A Review of the

Literature on Seed Fate 

in Whitebark Pine and 

the Life History Traits of

Clark’s Nutcracker and

Pine Squirrels 

Teresa J. Lorenz, Carol Aubry, and Robin Shoal

United States 

Department of 

Agriculture

Forest Service

Pacific Northwest

Research Station

General Technical

Report

PNW-GTR-742

April 2008
Authors

Teresa J. Lorenz is a wildlife biologist, Okanogan and Wenatchee National

Forests, Naches Ranger District, 10237 Highway 12, Naches, WA 98937; Carol

Aubry is a forest geneticist, and Robin Shoal is a forest ecologist, Pacific

Northwest Region, Olympic National Forest, 1835 Black Lake Blvd. SW, Olympia,

WA 98512-5623. The research supporting this publication is being conducted in 

collaboration with the Pacific Northwest Research Station.

Cover photo

Clark’s nutcracker foraging on whitebark pine seeds. Mount Baker-Snoqualmie

National Forest, Washington. (Photo by Robin Shoal)

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Abstract

Lorenz, Teresa J.; Aubry, Carol; Shoal, Robin. 2008. A review of the literature

on seed fate in whitebark pine and the life history traits of Clark’s nutcracker

and pine squirrels. Gen. Tech. Rep. PNW-GTR-742. Portland, OR: U.S.

Department of Agriculture, Forest Service, Pacific Northwest Research Station.

62 p.

Whitebark pine is a critical component of subalpine ecosystems in western North

America, where it contributes to biodiversity and ecosystem function and in some

communities is considered a keystone species. Whitebark pine is undergoing

rangewide population declines attributed to the combined effects of mountain pine

beetle, white pine blister rust, and fire suppression. The restoration and maintenance of whitebark pine populations require an understanding of all aspects of 

seed fate. In this paper, we review the literature on seed dispersal in whitebark

pine. Clark’s nutcracker, pine squirrels, and scatter-hoarding rodents are all known

to influence whitebark pine seed fate and ultimately affect the ability of whitebark

pine populations to regenerate. We also provide a general overview of the natural

histories of the most influential species involved with whitebark pine seed fate:

Clark’s nutcracker and the pine squirrel. 

Keywords: Whitebark pine, seed dispersal, Clark’s nutcracker, pine squirrels,

scatter-hoarding.
Contents

1 Introduction

3 Whitebark Pine Seed Fate

5 Primary Seed Dispersal by Clark’s Nutcracker

14 Seed Predation by Pine Squirrels

18 Secondary Seed Dispersal by Scatter-Hoarding Rodents

22 Life History Traits

22 Clark’s Nutcracker

34 Pine Squirrels

41 Conclusion

42 Glossary

43 Acknowledgments

44 English Equivalents

44 Literature Cited

59 Appendix 1

62 Appendix 2
Introduction

Whitebark pine (a list of common and scientific names off all species referred to

in this document can be found in app. 1) is a five-needle pine that is restricted in

distribution to upland and subalpine habitats in western North America (Arno and

Hoff 1989) (fig. 1). It plays a critical role throughout its range as a pioneer species

and in stabilizing soils and providing forage and cover for wildlife (Tomback et al.

2001). Whitebark pine communities are typically remote and consequently have 

not been considered as threatened by human-induced changes as lowland forests.

Within the last century, however, whitebark pine throughout its range has been

undergoing population declines attributed to the combined effects of mountain pine

beetle, white pine blister rust, and fire suppression (Keane 2001, Tomback et al.

2001) (fig. 2). 

1

A Review of the Literature on Seed Fate in Whitebark Pine and the Life History Traits of Clark’s Nutcracker and Pine Squirrels

Figure 1—Range of whitebark pine (U.S. Geological Survey 1999).

These three factors have affected whitebark pine in different ways. Mountain

pine beetles kill trees rapidly, and most commonly infect diseased and stressed

trees (Cole and Amman 1969, Larsson et al. 1983). During periodic epidemics,

healthy trees are also affected, and this causes rapid standwide mortality (Safranyik

and Carroll 2006). Climate models predict that conditions are favorable for more
General Technical Report PNW-GTR-742

2

Figure 2—Whitebark pine stand showing damage and mortality from blister rust and mountain pine

beetle. Okanogan and Wenatchee National Forests, Washington. 

extensive and prolonged outbreaks in the future (Logan and Powell 2001). White

pine blister rust is an exotic disease that kills trees slowly; infected trees often persist for years after initially being infected. The cone-bearing branches may be killed

many years before the tree itself is killed (McDonald and Hoff 2001), so regeneration in infected stands may be limited. Fire suppression has led to slow population

declines over the last century by altering the dynamics of stands in fire-prone

regions (Kendall and Keane 2001). In the absence of fire, whitebark pine in these

stands is replaced by more shade-tolerant, fire-intolerant species such as subalpine

fir and Engelmann spruce.

One or more of these agents have caused mortality in stands throughout the

range of whitebark pine. Mortality rates are as high as 90 percent in some stands in

the Cascade Range, while disease infects 100 percent of trees in some stands in the

Rocky Mountains (Kendall and Keane 2001, Ward et al. 2006). This decline has

ramifications for many taxa, most notably Clark’s nutcracker, which has evolved a

mutualistic relationship with whitebark pine and would be directly affected by its

decline (Lanner 1982; Tomback 1978, 1982). Nutcrackers forage on large seeds

produced by whitebark pine in autumn and scatter-hoard up to tens of thousands 

of seeds in subterranean caches to be retrieved later in the year (Tomback 1982).

Teresa Lorenz
Seeds that are not retrieved by nutcrackers are able to germinate (Lanner 1982,

1996; Tomback 1983; Tomback and Linhart 1990). Whitebark pine cones are indehiscent (see glossary for definitions) (Price et al. 1998) and therefore nutcracker

seed dispersal is required by whitebark pine for population-wide regeneration

(Hutchins and Lanner 1982); whitebark pine is an obligate mutualist of Clark’s nutcracker. Ultimately, it is possible that declines in whitebark pine may affect this

mutualism by offsetting a critical balance between pine and nutcracker populations,

resulting in reduced seed dispersal and regeneration (Tomback and Kendall 2001). 

Because of their large size and high nutritive value (Lanner 1996, Lanner and

Gilbert 1994), whitebark pine seeds are consumed by more than 20 species of vertebrates, in addition to the nutcracker. The interactions between all of these vertebrates and whitebark pine contribute to complexities in seed dispersal because

these animals have the potential to influence pine regeneration at the stand level.

Although some of these species play an important role in seed dispersal, their role

in whitebark pine population dynamics has received little study.

This paper provides a review of the current literature relating to seed fate in

whitebark pine. We review the seed harvest, caching, and retrieval behaviors of the

vertebrates that are known to directly influence the fate of whitebark pine seeds.

We then summarize the life history traits of the species that harvest most whitebark

pine seed—Clark’s nutcracker, the red squirrel, and the Douglas’ squirrel (hereafter

collectively referred to as pine squirrels). 

Whitebark Pine Seed Fate

Seed fate has a profound effect on population dynamics in plants (Schupp and 

Fuentes 1995, Vander Wall et al. 2005a, Wang and Smith 2002). Seeds are the 

primary means by which many species of plants, including whitebark pine, move

across landscapes, colonize new areas, and ultimately maintain populations. An

understanding of the fates of individual seeds is therefore important for the conservation and restoration of whitebark pine populations.

The seeds of whitebark pine are larger than the seeds produced by most other

conifers that occur within its range (table 1). Owing, in part, to their large size,

whitebark pine seeds are highly valued as food by many avian and mammalian

granivores (table 2). Seeds are rapidly harvested from trees every autumn (Hutchins

and Lanner 1982, Tomback 1978). To preclude widespread predation of seeds by

such granivores many species of pine, including whitebark pine, have evolved a

strategy called masting in which popu-lations synchronize their reproductive activity. Masting results in variations in proportion of seeds effectively dispersed

(Kelly 1994). In years of high seed pro-duction, granivores are satiated enabling 

Nutcrackers provide

the sole mechanism

of primary seed 

dispersal for whitebark pine, whose

cones are unique in

that they are essentially indehiscent.

3

A Review of the Literature on Seed Fate in Whitebark Pine and the Life History Traits of Clark’s Nutcracker and Pine Squirrels
Table 1—Mean seed weights of North American conifers 

within the range of whitebark pine

Species of conifer Mean seed weight

Pine: Milligrams

digger pine 782.6

foxtail pine 26.8

Great Basin bristlecone pine 10.7

Jeffery pine 122.6

knobcone pine 17.9

limber pine 92.6

lodgepole pine 4.0

ponderosa pine 37.4

Rocky Mountain bristlecone pine 25.1

single-leaf pinyon pine 408.6

sugar pine 216.0

western white pine 16.0

whitebark pine 174.5

Fir:

grand fir 24.7

noble fir 33.6

Pacific silver fir 41.2

red fir 70.9

subalpine fir 13.0

white fir 40.9

Larch:

subalpine larch 3.2

western larch 3.3

Spruce:

Engelmann spruce 3.4

Sitka spruce 2.2

white spruce 2.0

Hemlock:

mountain hemlock 4.0

western hemlock 1.7

Other:

Douglas-fir 11.5

western redcedar 1.1

Source: Conner and Lanner 1991, Schopmeyer 1974.

