59:156-165.

Dorward, D. F.

1962. Comparative Biology of the White Booby and the

Brown Booby Sula spp. at Ascension. Ibis, 103b:

174-220.

Drent, R., G. F. van Tets, F. Tompa, and K. Vermeer

1964. The Breeding Birds of Mandarte Island, British

Columbia. Canadian Field Naturalist, 78:208-

263.

Drury, W. H.

1961. Studies of the Breeding Biology of Horned Lark,

Water Pipit, Lapland Longspur, and Snow Bunting

on Bylot Island, Northwest Territories, Canada.

Bird-Banding, 32:1-44.

Dunnet, G. M.

1955. The Breeding of the Starling Sturnus vulgaris in

Relation to Its Food Supply. Ibis, 97.619-662.

Eisenmann, E., and H. Loftin

1967. Field Checklist of the Birds of the Panama Canal

Zone Area, 1967. Florida Audubon Society.

Emlen, J. T., Jr.

1956. Juvenile Mortality in a Ring-billed Gull Colony.

Wilson Bulletin, 68:232-238.

Errington, P. L., and F. N. Hammerstrom

1937. The Evaluation of Nesting Losses and Juvenile

Mortality of the Ring-necked Pheasant. Journal

of Wildlife Management, 1:3-20.

Evenden, F. G.

1957. Observations on Nesting Behavior of the House

Finch. Condor, 59:112-117.

Fautin, R. W.

1941. Incubation Studies of the Yellow-headed Blackbird.

Wilson Bulletin, 53:107-122.

Fitch, H. S., F. Swenson, and D. F. Tillotson

1946. Behavior and Food Habits of the Red-tailed Hawk.

Condor, 48:205-237.

Fretwell, S.

1968. Density Dependent Nest Mortality in Birds. Manuscript.

Glading, B.

1938. Studies on the Nesting Cycle of the California

Valley Quail in 1937. California Fish and Game,

24:318-340.

Glover, F. A.

1953. Nesting Ecology of the Pied-billed Grebe in Northwestern Iowa. Wilson Bulletin, 65:32-39.

Goddard, S. V., and V. V. Board

1967. Reproductive Success of Red-winged Blackbirds in

North Central Oklahoma. Wilson Bulletin, 79:

283-289.

Grinnell, L. I.

1943. Nesting Habits of the Common Redpoll. Wilson

Bulletin, 55:155-163.

Gross, A. O.

1923. The Black-crowned Night Heron {Nycticorax

nycticorax naevius) of Sandy Neck. Auk, 40:

1-30, 191-214.

Gullion, G. W.

1954. The Reproductive Cycle of American Coots in

California. Auk, 71:366-412.

Hagan, J. A.

1937. Least Tern Studies—1935 and 1936. Bulletin of

the Massachusetts Audubon Society, 21:5—8.

Hamerstrom, F. N., Jr.

1936. A Study of the Nesting Habits of the Ring-necked

Pheasant in Northwest Iowa. Iowa State College

Journal of Science, 10:173-203.

1939. A Study of Wisconsin Prairie Chicken and Sharptailed Grouse. Wilson Bulletin, 51:105-120.

Hammond, M. C, and C. J. Henry

1949. Success of Marsh Hawk nests in North Dakota.

Auk, 66:271-274.
NUMBER 9 45

Hann, H. W.

1937. Life History of the Ovenbird in Southern Michigan.

Wilson Bulletin, 49:145-237.

Harris, R. D.

1944. The Chestnut-collared Longspur in Manitoba.

Wilson Bulletin,, 56:105-115.

Hartman, F. A.

1946. Adrenal and Thyroid Weights in Birds. Auk,

63:42-64.

1955. Heart Weight in Birds. Condor, 57:221-238.

Hartman, F. A., and K. A. Brownell

1961. Adrenal and Thyroid Weights in Birds. Auk,

78:397-422.

Hawksley, O.

1957. Ecology of a Breeding Population of Arctic Terns.

Bird-Banding, 28:57-92.

Heinroth, O.

1922. Die Beziehungen zwischen Vogelgewicht, Eigewicht, Gelegewicht und Brutdauer. Journal fur

Ornithologie, 70:172-185.

Hensel, R. J., and W. A. Troyer

1964. Nesting Studies of the Bald Eagle in Alaska.

Condor, 66:282-286.

Hickey, J. J.