General Technical Report PNW-GTR-742

4

a proportion of seeds to escape predation. In years of low seed production, most

seeds are predated and populations of granivores are culled. This has important

implications for the number of seeds dispersed within populations and therefore the

adaptive evolution of whitebark pine (Siepielski and Benkman 2007b). Overall, the

number of seeds effectively dispersed differs in each year owing to many factors:

the number of seeds produced within a population of whitebark pine, the size and

mobility of populations of several species of granivore, and the availability of alternate foods for such granivores. 
Table 2—Vertebrates known to forage on whitebark pine seed

Birds:

Cassin’s finch (Carpodacus cassinii)

Clark’s nutcracker (Nucifraga columbiana)

common raven (Corvus corax)

mountain chickadee (Poecile gambeli)

pine grosbeak (Pinicola enucleator)

red-breasted nuthatch (Sitta anadensis)

red crossbill (Loxia curvirostra)

Steller’s jay (Cyanocitta stelleri)

white-breasted nuthatch (Sitta carolinensis)

Williamson’s sapsucker (Sphyrapicus thyroideus)

woodpeckers (multiple species) (Picoides spp.)

Mammals:

black bear (Ursus americanus)

chipmunk (multiple species) (Tamias spp.)

deer mouse (multiple species) (Peromyscus spp.)

Douglas’ squirrel (Tamiasciurus douglasii)

golden-mantled ground squirrel (Spermophilus lateralis)

grizzly bear Ursus arctos

red squirrel Tamiasciurus hudsonicus

vole (multiple species) (Clethrionomys spp.)

Sources: Hutchins 1990, Tomback 1978, Tomback and Kendall 2001.

5

A Review of the Literature on Seed Fate in Whitebark Pine and the Life History Traits of Clark’s Nutcracker and Pine Squirrels

We review the current literature on the attributes of granivores that affect their

effectiveness as either seed dispersers or seed predators for whitebark pine. We

describe three aspects of whitebark pine seed dispersal and predation. The first two

sections review the influence of Clark’s nutcracker and pine squirrels on whitebark

pine primary seed dispersal and seed predation. These two species account for the

majority of seed dispersal and seed predation, respectively (Hutchins and Lanner

1982, Tomback 1978). Information is also provided on the seed harvesting and

caching behaviors of nutcrackers and squirrels in other forest types. Also discussed

is secondary seed dispersal by mice and chipmunks in other species of North

American pine. Although the effectiveness of these rodents as secondary seed 

dispersers in whitebark pine has not been studied, it has been suggested that they

may contribute to seed dispersal (McCaughey and Tomback 2001).

Primary Seed Dispersal by Clark’s Nutcracker

Of all of the species involved with whitebark pine seed fate, Clark’s nutcracker 

is the only species that is known to be required for effective seed dispersal. As a

result of this relationship, whitebark pine has evolved several unique features

among North American pines that facilitate seed harvest by nutcrackers. Whitebark

pine seeds are retained in indehiscent cones. The seeds can only be extracted from
the cones by nutcrackers. The seeds of whitebark pine are also wingless, which

enables nutcrackers to more rapidly harvest and transport seeds. Additionally, the

cones of whitebark pine are borne conspicuously at the ends of upswept branches,

making it relatively easy for birds to locate cones when flying overhead (Lanner

1982) (fig. 3). The focus of this section is the behaviors of nutcrackers that drive

the mutualism between Clark’s nutcracker and whitebark pine. 

Seed harvest— 

Nutcrackers (fig. 4) forage on fresh whitebark pine seeds from July through

October. They forage on unripe seeds prior to the onset of cone maturation, as early

as mid-July in some years (table 3). Unripe seeds are soft and cannot be removed

entirely (Hutchins and Lanner 1982, Tomback 1978), so nutcracker seed harvest

rates are low at this time; they harvest whitebark pine seeds at a rate of 4.5 seeds

per minute in mid-August, compared to 12.3 seeds per minute in mid-September

when cones are ripe (Hutchins and Lanner 1982). Yet in years of poor or moderate

General Technical Report PNW-GTR-742

6

Figure 3—Mature whitebark pine tree showing rounded, upswept crown. Okanogan and Wenatchee

National Forests, Washington. Cones (inset photo) are borne at the ends of the branches. 

Robin Shoal
The central role of

nutcrackers in the

regeneration of

whitebark pine 

populations cannot

be overestimated:

the harvest and 

scatterhoarding

behaviors of Clark’s

nutcracker provide

the only mechanism

of primary seed 

dispersal in whitebark pine.

7

A Review of the Literature on Seed Fate in Whitebark Pine and the Life History Traits of Clark’s Nutcracker and Pine Squirrels

Figure 4—Clark’s nutcracker. 

John and Karen Hollingsworth, U.S. Fish and Wildlife Service

seed production, the majority of the seeds harvested by nutcrackers may be unripe.

In a study of limber pine seed harvest, Lanner and Vander Wall (1980) observed

that in one year of moderate seed production, 91 percent of the cones harvested by

nutcrackers were not fully ripe. 

Whitebark pine seeds mature throughout the month of August. In late August

and September, nutcrackers congregate in loose flocks in whitebark pine stands

where they forage predominately on the ripening seeds (Tomback 1978). Caching

activity begins with the onset of cone maturation, and nutcrackers typically cache

only mature whitebark pine seeds (Tomback 1978). When harvesting and caching

seeds, there may be heavy intra- and interspecific competition for seeds. Under

such circumstances, nutcrackers may store seeds temporarily within a stand. In

locations where pine squirrels were observed actively harvesting whitebark pine

cones, Hutchins and Lanner (1982) observed nutcrackers placing seeds in temporary caches within 100 m of the harvest trees. After most seeds had been harvested

from cones, nutcrackers were observed retrieving these caches and moving them to

cliff faces several kilometers distant. Christensen et al. (1991) observed nutcrackers

discriminately harvesting pinyon pine seeds from trees in stands containing the

most cones, which would increase foraging efficiency and enable them to more

effectively compete with other species for seeds. 
Table 3—Timing of nutcracker harvest and caching in different locations and for different species of pine

Nutcracker Nutcracker Nutcracker Seed Size of

harvest caching caching essentially Location Elevation of Year of Species of cone

starteda starteda completeda maturea of study study site study pine studied cropb Data source

Meters

Late Aug - Mid-Dec Late Aug Arizona 2000–2350 1969 Colorado L Vander Wall

pinyon and Balda

1977

Late Aug - Late Oct Late Aug Arizona 2000–2350 1973 Colorado M Vander Wall

pinyon and Balda

1977

- 20 Sept - - California 2670–3000 1974 Jeffrey M-L Tomback 1977

12 Sept 16 Oct 7 Dec - California 2670–3000 1975 Jeffrey M-L Tomback 1977

3 Aug 18 Sept - Mid-Sept California 2670–3000 1976 Jeffrey M-L Tomback 1977

- 26 Aug Late Nov - Wyoming 2525 1978 Limber - Vander Wall

and Hutchins

1983

Late July Late Aug Late Oct Mid-Sept Utah 2500–2550 1978 Limber M-L Vander Wall

1988

none none none Mid-Sept Utah 2500–2550 1980 Limber S Vander Wall

1988

20 Aug 28 Aug Late Oct Mid-Sept Colorado 1800–3350 1991 Limber - Baud 1993- - Mid-Oct Mid-Sept Colorado 2780–2810 1991 Limber L Torick 1995Early Nov Early Nov - - Colorado 1800–2400 1991 Ponderosa L Baud 1993Early Oct - Mid-Nov Mid-Oct Colorado 2450–2680 1991 Ponderosa L Torick 1995

Rocky Mt

Mid-Oct - - - Colorado - 1991 bristlecone - Baud 1993

Late July 27 Aug Late Nov - Wyoming 1825–2450 1978 Singleleaf - Vander Wall

pinyon and Hutchins

1983

General Technical Report PNW-GTR-742 8
Ta

ble

3

—

Timin

g

of n

utcra

c

k

er h

arv

e

st a

n

d

c

a

c

hin

g in

differe

nt lo

c

atio

n

s

a

n

d for differe

nt s

p

e

cie

s

of pin

e

N

utcracker N

utcracker N

utcracker Seed

Size of

harvest caching caching essentially

Location

Elevation of Year of S

pecies of cone

starteda starteda com

pleteda maturea of stu

d

y stu

d

y site stu

d

y

pine stu

died cropb Data source

Meters

Late July 2 Sept Late Nov - Wyomin

g 1825–2450 1980 Sin

gleleaf - Van

der Wall

pin

yon an

d

H

utchins

1983

Late July 28 A

u

g

Late Oct Late Sept Utah 2000–2200 1978 Sin

gleleaf M-L

Van

der Wall

pin

yon 1988

Early

A

u

g 31 A

u

g - Late Sept Utah 2000–2200 1980 Sin

gleleaf L

Van

der Wall

pin

yon 1988

19 July 29 A

u

g - - California 2670–3000 1973 W

hitebark

M-L

Tom

back 1977

2 A

u

g 25 A

u

g 11 Oct - California 2670–3000 1974 W

hitebark

M-L

Tom

back 1977

1 A

u

g 8 Sept 16 Oct Early Sept California 2670–3000 1975 W

hitebark

M-L

Tom

back 1977

28 July - 10 Oct - California 2670–3000 1976 W

hitebark

M-L

Tom

back 1977

13 July

Mid-A

u

g

Late Oct Late A

u

g

Wyomin

g 2805–3140 1979 W

hitebark - H

utchins an

d

Lan

ner 1982

23 July

Mid-A

u

g

Late Oct Late A

u

g

Wyomin

g 2805–3140 1980 W

hitebark

L

H

utchins an

d

Lan

ner 1982

4 A

u

g

Mid-A

u

g

Late Oct Late A

u

g

Wyomin

g 2805–3140 1980 W

hitebark

L

H

utchins an

d

Lan

ner 1982

Early Sept 31 A

u

g

Late Oct Early Sept California 3131-3978 1990 W

hitebark

L

Dim

mick 1993

Early

A

u

g 12 Sept Late Oct Early Sept California 3131-3978 1991 W

hitebark

M-L

Dim

mick 1993

Dashes indicate that source does not provide this information.a Dates are approximate.b L

= large, M

=

moderate, S = small.

9

A Review of the Literature on Seed Fate in Whitebark Pine and the Life History Traits of Clark’s Nutcracker and Pine Squirrels
Table 4—Number of seeds stored by Clark’s nutcracker per cache and cache

depth for different species of pine

Species of Range of Mean Cache 

pine cache size cache size depth Source

Centimeters

Jeffrey pine 1–9 4.21a - Tomback 1977, 1978

(n=42)

limber pine 5–9 7.0 - Vander Wall and Balda 

(n=2) 1977

limber pine 2–5 3.5 - Baud 1993

(n=11)

ponderosa pine 31 - - Torick 1995

(n=1)

whitebark pine 1–15 4.21a - Tomback 1977, 1978 

(n=172)

whitebark pine 1–14 3.2 - Hutchins and Lanner 1982

(n=157)

whitebark pine 1–15 3.7 2.0 Tomback 1982

(n=54) (n=30)

whitebark pine 1–14 3.4 2.5 Vander Wall and Hutchins 

(n=155) (n=14) 1983

whitebark pine 1–24 2–4.2b 3.1 Dimmick 1993

(n=890) (n=16)

a Cache size of Jeffrey and whitebark pine were lumped in these studies. 

b Mean differed between years and between nutcracker age classes.

General Technical Report PNW-GTR-742

10

Seed storage—

Nutcrackers store seeds in scattered caches. Caches are placed in a variety of

above- and belowground locations. Seeds are commonly placed in subterranean

caches, 1 to 5 cm deep (table 4). Caches are made either by side-swiping the bill 

to create a slight depression or by directly probing into the ground with the bill

(Tomback 1998). Caches are always concealed, either by the bird brushing dirt

over the seeds or by placing pebbles, twigs, or other objects on top of the cached

seeds (Dimmick 1993, Tomback 1978). When a cache is complete, there is no sign

of a disturbance (Tomback 1978, Vander Wall and Balda 1977). Nutcrackers have

been observed caching the seeds of many different species of conifer, ranging from

the large seeds produced by pinyon pines to seeds as small as those produced by

Douglas-fir (table 3). Cache size differs and may be correlated with the size of the

seed cached (Vander Wall and Balda 1981). Caches of whitebark pine seeds on

average contain 1 to 5 seeds compared to ponderosa pine seed caches, which may

contain as many as 31 seeds (table 4). 
Table 5—Aspect and elevation of communal caching 

grounds of Clark’s nutcracker

Aspect Elevation Source

Meters

South 2525–2750 Vander Wall and Balda 1977

South 2450 Vander Wall and Balda 1977

Southeast 2805–2880 Tomback 1978

Southwest 2265–2325 Tomback 1978

West-southwest 2550–2670 Tomback 1978

South 2805–2985 Hutchins and Lanner 1982

West-northwest 3100–3500 Dimmick 1993

11

A Review of the Literature on Seed Fate in Whitebark Pine and the Life History Traits of Clark’s Nutcracker and Pine Squirrels

Table 6—Distances and elevations between seed harvest and cache locations 

of Clark’s nutcracker

Elevation

Species Distancea change Source

Meters

Colorado pinyon pine 7.5–22 km 530–600 Vander Wall and Balda 1977

Limber pine 4–5 km - Vander Wall and Balda 1977

Limber pine 100 m–1 km - Vander Wall 1988

Singleleaf pinyon pine 1 m–5 km - Vander Wall 1988

Whitebark pine 2.5–12.5 km 500b Tomback 1978

Whitebark pine 100 m–3.5 km - Hutchins and Lanner 1982

Whitebark pine 1 m–3 km - Dimmick 1993

a Numbers are not representative of the mean distance between harvest and cache locations because

most sources give the maximum distance and elevation traveled rather than the range of distances.

b Represents a negative change (decrease) in elevation.

Flocks of up to 150 nutcrackers may use the same area for caching (Hutchins

and Lanner 1982). Many of such communal caching areas (Vander Wall and Balda

1977) have been found on steep, south-facing slopes (table 5), which may demonstrate a preference for sites where caches would be available earlier in the season

because of rapid snowmelt. In such communal areas, nutcrackers cache seeds independently and without aggression (Hutchins and Lanner 1982, Vander Wall and

Balda 1977). 

Nutcrackers commonly cache seeds within a few hundred meters of the trees

from which they were harvested (Dimmick 1993). However, they may transport

seeds to distant and higher or lower elevation cache sites (table 6). Nutcrackers

have been known to transport pinyon pine seeds up to 22 km in distance and 600 m

in elevation between seed sources and caching grounds (Vander Wall and Balda

1977, 1981). 
Nutcrackers bury seeds in a variety of locations, including but not limited to

meadows, closed-canopy forests, puddles, and dry slopes (Hutchins and Lanner

1982, Tomback 1978, Torick 1995). Nutcrackers also place caches aboveground in

crevices in trees and rocks (Dimmick 1993, Tomback 1978). Observational studies

have noted that nutcrackers in the Sierra Nevada Mountains in California appear to

place most of their whitebark pine seed caches at the bases of large trees and rocks

and in open pumice (Dimmick 1993, Tomback 1978). Nutcrackers also use a variety of substrates for burying caches. In the most comprehensive study of nutcracker

caching to date, needle litter and gravel were the most commonly used substrates

for whitebark pine seed storage (Dimmick 1993). There has been no research published to support the widely cited perception that Clark’s nutcrackers preferentially

cache in recent burns or clearings.

Many of the caching behaviors of nutcrackers are advantageous for population-wide regeneration in whitebark pine. Because of their wide-ranging caching

behavior, nutcrackers give whitebark pine a pioneering advantage over trees with

Many of the caching

behaviors of nutcrackers are advantageous for populationwide regeneration in

whitebark pine.

General Technical Report PNW-GTR-742

12

Figure 5—A clump of whitebark pine seedlings resulting from a multiseed cache.

Okanogan and Wenatchee National Forests, Washington. 

Teresa Lorenz
13

A Review of the Literature on Seed Fate in Whitebark Pine and the Life History Traits of Clark’s Nutcracker and Pine Squirrels

wind-dispersed seeds. The number of seeds placed by nutcrackers in one cache

often results in a multigenet growth form because whitebark pine seedlings tolerate

crowding (Furnier et al. 1987, Linhart and Tomback 1985) (fig. 5). Additionally,

the depths of nutcracker caches overlap the range of depths from which seedlings

can successfully germinate (Tomback 1982).

Conversely, the sites selected by nutcrackers for caches are not always optimal

for seedling survival. Nutcrackers most commonly place caches in needle litter or

in microsites that are unshaded (Dimmick 1993). Whitebark pine seedlings are susceptible to death by heat stress and desiccation in their first summer, and seedling

survivorship in whitebark pine is significantly lower for seeds growing in needle

litter and full sun (McCaughey 1990). Nutcrackers also commonly place caches in

cliffs and rocky slopes where vegetation is minimal (Dimmick 1993, Hutchins and

Lanner 1982, Tomback 1978). Because rodent cache theft is likely to be lower in

such places, nutcrackers may be selecting these habitats for their caches to minimize cache theft by rodents (Hutchins and Lanner 1982). However, whitebark pine

seed caches placed by nutcrackers on cliffs or rocky areas likely face relatively

high mortality rates as seedlings and may be unlikely to mature into cone-bearing

adults. 