1955. Some American Population Research on Gallinaceous Birds. In Recent Studies in Avian Biology,

edited by A. Wolfson. Urbana: University of Illinois Press.

Holmes, R. T.

1966. Breeding Ecology and Annual Adaptations of the

Red-backed Sandpiper (Calidris alpina) in Northern Alaska. Condor, 68:3-46.

Horn, H.

1968. The Adaptive Significance of Colonial Nesting in

the Brewer's Blackbird (Euphagus cyanocephalus).

Ecology, 49:682-694.

Howell, J. C.

1942. Notes on the Nesting Habits of the American Robin

(Turdus migratorius L.). American Midland Naturalist, 28:529-603.

Imler, R. H.

1937. Weights of Some Birds of Prey of Western Kansas.

'Bird-Banding, 8:166-169.

Irving, L., and J. Krog

1956. Temperature During the Development of Birds in

Arctic Nests. Physiological Zoology, 29:195-205.

Kale, H. W. II

1965. Ecology and Bioenergetics of the Long-billed Marsh

Wren in Georgia Salt Marshes. Publications Nuttall Ornithological Club, number 5.

Kalmbach, E. R.

1939. Nesting Success: Its Significance in Waterfowl Reproduction. Transactions North American Wildlife Conference, 4-591-604.

Keller, R. J., H. R. Shephard, and R. N. Randall

1941. Report of the Sage Grouse Survey: Pittman-Robertson Project, Colorado 4-R, Season 1941, with

Comparative Data of Previous Seasons. Sage

Grouse Survey Colorado, 3:31.

Kendeigh, S. C.

1942. Analysis of Losses in the Nesting of Birds. Journal Wildlife Management, 6:19-26.

King, J. R.

1954. Victims of the Brown-headed Cowbird in Whitman

County, Washington. Condor, 56:150-154.

Klimstra, W. D.

1950. Bob-White Quail Nesting and Production in southeastern Iowa. Iowa State College Journal of

Science, 24:385-395.

Klopman, R. B.

1958. The Nesting of the Canada Goose at Dog Lake,

Manitoba. Wilson Bulletin, 70:168-183.

Kozicky, E. L., and F. V. Schmidt

1949. Nesting Habits of the Clapper Rail in New Jersey.

Auk, 66:355-364.

Kuerzi, R. G.

1941. Life-History Studies of the Tree Swallows. Proceedings of the Linnean of Society of New York,

52-53:1-52.

Lack, D.

1954. The Natural Regulation of Animal Numbers.

Oxford, Clarendon Press.

1965. Evolutionary Ecology. Journal of Animal Ecology,

34:223-231.

Lack D., and E. Lack

1951. The Breeding Biology of the Swift Apus apus.

Ibis, 93:501-546.

La Rivers, I.

1944. Observations on the Nesting Mortality of the

Brewer Blackbird, Euphagus cyanocephalus.

American Midland Naturalist, 32:417-437.

Laskey, A. R.

1940. The 1939 Nesting Season of Bluebirds at Nashville,

Tennessee. Wilson Bulletin, 52:183-190.

1943. The Nesting of Bluebirds Banded as Nestlings.

Bird-Banding, 14:39-43. [See also Nice, 1957.]

Laven,H.

1940. Beitrage zur Biologie des Sandregenpfeifers (Charadrius hiaticula L.). Journal fur Ornithologie,

88:183-287.

Leedy, D. L., and L. E. Hicks

1945. The Pheasant in Ohio. In The Ring-necked Pheasant and Its Management in North America, edited

by W. L. McAtee. Washington: American Wildlife

Institute, pages 57-130.

Lehmann, V. W.

1941. Attwater's Prairie Chicken: Its Life History and

Management. North American Fauna, 57:1-65.

1946. Bobwhite Quail Reproduction in Southwestern

Texas. Journal of Wildlife Management, 10:111-

123.

Lloyd, M.

1960. Statistical Analysis of Marchant's Data on Breeding Success and Clutch-size. Ibis, 102:600-611.

Lord, R. D., Jr.

1960. Litter Size and Latitude in North American Mammals. American Midland Naturalist, 64:488-499.
46 SMITHSONIAN CONTRIBUTIONS TO ZOOLOGY

Low, J. B.

1941. Nesting of the Ruddy Duck in Iowa. Auk, 58:

506-517.

Low, S.

1933. Notes on the Nesting of Bluebirds. Bird-Banding,

4:109-111.