Cache retrieval—

Many granivores, especially granivorous rodents, use their sense of smell to locate

buried seeds. Nutcrackers have a poor sense of smell, however, and retrieve their

buried caches by memorizing the exact locations in which they were placed (Balda

and Kamil 1992, Kamil and Balda 1985, Vander Wall 1982). In captivity, nutcrackers retain memory of their caches with high accuracy for 180 days and with diminishing accuracy for nearly 300 days (Balda and Kamil 1992). 

While the abilities of nutcrackers to remember the locations of their caches

have been well studied in laboratory settings, cache recovery in the wild has not

been studied in as much depth. Cache recovery of pine seed has been observed

mostly in early spring through summer (Tomback 1977, 1978; Vander Wall and

Hutchins 1983). Nutcrackers in the Sierra Nevada have been observed recovering

seeds from ground caches in February and March at mid-elevation sites (2100 m)

and in late May and early June at high-elevation sites (2650 to 3150 m) (Tomback

1978). In Wyoming, nutcrackers have been recorded recovering seeds in June and

July (Vander Wall and Hutchins 1983). 

Nutcrackers recover their own caches by flying directly to cache sites and

immediately removing seeds (Vander Wall 1982). Nutcrackers also pilfer the

caches of other individuals by engaging in extensive or prolonged searches of the
ground surface (Vander Wall 1982, Vander Wall and Hutchins 1983). Nutcrackers

increase their chances of locating such seed stores by searching for germinating

seedlings (Vander Wall and Hutchins 1983; Lorenz, unpublished data) or by concentrating search effort near landmarks where nutcrackers in general are more 

likely to concentrate caches (Vander Wall 1982). There are no data on the proportion of seeds acquired by individual nutcrackers from personal caches as opposed

to pilfered caches.

It is widely known that nutcrackers are pine seed specialists who consume

pine seed throughout the year (Bent 1946, Bradbury 1917, Giuntoli and Mewaldt

1978, Mewaldt 1956, Tomback 1977, Vander Wall and Hutchins 1983). However

there is no information on the degree to which nutcrackers rely specifically on

cached seed because nutcrackers have been observed foraging on seeds retained in

cones throughout the year. The cones of large-seeded pines, such as whitebark and

pinyon pine, provide seeds from July though mid-autumn (Lanner and Vander Wall

1980, Tomback 1978, Vander Wall and Balda 1977). After this time, nutcrackers

may forage on seeds of wind-dispersed conifers at lower elevations through June 

of the following year (Tomback 1977, 1978; Lorenz, unpublished data). Despite

current gaps in the literature, there is likely considerable variation in an individual’s

reliance on seed caches among years depending on factors such as the availability

of alternate foods, winter severity, and breeding status. 

Seed Predation by Pine Squirrels

Research on seed dispersal in large-seeded pines has focused on the harvesting and

caching behaviors of birds. Benkman (1995) argued that such a focus is unwarranted and regardless of the mutualistic associations between birds and pines, predatory

interactions strongly influence all aspects of plant demographics (Crawley 2000,

Fleming and Estrada 1993). A shift in focus to encompass all aspects of whitebark

pine seed fate is thus necessary for a comprehensive understanding of regeneration

in whitebark pine. 

Major invertebrate predispersal seed predators include cone worms and cone

beetles (Bartos and Gibson 1990). Predispersal predators of whitebark pine seed

also include the avian and mammalian granivores that forage on cones and collect

fallen seed from the ground. Postdispersal predators include granivores such as

Clark’s nutcracker and the pine squirrel, and less specialized omnivores such as

bears that consume seeds opportunistically. 

Pine squirrels (red and Douglas’ squirrels) are the most effective predispersal

seed predators of whitebark pine because of their methods of seed harvest

General Technical Report PNW-GTR-742

14
Pine squirrels 

harvest whitebark

pine cones up to 6

weeks before seeds

are ripe and several

weeks before 

nutcrackers begin

harvesting seeds.

15

A Review of the Literature on Seed Fate in Whitebark Pine and the Life History Traits of Clark’s Nutcracker and Pine Squirrels

(Hutchins and Lanner 1982), and nearly all of the seeds harvested by pine squirrels

are consumed or unable to germinate (Hutchins and Lanner 1982, McCaughey and

Tomback 2001). Thus predation of whitebark pine cones by pine squirrels affects

the dispersal probabilities of whitebark pine seed. In regions facing strong selective

pressure from pine squirrels, nutcrackers disperse fewer seeds compared to regions

where pine squirrels are absent (Siepielski and Benkman 2007a). This is because

pine squirrel predation in stands results in trees with fewer seeds per cone and an

increase in cone defenses such as an increase in cone scale thickness and a

decrease in the number of seeds per cone (Siepielski and Benkman 2007a).

Consequently, pine squirrels are able to exert selection pressures on whitebark pine

that are arguably as strong as the selective pressures exerted by whitebark pine’s

mutualistic partner, Clark’s nutcracker. The influence of pine squirrels on seed fate

in whitebark pine is considerable and should not be underestimated. However,

because there is limited information on the influence of pine squirrels on whitebark

pine seed fate, most of the information presented in this section is from studies of

their harvest, storage, and retrieval behaviors in other forest types. 

Seed harvest— 

Red squirrels and Douglas’ squirrels forage extensively on conifer seed, and their

life history traits revolve around seed availability. Although whitebark pine seeds

are valued by pine squirrels because of their large size (Hutchins and Lanner 1982,

Smith 1968b, Tomback and Linhart 1990,), the range of whitebark pine overlaps

only a small portion of the range of the pine squirrel. Consequently, squirrels in

most parts of their range forage on the smaller seeds of wind-dispersed conifers

(Shaw 1936; Smith 1968b, 1970; Tevis 1953). 

Pine squirrels harvest whitebark pine cones up to 6 weeks before seeds are

ripe and several weeks before nutcrackers begin harvesting seeds (Hutchins and

Lanner 1982). Hutchins (1982, 1994) observed that squirrels harvest whitebark

pine cones more intensively before seeds have ripened. Seeds in cut unripe cones

are not fully developed, and consequently their predation is a reproductive loss for

the tree (McCaughey and Tomback 2001). 

Pine squirrels generally inhabit stands in which there are multiple species of

conifer. This is because different species of conifer generally produce mast crops of

cones in different years, making it more likely that squirrels will have a consistent

supply of food year to year. Mixed stands of whitebark pine, spruce, and fir are

good habitat for pine squirrels because they provide a more reliable food source

and protective shelter (Reinhart and Mattson 1990). Pine squirrels are often uncommon in or absent from pure whitebark pine stands because cone production is not
reliable between years (Mattson and Reinhart 1997, Reinhart and Mattson 1990).

Overall, whitebark pine trees in mixed stands are likely to have more cones harvested by squirrels than whitebark pine trees in pure stands or isolated in meadows

(Hutchins 1982). 

The amount of seeds harvested by squirrels differs and is dependent on stand

characteristics. Benkman et al. (1984) found that in mixed limber pine stands, as

much as 89 percent of the cone crop was harvested by squirrels, with as little as 1

percent remaining for dispersal by Clark’s nutcrackers. In nearby pure stands, most

cones remained on the tree and available for nutcrackers. Similarly, Hutchins

(1982) found that squirrels harvested nearly 64 percent of the available whitebark

pine seed in forests of whitebark pine, subalpine fir, and Engelmann spruce, but

harvested no cones from isolated whitebark pines in meadows. 

Seed storage— 

Because the availability of conifer seed differs yearly, squirrels store food to survive periods of food shortage. When harvesting for storage purposes, squirrels 

rapidly cut and drop large numbers of cones; they have been observed cutting 

the small cones of giant sequoia trees at a rate of 500 cones in 30 minutes

(Shellhammer 1966). Squirrels gather such cut cones within days of harvest and

store them for later use. 

Squirrels usually cache whole conifer cones. However, in some cases they

extract the seeds from the cones of whitebark and limber pines before storage

(Benkman et al. 1984, Hutchins and Lanner 1982). Unlike the scatter-hoarding

behaviors of nutcrackers, pine squirrels typically larder-hoard, storing all of their

seeds in a few locations within their territory (Finley 1969, Hurly and Lourie

1997). It is common for cones and seeds to be buried within middens, which are

piles of loose cone scales that have accumulated beneath favored feeding perches

(Finley 1969, Vahle and Patton 1983) (fig. 6). To be effective for cone storage,

middens must have a cool and moist interior that prevents cones from opening

(Finley 1969, Shaw 1936, Smith and Mannan 1994). Where moisture is limited,

squirrels select middens for cone storage that are on north-facing slopes or heavily

shaded by the canopy. Middens on south-facing slopes or in the open are not typically used for cone storage (Finley 1969, Smith and Mannan 1994). 