1934. Nest Distribution and Survival Ratio of Tree

Swallows. Bird-Banding, 5:24-30.

Lunk, W. A.

1962. The Rough-winged Swallow. A Study Based on Its

Breeding Biology in Michigan. Publications of the

Nuttall Ornithological Club, Number 4.

Marchant, S.

1959. The Breeding Season in S. W. Ecuador. Ibis,

101.137-152.

1960. The Breeding of Some S. W. Ecuadorian Birds.

Ibis, 102:349-382, 584-599.

Marshall, A. J.

1961. Breeding Seasons and Migration. In Biology and

Comparative Physiology of Birds, 2:307-339, edited

by Marshall, A. J., New York: Academic Press.

Mayfield, H.

1961. Nesting Success Calculated from Exposure. Wilson

Bulletin, 73:255-261.

McCabe, R. A., and A. S. Hawkins

1946. The Hungarian Partridge in Wisconsin. American

Midland Naturalist, 36:1-75.

McClure, H. E.

1946a. Mourning Doves in Nebraska and the West. Auk,

63:24-42.

1946b. Phoebes in Central Nebraska. Auk, 63:211-215.

McGahan, J.

1968. Ecology of the Golden Eagle. Auk, 85:1-12.

Meanley, B.

1953. Nesting of the King Rail in the Arkansas Rice

Fields. Auk, 70:261-269.

1955. A Nesting Study of the Little Blue Heron in Eastern

Arkansas. Wilson Bulletin, 67:84-99.

Mickey, F. W. •

1943. Breeding Habits of McCown's Longspur. Auk,

60:181-209.

Monk, H. C.

1949. Nesting of the Mourning Dove at Nashville, 1949.

Migrant, 20:1-9.

Morehouse, E. L., and R. Brewer

1968. Feeding of Nestling and Fledgling Eastern Kingbirds. Auk, 85:44-54.

Nelson, J. B.

1966. The Behaviour of the Young Gannet. British Birds,

59:393-419.

Nice, M. M.

1931. Birds of Oklahoma. University of Okla. Biological

Survey, 3:1-224.

1937. Studies in the Life History of the Song Sparrow. 1.

A Population Study of the Song Sparrow. Transactions Linnean Society of New York, 4:1-247.

1954. Problems of Incubation Periods in North American

Birds. Condor, 56:173-197.

1957. Nesting Success in Altricial Birds. Auk, 74:305-

321.

1962. Development of Behavior in Precocial Birds.

Transactions of the Linnean Society of New York,

8:1-211.

Nolan, V.

1963. Reproductive Success of Birds in a Deciduous Scrub

Habitat. Ecology, 44:305-313.

Norris, R. A., and D. W. Johnston

1958. Weights and Weight Variations in Summer Birds

from Georgia and South Carolina. Wilson Bulletin, 70:114-129.

Norris, R. T.

1947. The Cowbirds of Preston Frith. Wilson Bulletin,

59:83-103.

Oakeson, B. B.

1954. The Gambel's Sparrow at Mountain Village, Alaska.

Auk, 71:351-365.

Odum, E. P.

1941. Annual Cycle of the Black-capped Chicadee. Auk,

58:314-333, 518-535.

Orians, G. H.

1966. Food of Nestling Yellow-headed Blackbirds, Cariboo

Parklands, British Columbia. Condor, 68:321-

337.

Orians, G., and Kuhlman, F.

1956. Red-tailed Hawk and Horned Owl Populations in

Wisconsin. Condor, 58:371-385.

Owen, D. F.

1960. The Nesting Success of the Heron Ardea cinerea

in relation to the Availability of Food. Proceedings of the Zoological Society of London, 133:597—

617.

Palmer, R. S., editor

1962. Handbook of North American birds. I, Loons

Through Flamingos. New Haven: Yale University Press.

Paludan, K.

1951. Contributions to the Breeding Biology of Larus

argentatus and Larus fuscus. Dansk Natural Historisk forening, Videnskabelige meddelelser, 114:

1-128.

Patterson, R. L.

1949. Sage Grouse Along the Oregon Trail. Wyoming

Wildlife, 13:1-16.

Paynter, R. A., Jr.

1949. Clutch-size and the Egg and Chick Mortality of

Kent Island Herring Gulls. Ecology, 30:146-166.

Pearson, A. M., and G. C. Moore

1939. Nesting Habits of the Mourning Dove in Alabama.

Transactions of the North American Wildlife Conference, 4:468-473.

Petersen, A., and H. Young

1950. A Nesting Study of the Bronzed Grackle. Auk,

67:466-476.

Pettingill, O. S., Jr.

1939. History of One Hundred Nests of Arctic Tern.

Auk, 56:420-428.
NUMBER 9 47

Pickwell, G. B.

1931. The Prairie Horned Lark. Transactions of the

Academy of Science of St. Louis, 27:1-153.

Pole, E. L.

1938. Weights and Wing Areas in North American Birds.

Auk, 55:511-517.

Preston, F. W., and R. T. Norris

1947. Nesting Heights of Breeding Birds. Ecology, 28:

241-273.

Putnam, L. S.

1949. The Life History of the Cedar Waxwing. Wilson

Bulletin, 61:141-182.

Randall, P. W.

1940. The Life Equation of the Ring-Necked Pheasant in

Pennsylvania. Transactions of the North American Wildlife Conference, 5:300-320.

Rasmussen, D. I., and L. A. Griner

1938. Life History and Management of the Sage Grouse

in Utah, with Special Reference to Nesting and

Feeding Habits. Transactions of the North American Wildlife Conference, 3:852-864.

Rasmussen, D. I., and W. T. McKean

1945. The Pheasant in the Inter-mountain Irrigated Region. In The Ring-Necked Pheasant and Its Management in North America, edited by W. L. McAtee,

American Wildlife Institute, Washington, pages

234-253.

Ratcliffe, D. A.

1962. Breeding Density in the Peregrine Falco peregrinus

and Raven Corvus corax. Ibis., 104:13-39.

Ricklefs, R. E.

1965. Brood Reduction in the Curve-billed Thrasher.

Condor, 67:505-510.

1966. The Temporal Component of Diversity Among

Species of Birds. Evolution, 20:235-242.

1968a. On the Survival Rate of Juvenile Cactus Wrens.

Condor, 70:388-389.

1968b. Patterns of Growth in Birds. Ibis, 110:419-451.

Ricklefs, R. E., and F. R. Hainsworth

1968. Temperature Regulation in Nestling Cactus Wrens.

The Nest Environment. Condor, 70:121-127.

Royama, T.

1966. Factors Governing Feeding Rate, Food Requirement and Brood Size of Nestling Great Tits Parus

major. Ibis, 108:313-347.

Schafer, E.

1957. Les Conotos. Bonner Zoolgische Beitdge, Special

Supplement 5, 148 pages.

Schrantz, F. G.

1943. Nest Life of the Eastern Yellow Warbler. Auk,

60:367-387.

Shelley, L. O.

1937. Further Tree Swallow Notes. Bird-Banding, 8:80-

81.

Skutch, A. F.

1949. Do Tropical Birds Rear As Many Young As They

Can Nourish? Ibis, 91:430-455.

1954, 1960. Life Histories of Central American Birds.

Pacific Coast Avifauna, 31:1-448; 34:1-593.

1956. Life History of the Ruddy Ground-Dove. Condor,

58:188-205.

1966. A Breeding Census and Nesting Success in Central

America. Ibis, 108:1-16.

1967. Adaptive Limitation of the Reproductive Rate of

Birds. Ibis, 109:579-599.

Smith, H. M.

1943. Size of Breeding Populations in Relation to Egglaying and Reproductive Success in the Eastern

Red-wing (Agelaius p. phoeniceus). Ecology,

24: 183-207.

Smith, N. G.

1968. The Advantage of Being Parasitized. Nature,

219:690-694.

Smith, S. M.

1967. Seasonal Changes in the Survival of the Blackcapped Chickadee. Condor, 69:344-359.

Snow, B.

1960. The Breeding Biology of the Shag Phalacrocorax

aristotelis on the Island of Lundy, British Channel.

Ibis, 102:554-575.

Snow, D. W.

1958. The Breeding of the Blackbird Turdus merula at

Oxford, Ibis, 100:1-30.

1962. A Field Study of the Black-and-White Manakin,

Manacus manacus, in Trinidad. Zoologica, 47:

65-104.

Snow, D. W., and B. K. Snow

1963. Breeding and the Annual Cycle in Three Trinidad

Thrushes. Wilson Bulletin, 75:27-41.

Soikkeli, M.