The number of conifer cones stored by squirrels in middens differs depending

on the presence of potential competitors and food availability (Finley 1969, Gurnell

1984). Middens may contain enough food to last 1 year (Ferron et al. 1986, Gurnell

1984, Smith 1968b). Middens as large as 85 m2 in diameter and 0.4 m deep have

been found. Such middens often reflect the work of many generations of squirrels

(Finley 1969, Rusch and Reeder 1978). Estimates of 16,000 spruce cones, 3,000

General Technical Report PNW-GTR-742

16
Estimates of 16,000

spruce cones, 3,000

whitebark pine

cones, and 8,800

lodgepole pine cones

in individual middens

exemplify the industriousness of pine

squirrels.

17

A Review of the Literature on Seed Fate in Whitebark Pine and the Life History Traits of Clark’s Nutcracker and Pine Squirrels

Figure 6—A red squirrel pausing near its midden. Coronado National Forest, Arizona. 

Anne L. Casey, U.S. Forest Service

whitebark pine cones, and 8,800 lodgepole pine cones in some middens exemplify

the industriousness of pine squirrels (Hurly and Lourie 1997, Schmidt and

McCaughey 1990, Smith 1968b). 

Squirrels without access to suitable middens may store cones under piles 

of brush or logs or within tree cavities (Finley 1969, Gurnell 1984, Hatt 1943,

Shellhammer 1966). Pine squirrels also scatter-hoard cones in subterranean caches

(1 to 30 cones) (Carey 1991, 1995; Hurly and Lourie 1997). Hurly and Lourie

(1997) found that pine squirrels in Alberta cached lodgepole pine cones both in

large, central middens (1,000 to 8,000 cones) and in numerous scatter-hoards (1 to

5 cones). Approximately 40 percent of the cones stored by squirrels in this population were scatter-hoarded. The scatter-hoarding behavior of pine squirrels may be

underreported because scatter-hoarded cones are less visible than cones stored in

larders.

For multiple reasons, seeds stored by pine squirrels are unlikely to establish 

as seedlings. Seeds are usually not extracted from cones before being stored, which

precludes germination. When caching individual seeds, squirrels larder-hoard

whitebark pine seeds in caches between 6.5 and 40 cm deep and ranging from 14 

to 55 seeds (Hutchins and Lanner 1982). This inhibits regeneration by preventing

germinants from being able to grow upwards enough to penetrate the surface of 

the midden and by causing overcrowding. The squirrels’ tendency to place caches
The conventional

view of seed dispersal as a short-term,

discrete event is

being replaced by 

an understanding of

the complex interactions between 

animals and plants

that occur for an

extended period after

a seed arrives on a

substrate.

General Technical Report PNW-GTR-742

18

at the bases of large, live trees and under the canopy of forests inhibits seedling

growth. Moreover, as squirrels cache and retrieve cones from their larders, materials are continually turned over, making establishment impossible for most seeds

(Gurnell 1984, Hatt 1943). Hutchins and Lanner (1982) found significantly fewer

surviving whitebark pine seedlings on middens compared to randomly selected 

portions of the forest floor. 

Cache retrieval— 

Pine squirrels recover stored seed mainly in winter when fresh seed is unavailable

(Smith 1968b). Squirrels will retrieve seeds up to several years after they have been

cached (Smith 1968b). The amount of seed recovered from caches in a given year

depends on a number of factors, especially the size of the previous year’s cone crop

and the amounts and availability of alternate food sources. Consecutive years of

cone crop failure may result in the consumption of all cached seeds (Smith 1968b).

Secondary Seed Dispersal by Scatter-Hoarding Rodents

For many systems with vertebrate-mediated seed dispersal, research has historically

focused on the relationship between the plant and the animal most directly involved

with the transport of the seed away from the parent plant. However, in many systems, the travels of seeds do not always end with this first phase of dispersal

(Chambers and MacMahon 1994, Vander Wall et al. 2005b). Moreover, the secondary phases of dispersal are often as influential to plant population dynamics as the

first phase (Chambers and MacMahon 1994). It is now apparent that many plantdisperser interactions involve multiple stages of varying degrees of effectiveness,

occurring in two or more phases. The conventional view of seed dispersal as a

short-term, discrete event is being replaced by an understanding of the complex

interactions between animals and plants that occur for an extended period after a

seed arrives on a substrate.

Seed dispersal in whitebark pine has not been studied in detail, but it has traditionally been considered a single-phase event in which nutcrackers disperse seeds

from a parent plant to favorable sites. In other species of western pine with both

wind- and bird-dispersed seeds, recent work has changed traditional perceptions of

seed dispersal. For example, the seeds of ponderosa and Jeffery pines were traditionally considered to be dispersed by the wind. The seeds of Colorado pinyon and

single-leaf pinyon pine have been described as dispersed entirely by birds. Studies

tracking the movements of such seeds over time have revealed that granivorous

rodents are involved, and at times required, for effective seed dispersal in these

species. 
Many species of rodents store seeds in small (one to three seeds), shallow (5

to 20 mm deep) subterranean caches. The term “scatter-hoarding rodents” is used

widely as a term for this guild. Some examples of scatter-hoarding rodents that participate in the harvest of pine seeds include chipmunks, ground squirrels, kangaroo

rats, pocket mice, and deer mice. Their scatter-hoarding behavior enables population regeneration in some species of North American pines (Vander Wall 1992b).

Whitebark pine is different from other North American pines because it is the

only species with indehiscent cones. Effective seed dispersal requires Clark’s nutcracker and this has been well documented (Hutchins and Lanner 1982, Tomback

1978, Tomback and Linhart 1980). However, scatter-hoarding rodents demonstratedly affect seed fate in whitebark pine (Hutchins and Lanner 1982, McCaughey

and Weaver 1990, Tomback 1980). The outcome of their interactions with whitebark pine seed and their overall role in whitebark pine seed fate has not been 

studied (McCaughey and Tomback 2001).

This section provides a review of the current literature on whitebark pine seed

harvest by scatter-hoarding rodents and their seed storage and cache retrieval

behaviors with other pines. Seed storage and cache retrieval by such rodents has

been studied in Jeffery pine, sugar pine, and pinyon pines in western North

America. Although storage and retrieval behaviors of these rodents have not been

formally studied in whitebark pine, information on their behaviors in other systems

may contribute to an understanding of the complexities of whitebark pine seed fate.

Seed harvest— 

Chipmunks and deer mice have weak jaw muscles compared to pine squirrels, and

they therefore are not as efficient in harvesting seeds compared to nutcrackers and

pine squirrels (Hutchins and Lanner 1982, Tomback 1978). A relatively small proportion of the whitebark pine cones are cut down by these scatter-hoarding rodents

(Hutchins and Lanner 1982) (fig. 7). 

Scatter-hoarding rodents more commonly forage on and collect seed that has

fallen on open ground. In experimental settings, whitebark pine seed that was surface sown in stands was removed by rodents within 24 hours (Hutchins and Lanner

1982, McCaughey and Weaver 1990). It is important to note, however, that relatively little whitebark pine seed naturally drops to the ground (Hutchins and Lanner

1982). Scatter-hoarding rodents likely exert more influence on whitebark pine seed

dispersal by pilfering seeds from nutcracker and pine squirrel caches. Over the

course of 9 months, theft of simulated nutcracker caches ranged from 57 percent in

subalpine, Engelmann spruce forests, to 90 percent in whitebark pine stands (Baud

19

A Review of the Literature on Seed Fate in Whitebark Pine and the Life History Traits of Clark’s Nutcracker and Pine Squirrels
1993, Hutchins and Lanner 1982). Rodent theft of seeds from pine squirrel larders

has not been studied.

Seed storage—

In North America, scatter-hoarding rodents contribute to population-wide regeneration in several species of pine. Their seed storage behaviors have been studied most

thoroughly in Jeffery, sugar, and pinyon pine. Jeffery and sugar pines have seeds

with wings. Although primary dispersal is enabled by the wind, recent studies have

shown that secondary dispersal by rodents may be required for germination and

seedling establishment (Vander Wall 1992b, Vander Wall et al. 2005b). Pinyon pine

seeds are wingless and are dispersed by birds. The first phase of seed dispersal in

pinyon pines is enabled by Clark’s nutcracker and the pinyon jay. Rodents are

involved in secondary seed dispersal in pinyon pines both by collecting seeds

dropped during seed harvest by birds, and by stealing cached seeds (of birds and

other rodents) that they are able to smell buried under the soil surface. 

A comparative study of the effectiveness of six species of rodent as dispersers

for pinyon pines found that seeds harvested by deer mice and least chipmunks were

General Technical Report PNW-GTR-742

20

Figure 7—A chipmunk harvesting seeds from a whitebark pine cone. Okanogan and Wenatchee

National Forests, Washington.

Robin Shoal
10 times more likely to germinate compared to seeds dispersed by kangaroo rats

and pocket mice (Hollander and Vander Wall 2004). This is because most of the

pine seeds obtained by deer mice and chipmunks are immediately cached in shallow, subterranean caches (Hollander and Vander Wall 2004; Vander Wall 1993,

2003; Vander Wall and Joyner 1998). Seeds are buried mostly in caches of one to

four seeds and are transported between 1 and 69 m from the seed source (Vander

Wall 1992b, 1993). Cache depth of scatter-hoarded seeds ranges from just below

the soil surface to 8.0 cm, and most caches are buried approximately 5 to 15 mm

deep (Thayer and Vander Wall 2005; Vander Wall 1992b, 1993, 1997). 