1967. Breeding Cycle and Population Dynamics in the

Dunlin (Calidris alpina). Annales Zoologica Fennici, 4:158-198.

Stoddard, H. L.

1931. The Bobwhite Quail: Its Habits, Preservation and

Increase. New York: Charles Scribner's Sons.

Stokes, A. W.

1950. Breeding Behavior of the Goldfinch. Wilson Bulletin, 62:107-127.

Stonehouse, B.

1962. The Tropic-birds (genus Phaethon) of Ascension

Island. Ibis, 103b: 124-161.

S toner, D.

1936. Studies on the Bank Swallow Riparia riparia riparia

(Linneaus) in the Oneida Lake Region. Roosevelt Wildlife Annual, 4:126-233.

1945. Temperature and Growth Studies of the Northern

Cliff Swallow. A uk, 62:207-216.

Sutton, G. M., and D. F. Parmelee

1954. Nesting of the Snow Bunting on Baffin Island.

Wilson Bulletin, 66:159-179.

1955. Summer Activities of the Lapland Longspur on

Baffin Island. Wilson Bulletin, 67:110-127.

Taylor, W. K.

1965. Nesting Heights of Some Louisiana Birds. Wilson

Bulletin, 77:146-150.

Teal, J. M.

1965. Nesting Success of Egrets and Herons in Georgia.

Wilson Bulletin, 77:257-263.
48

SMITHSONIAN CONTRIBUTIONS TO ZOOLOGY

Thomas, R. H.

1946. A Study of Eastern Bluebirds in Arkansas. Wilson

Bulletin, 58:143-183.

Thoresen, A. C.

1964. The Breeding Behavior of the Cassin Auklet.

Condor, 66:456-476.

Tuck, L. M.

1960. The Murres. Canadian Wildlife Series, 1:1-260.

Walkinshaw, L. H.

1940. Summer Life of the Sora Rail. Auk, 57:153-168.

1941. The Prothonotary Warbler, a Comparison of Nesting Conditions in Tennessee and Michigan. Wilson Bulletin, 53:1-21.

1952. Chipping Sparrow Notes. Bird-Banding, 23:101—

108.

1953. life-history of the Prothonotary Warbler. Wilson

Bulletin, 65:152-168.

Weins, J. A.

1965. Behavioral Interactions of Red-winged Blackbirds

and Common Grackles on a Common Breeding

Ground. Auk, 82:356-374.

Weller, M. W.

1961. Breeding Biology of the Least Bittern. Wilson

Bulletin, 73:11-35.

Wetherbee, K. B.

1934. Some Measurements and Weights of Live Birds.

Bird-Banding, 5:55-64.

Weydemeyer, W.

1935. Efficiency of Nesting of the Tree Swallow. Condor,

37:216-217.

Wilcox, L.

1959. A Twenty Year Banding Study of the Piping Plover.

Auk, 76:129-152.

Williams, C. S., and W. H. Marshall

1938. Duck Nesting Studies, Bear River Migratory Bird

Refuge, Utah, 1937. Journal of Wildlife Management, 2:29-48.

Williams, G. R.

1959. Aging, Growth-rate and Breeding Season Phenology

of Wild Populations of California Quail in New

Zealand. Bird-Banding, 30:203-216.

Williams, J. F.

1940. The Sex Ratio in Nestling Eastern Red-Wings.

Wilson Bulletin, 52:267-277.

Williamson, F. S. L., M. C. Thompson, and J. Q. Hines

1966. Avifaunal investigations. In Environment of the

Cape Thompson Region Alaska, edited by N. J.

WUimovsky and J. N. Wolff, U.S. Atomic Energy

Commission, pages 437-480.

Willson, M. F.

1966. Breeding Ecology of the Yellow-headed Blackbird.

Ecological Monographs, 36:51-77.

Winn, H. E.

1950. The Black Guillemots of Kent Island, Bay of Fundy.

Auk, 67:477-485.

Woods, R. S.

1923. Further Observations on the Costa Hummingbird.

Condor, 25:195-198.

Yeatter, R.

1934. The Hungarian Partridge in the Great Lakes region.

University of Michigan School of Forestry and Conservation Bulletin, 5:1-92.

Young, H.

1955. Breeding Behavior and Nesting of the Eastern

Robin. American Midland Naturalist, 53:329-

352.

1963. Age Specific Mortality in the Eggs and Nestlings of

Blackbirds. Auk, 80: 145-155.

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