Microsites chosen by scatter-hoarding rodents for caching are different than

the microsites chosen by avian dispersers. In studies of seed dispersal in sugar and

pinyon pines, rodents placed the majority of their caches near or under shrubs

(Thayer and Vander Wall 2005, Vander Wall and Joyner 1998). Rodents are subject

to predation by aerial foraging raptors, so rodents may be more likely to cache

under the protection of shrubs to reduce predation risk (Thayer and Vander Wall

2005). In a comparative study of caching behaviors in Steller’s jays and yellow

pine chipmunks on sugar pine seeds, 100 percent of rodent caches were associated

with potential nurse shrubs, compared to 42 percent of jay caches (Thayer and

Vander Wall 2005). Rodents also most commonly place caches in mineral soil,

whereas Steller’s jays and Clark’s nutcrackers most commonly place caches in needle litter (Dimmick 1993, Thayer and Vander Wall 2005, Vander Wall 1993, Vander

Wall and Joyner 1998). Seedlings of many species of pine, including whitebark

pine, show increased survivorship when grown in mineral soil and protected from

full sun (McCaughey and Weaver 1990; Vander Wall 1992a, 1997). 

Cache retrieval— 

Chipmunks and deer mice use both spatial memory and scent to locate buried

seeds. They may use their spatial memory to locate their own caches, but they rely

on scent to raid the caches of other animals (Vander Wall 1991, 2000). The ability

of chipmunks and deer mice to successfully locate a cache by scent is affected by

soil type and moisture (Briggs and Vander Wall 2004; Vander Wall 1998, 2000). 

Few of the seeds that chipmunks and deer mice encounter and steal are immediately consumed. Vander Wall and Joyner (1998) found that 75 percent of Jeffrey

pine seed caches that are pilfered by chipmunks are recached immediately in new

sites. Individuals aid their own fitness by stealing and recaching seeds previously

cached by neighbors (Vander Wall 1995a). Individuals also monitor their own seed

stores for evidence of pilferage and recache their own stores multiple times. Caches

of larger seeds contain fewer seeds, and are placed farther apart than the caches of

21

A Review of the Literature on Seed Fate in Whitebark Pine and the Life History Traits of Clark’s Nutcracker and Pine Squirrels
smaller seeds. Both of these strategies reduce the likelihood of cache theft, suggesting that rodents give preferential treatment to large seeds (Vander Wall 1995b,

2003). 

As seeds are monitored, pilfered, and repilfered by different individuals, they

may be moved more than four times before germinating (Vander Wall and Joyner

1998). In a study of seed dispersal in Jeffery pine, Vander Wall and Joyner (1998)

found that only 1 percent of seeds germinating from chipmunk caches had

remained in their original cache site through the winter. 

The proportion of seeds that successfully germinate and establish from the

caches of scatter-hoarding rodents and birds differs between years depending on

factors such as population size, the availability of alternate foods, and the number

of seeds produced. Actual germination rates from caches made by birds are not 

currently known because birds may cache over many square kilometers and these

seeds are not easily tracked. Conversely, multiple studies have examined germination rates from seeds scatter-hoarded by rodents; rodent caches are easier to track

because rodents cache within 100 m of the seed source. Of Jeffrey pine and bitterbrush seeds experimentally presented to and cached by chipmunks in two studies,

approximately 14 percent of seeds scatter-hoarded and 69 percent of caches made

by scatter-hoarding rodents survived to germination (Vander Wall 1995a, Vander

Wall and Joyner 1998). 

Life History Traits

Clark’s nutcracker and the pine squirrel have the most direct influence on the fate 

of whitebark pine seed. This section reviews key aspects of the evolutionary biology, annual cycle, and natural histories of Clark’s nutcracker and pine squirrel. 

Clark’s Nutcracker

Clark’s nutcracker is a conifer-seed specialist (Lanner 1996, Tomback 1983, 

Tomback and Linhart 1990). On a landscape scale, conifers do not produce the

same amounts of seed every year. Years of heavy seed production, or ‘’mastyears,’’ are often followed by 1 to 3 years of low or moderate production. Nutcrackers therefore must be highly opportunistic and adaptable in order to survive

years of low seed production. Many aspects of the nutcracker’s life history, such as

their varied diet and their yearly movements, reflect this opportunistic nature. Other

aspects, including morphology and the timing of the breeding season, reflect their

dependence on conifer seed. To understand the life history of Clark’s nutcrackers, 

it is important to consider both their adaptability and their dependence on conifer

seed. 

To understand the

life history of Clark’s

nutcrackers, it is

important to consider

both their conifer

seed dependence

and their adaptability.

General Technical Report PNW-GTR-742

22
Morphology—

Clark’s nutcracker is a gray bird with contrasting black-and-white markings on its

wings and tail (fig. 4). It is sexually monomorphic in plumage, but males are heavier (mean mass 137 g) than females (mean mass 123 g) (Mewaldt 1948), and males

have longer tails and wings and a longer, wider bill (Mewaldt 1958). Adult nutcrackers have varying amounts of white plumage on their faces—some individuals

have white extending in a broad patch from the bill to the eye while others have

only a small patch of white near the bill. Juveniles in their first summer have a gray

face and can reliably be distinguished from adults by the color of the mouth; juveniles have a pink mouth, and adults have a black mouth (Mewaldt 1958) (fig. 8).

Juveniles molt their plumage during the postjuvenile molt, which extends from July

to January in their first year. After this time, hatch-year birds are distinguishable

from adults only by faded, brown-tinged wing and tail feathers (which are glossy

black in adults). This usually requires scrutiny of the bird in hand. By midsummer

of the following year, adults and second-year birds are indistinguishable even in 

the hand (Mewaldt 1958). 

Clark’s nutcracker is unusual among passerines in having an extended molt.

Most songbirds initiate molt after their breeding season, and the molt is completed

within weeks. Nutcrackers begin the annual molt of all feathers in late winter. Molt

lasts for up to 9 months, extending from the start of the breeding season throughout

23

A Review of the Literature on Seed Fate in Whitebark Pine and the Life History Traits of Clark’s Nutcracker and Pine Squirrels

Figure 8—A Clark’s nutcracker in its first year, showing the pink mouth lining, which contrasts

with a black mouth lining in adults. Deschutes National Forest, Oregon.

Dave Herr, U.S. Forest Service
the postfledging period and well into autumn (Mewaldt 1958). The extended molt

enables nutcrackers to be active during all times of the year, but especially in the

fall, while harvesting conifer seeds, at a time when many passerines are molting

and have reduced flight capabilities.

Clark’s nutcrackers have several distinctive morphological adaptations for

their conifer seed diet. Their bill is long, decurved, and sharply pointed, which

enables them to chisel into cones and extract seeds from narrow cracks in cone

scales (Vander Wall and Balda 1981). Nutcrackers have evolved strong flight capabilities relative to sympatric species, which aids them when searching for conebearing trees and when transporting seeds (Vander Wall and Balda 1981). They are

capable of transporting more than one seed at a time because of a unique, expandable pouch below the tongue (Bock et al. 1973). This sublingual pouch has an

approximate volume of 28 ml (Vander Wall and Balda 1981), and an individual

nutcracker can carry up to 20 percent of its body weight in seeds. A nutcracker is

therefore capable of transporting approximately 150 whitebark pine seeds at a time,

although it is more common for individuals to carry 50 to 70 seeds (Tomback

1977) (table 7). 

The nutcracker’s brain is also morphologically specialized for its reliance on

conifer seed. Within the lower section of bird and mammal brains is a complex 

that controls certain memory-related tasks. This portion of the brain, called the hippocampal complex, controls spatial memory in birds. The hippocampus of Clark’s

nutcracker is larger than those of noncaching birds of similar size (Basil et al.

1996). This enlarged hippocampus enables nutcrackers to remember the locations

of tens of thousands of individually buried seeds for nearly a year (Balda and

Kamil 1992). Nutcrackers remember the locations of caches by memorizing the

positions of objects surrounding their caches; the positioning of landmarks, rather

than the distances to them, is the tool used to locate hidden caches (Kamil and

Cheng 2001, Vander Wall 1982). There is also evidence that nutcrackers are able to

A nutcracker is capable of transporting

approximately 150

whitebark pine seeds

at a time.

General Technical Report PNW-GTR-742

24

Table 7—Estimates of the number of seeds of different species of pine 

carried in the sublingual pouch of Clark’s nutcracker

Mean number of

Species of conifer seeds in 

seeds carried sublingual pouch Source

Colorado pinyon pine 55 (n=5) Vander Wall and Balda 1977

Ponderosa pine 34 (n=1) Grinnell et al. 1930

Singleleaf pinyon pine 72 (n=1) Grinnell and Storer 1924

Whitebark pine 65 (n=1) Grinnell and Storer 1924

Whitebark pine 77 (n=13) Tomback 1978
remember not only the locations but also the sizes of the seeds that they place in

each cache (Möller et al. 2001). 

Distribution— 

The range of Clark’s nutcracker is restricted to west-central North America (fig. 9).

Its breeding range extends northward to central British Columbia, east to the front

range of the Rocky Mountains, westward to the Coast Ranges, and south nearly to

the border of the United States and Mexico (Tomback 1998). Many populations of

nutcrackers are relatively isolated. Populations inhabit and likely regularly breed in

isolated high-elevation forests in the Great Basin of Nevada; on Cerro Potosi in

Nuevo Leon, Mexico; in the Black Hills of South Dakota; and in the Olympic

Mountains of Washington (Peterson 1988, Smith et al. 1997, Tomback 1998). In

years of low conifer seed production, nutcrackers have been observed far outside

their typical range. They are reported irregularly in Alaska and eastern parts of

North America, and these extralimital movements have been attributed to widespread cone crop failures (Davis and Williams 1957, Fisher and Myres 1979,

Vander Wall et al. 1981).

Nutcrackers remember the locations of

caches by memorizing the positions of

objects surrounding

their caches.

25

A Review of the Literature on Seed Fate in Whitebark Pine and the Life History Traits of Clark’s Nutcracker and Pine Squirrels

Figure 9—Range of Clark’s nutcracker (Ridgeley et al. 2005) in gray. Range of whitebark pine (U.S.

Geological Survey 1999) overlaid in black crosshatch.
Systematics and related species—

Within the class Aves, Clark’s nutcracker is a songbird, in the order Passeriformes,

family Corvidae, genus Nucifraga. Species in the family Corvidae are linked taxonomically by some common morphological traits. Overall, corvids are regarded as

social and highly intelligent birds (McGowan 2001). The only other member of the

genus Nucifraga is a Eurasian species, the spotted nutcracker, of which there are at

least seven recognized subspecies (Goodwin 1986, Madge and Burn 1994) (fig.

10). Many of the life history traits of the spotted nutcracker are similar or identical

to those of Clark’s nutcracker. Most notably, both species have coevolved with

pines in the subsection Cembrae (section Strobus, subgenus Strobus, genus Pinus),

and their annual cycles revolve around seed availability.

Within the family Corvidae, nutcrackers appear to be most closely related to

the crows (Goodwin 1986). However, several species of New World jays have convergently evolved traits similar to those of the nutcracker. The blue jay of eastern

North America and the western scrub jay of the Southwestern United States have

evolved traits for effective seed dispersal in oaks and beeches. The Steller’s jay of

western North America opportunistically harvests and disperses seeds of both

deciduous and coniferous trees (Lanner 1996). Of the jays, the pinyon jay (fig. 11)

is most specialized in conifer seed dispersal and has several morphological adaptations similar to those of the nutcracker: exceptional spatial memory, morphological

In years of low

conifer seed production, nutcrackers

have been observed

far outside their 

typical range.

General Technical Report PNW-GTR-742

26

Figure 10—The only other member of the genus Nucifraga is the spotted nutcracker, which disperses

seeds for several species of pine throughout Eurasia. 

Michel Rogg
adaptations for carrying multiple seeds, a long sharp pointed bill, and strong flight

capabilities for transporting seeds long distances (Vander Wall and Balda 1981). 

In general, however, nutcrackers are considered the most specialized food-hoarding

and seed-eating corvid (Vander Wall and Balda 1981).

Diet— 

Despite its evolutionary history with whitebark pine, Clark’s nutcracker does not

require whitebark pine for survival. The range of Clark’s nutcracker extends

beyond the range of whitebark pine in the Southwestern United States. Seeds from

other large-seeded pines including limber, southwestern white, and pinyon pine

replace those of whitebark pine as important food in the southern parts of the nutcracker’s range (Samano and Tomback 2003, Vander Wall 1988, Vander Wall and

Balda 1977). In western Montana, Giuntoli and Mewaldt (1978) found that even

where the ranges of nutcrackers and whitebark pine overlap, whitebark pine seed

on average accounted for less than 20 percent of the annual diet of nutcrackers

(app. 2). Ponderosa pine seed accounted for more than half of the annual diet over

3 years, compared to 19 percent whitebark pine seed, 13 percent insects and spiders, and 11 percent Douglas-fir seed. In other parts of their range, nutcrackers are

known to forage and sometimes to rely on seeds of Great Basin bristlecone pine,

Rocky mountain bristlecone pine, Monterey pine, ponderosa pine, Jeffery pine, and

Douglas-fir (table 8). The cones of these conifers ripen asynchronously throughout

Despite its evolutionary history with

whitebark pine,

Clark’s nutcracker

does not require

whitebark pine for

survival; local populations of nutcrackers are able to persist in the absence of

whitebark pine.

27

A Review of the Literature on Seed Fate in Whitebark Pine and the Life History Traits of Clark’s Nutcracker and Pine Squirrels

Figure 11—A sympatric species, the pinyon jay, which disperses the seeds of North American

pinyon pines. Clark’s nutcracker and the pinyon jay share many similarities in life history

traits and morphological adaptations that reflect their reliance on pine seeds. Deschutes

National Forest, Oregon. 

Dave Herr, U.S. Forest Service
the fall (Krugman and Jenkinson 1974). Within any given region, nutcrackers

therefore must be highly mobile and move throughout the landscape during the

year to exploit the available seeds of the different species of conifer. 

The amount of conifer seed in the diet of nutcrackers differs by month (app. 2).

Throughout the year, nutcrackers opportunistically consume foods other than

conifer seed. Insects may account for the majority of the nutcracker’s diet in summer (Giuntoli and Mewaldt 1978). Nutcrackers have readily adapted to human food

and are attracted to bird feeders, picnic grounds, and human habitations at moderate and high elevations. They also eat songbird eggs and young, consume carrion,

forage on pollen cones, and prey upon small mammals, amphibians, and birds

(Cottam 1945; Dixon 1956; French 1955; Giuntoli and Mewaldt 1978; Lorenz,

unpublished data; MacCracken 1949; Mulder et al. 1978). 

Territoriality and pair bonds—

Avian territories are most commonly defined as established, defended patches 

for breeding purposes. Territoriality is commonly exhibited in passerines during 

a short, 2- or 3-month breeding season during which there is competition among

individuals for food. Territories typically incorporate a nest site and sufficient

resources for reproduction. Territory size is correlated with body size (Schoener

1968) because resources need to be economically defensible (Brown 1964).

Trespassing by conspecifics is not tolerated, and food resources and mates are

General Technical Report PNW-GTR-742

28

Table 8—Species other than whitebark pine on which Clark’s nutcrackers forage

for seed 

Species used as foragea Source

Bush chinquapin Roth and Vander Wall 2005

Colorado pinyon pine Christensen et al. 1991, Tomback 1977, Vander Wall 

and Balda 1977, Vander Wall et al. 1981

Douglas-fir Giuntoli and Mewaldt 1978, Vander Wall et al. 1981

Great Basin bristlecone pine Lanner 1988

Jeffrey pine Tomback 1978

Limber pine Benkman et al. 1984, Lanner and Vander Wall 1980, 

Tomback and Kramer 1980, Torick 1995, Vander 

Wall 1988, Vander Wall and Balda 1977, 

Monterey pine Davis and Williams 1957

Ponderosa pine Baud 1993, Giuntoli and Mewaldt 1978, Torick 1995

Rocky Mountain bristlecone pine Baud 1993

Singleleaf pinyon pine Vander Wall 1988, Vander Wall et al. 1981 

Southwestern white pine Benkman et al. 1984, Samano and Tomback 2003, 

Vander Wall and Balda 1977

a Nutcrackers have been cited as making caches of all of these species except Monterey pine. 
actively defended. In most species, territories are established and defended by

singing males. Plumage coloration and songs in males aid in territorial defense and

mate acquisition. 

Rolando and Carisio (2003) found that the spotted nutcracker of Eurasia

restricts territorial behavior to its nest site, and the same appears to be true for

Clark’s nutcracker. Mewaldt (1956) reported three nests of Clark’s nutcracker in

Montana located within 500 m of each other. During the breeding season, conspecifics were observed traveling within 100 m of a nesting pair without being

challenged (Mewaldt 1956). 

Both male and female breeding nutcrackers often become quiet and secretive

during the breeding season (Mewaldt 1948). During the initial stages of the breeding season, males may act as sentinels while females are tending the nest or young.

Territorial defense is generally reserved for predators. Potential predators are often

met with aggressive mobbing by resident nutcrackers. Mobbing behavior by one

individual acts to attract other nutcrackers within range of hearing, and predators

may be driven away from nests by nutcrackers from multiple breeding territories

(Mewaldt 1948, Tomback 1998).

In the closely related spotted nutcracker and pinyon jay, males and females

form pair bonds that typically last multiple breeding seasons, and it is likely that

Clark’s nutcracker does the same (Marzluff and Balda 1992, Swanberg 1956).

Adult nutcrackers observed foraging outside of the breeding season and caching 

in autumn appear to be paired (Mewaldt 1956).

Breeding biology— 

Adult nutcrackers forage on freshly harvested and previously cached seeds and

insects during the spring breeding season (Giuntoli and Mewaldt 1978).

Observations of nesting nutcrackers across years suggest that adults attempt to

breed only in years that they have sufficiently large seed stores (Tomback 1998). 

In years when breeding occurs, nutcrackers initiate the breeding season in

midwinter, up to 3 months earlier than most passerines (Bendire 1889, Bradbury

1917, Mewaldt 1956). Not only is the breeding season early, it is also lengthy,

extending into late summer. This extended period enables young nutcrackers to

develop mentally and physically before being faced with the challenges of seed

harvesting, seed caching, and independence (Dimmick 1993, Vander Wall and

Hutchins 1983). 

Nutcrackers initiate the breeding season by building nests. Nest-building typically takes less than a week and occurs in February and March (Campbell et al.

1997, Mewaldt 1956). Nests are bulky to provide adequate insulation for the eggs

29

A Review of the Literature on Seed Fate in Whitebark Pine and the Life History Traits of Clark’s Nutcracker and Pine Squirrels
General Technical Report PNW-GTR-742

30

and nestlings during the early breeding season. Nests may be placed anywhere

from 2 to 24 m above ground, usually in conifers (Bradbury 1917, Mewaldt 1956).

Nutcracker nests have been found in ponderosa pine, Douglas-fir, pinyon pine,

western juniper, and mountain mahogany in mid-elevation coniferous forests, and

in subalpine fir and whitebark pine at high elevations (Bent 1946; Bradbury 1917;

Campbell et al. 1997; Mewaldt 1956; Quigley and Dixon 1956; Lorenz, unpublished data). Compared to most other songbirds, however, few nutcracker nests

have been reported, and more study is needed on habitat selection by breeding nutcrackers and productivity within populations. 

Two to five eggs are laid within days of completion of the nest (Mewaldt

1956). Unlike most passerines, male nutcrackers share incubation and brooding

duties with the female (Mewaldt 1952), which enables both sexes to retrieve seeds

from personal caches (fig. 12). Incubation lasts approximately 3 weeks. After

hatching, the nestlings remain in the nest for 20 days before fledging. As the young

approach fledging, they demand constant feeding by both adults. Nestlings are fed

regurgitated conifer seeds and insects (Bendire 1889, Bradbury 1917, Dixon 1934,

Mattes 1994, Mewaldt 1956). Stored seeds may be important for the survival of

nestling nutcrackers, whereas insect material is likely critical for growth and development (Dixon 1934, Mewaldt 1956). Nutcracker young typically fledge in April

and May. 

Figure 12—A Clark’s nutcracker brooding its nestlings. Okanogan and Wenatchee National

Forests, Washington. 

Nicholas T. Ernst
Shortly after the young fledge, snow begins melting in subalpine areas, especially on the south-facing slopes where adults placed caches the previous fall. The

adults with their newly fledged young move to these caching grounds to retrieve

stored seeds and forage on germinating seedlings (Vander Wall and Hutchins

1983). Young are proficient fliers by early summer, although they often remain

dependent on their parents for food. The young are highly vocal, and one or several

may follow their parents throughout each day, begging loudly (Vander Wall and

Hutchins 1983). 

Juvenile nutcrackers are dependent on their parents for 2 or 3 months after

fledging. Independence occurs gradually over several weeks. In mid-July, young

nutcrackers start learning to find caches on their own by locating germinating

seedlings (Vander Wall and Hutchins 1983). Young birds likely learn this technique

by following their parents as they search for cached food. Juvenile nutcrackers also

practice cache and retrieval behaviors by “playing,” in which they cache and

retrieve nonedible objects (Vander Wall and Hutchins 1983). 

Adult nutcrackers may harvest unripe seeds from whitebark pine cones as

early as July (Tomback 1978, Vander Wall and Hutchins 1983). First-year birds

also attempt to harvest unripe seeds from cones, although typically their rate of

seed retrieval is far slower than those of adults (Dimmick 1993), because the young

birds have difficulties perching and balancing on cones, prying scales apart, retrieving intact seeds, and balancing on branches (Tomback 1978). Aggression by juveniles toward adults may be high as seed stores are depleted and juveniles must subsist on unripe seeds; juveniles often successfully supplant adults foraging on cones

(Vander Wall and Hutchins 1983). As the seed harvest season progresses, juveniles

become more efficient at harvesting seeds (Vander Wall and Hutchins 1983). This

is facilitated by ripening of the cones, which increases seed harvesting rates for all

birds. 

Ripening of the cones coincides with the beginning of the seed-caching period

for adults. Juveniles do not begin caching seeds until late in the season, typically

after adults have been caching for weeks (Vander Wall and Hutchins 1983). This is

a critical time in the lives of newly independent young birds. Their ability to learn

harvesting and caching techniques may determine their likelihood of surviving to

adulthood and securing a mate in following years.

Winter survival— 

Clark’s nutcrackers forage primarily on conifer seeds in winter (Giuntoli and

Mewaldt 1978) (fig. 13), and the availability of seeds likely affects their movements in winter (Davis and Williams 1957, 1964; Fisher and Myres 1979; Vander

31

A Review of the Literature on Seed Fate in Whitebark Pine and the Life History Traits of Clark’s Nutcracker and Pine Squirrels
Wall et al. 1981). In years of moderate or high cone production, most nutcrackers

appear to be high-elevation residents or short-distance altitudinal migrants

(Campbell et al. 1997, Grinnell and Storer 1924, Tomback 1978). 

Widespread and simultaneous failures in cone crops can cause population

irruptions (migration to areas outside the usual range of Clark’s nutcracker) in

some years. Extralimital sightings include multiple reports of nutcrackers in the

Eastern United States (Bryant 1895, Coale 1911, Fisher 1943, Mitchell 1894) as

well as in southern and coastal California (Davis and Williams 1957, 1964), the

Desert Southwest (Presnall 1936, Westcott 1964), and eastern Canada (Fisher and

Myres 1979). Most of these sightings occur from August to December, at the time

that cone crops in low production years would be depleted. Population irruptions

are common in other North American birds that rely on conifer seeds, such as the

pine siskin, red crossbill, and white-winged crossbill (Gill 1995). The spotted nutcracker of Eurasia undergoes similar although less frequent population irruptions in

times of low seed availability (Formosof 1933, Mattes 1994, Swanberg 1956).

Irruptions of Clark’s nutcracker have been observed at 5- to 15-year intervals

(Davis and Williams 1957, 1964), although more recent work indicates that minor

extralimital movements may be occurring as frequently as every 3 or 4 years

(Fisher and Myres 1979). 

Although extralimital sightings have been well documented, seasonal migrations of nutcrackers within their normal range are poorly understood. There is 

Based on observations of returning

migrants, nutcrackers may spend up 

to 10 months, or the

majority of the year,

on wintering

grounds.

General Technical Report PNW-GTR-742

32

Figure 13—A Clark’s nutcracker perched during a midwinter snow storm. Okanogan and Wenatchee

National Forests, Washington. 

Nicholas T. Ernst
evidence that migrations occur regularly and perhaps even annually in some populations. Vander Wall et al. (1981) documented both northward and southward

migrations of thousands of nutcrackers through the Great Basin in the autumns of

1977, 1978, and 1979. Based on observations of returning migrants, nutcrackers

may spend up to 10 months, or the majority of the year, on such wintering grounds.

Although there is no evidence that nutcrackers transport seeds while on migration

(Vander Wall et al. 1981), they have been seen caching seeds while on wintering

grounds (Davis and Williams 1957). Migrants will participate in the harvest of

seeds with resident populations and may attempt to breed on their wintering

grounds (Vander Wall et al. 1981). Several species of bird follow such semiannual,

irregular migration patterns and are described as facultative partial migrants

(Podulka et al. 2004). Movement patterns in such species are unpredictable as 

they respond to changes in the abundance of specialized foods.

Population trends— 

Information on population trends in Clark’s nutcracker is available from two

sources, the North American Breeding Bird Survey (BBS; coordinated by the U.S.

Geological Survey), and the Christmas Bird Count (CBC; coordinated by the

National Audubon Society). Both counts are standardized across North America

and have been ongoing since 1966 and 1900, respectively. The BBS is conducted in

June, and the CBC is conducted in December of each year. Both counts are intended to provide a long-term data set for investigating general trends in populations. 

Results from the BBS show a slight overall increase in numbers of Clark’s

nutcracker across its range since 1966. In the past 20 years, counts in the Rocky

Mountains and Great Basin have recorded fairly stable or slight increases in numbers. Conversely, counts have recorded consistent declines in the Cascade Range

and Sierra Nevada (Sauer et al. 2005). Results from the CBC suggest that populations of nutcrackers rangewide have remained stable in the past 40 years (National

Audubon Society 2002).

Although these data are available and currently provide the only measure of

population trends in Clark’s nutcracker, there are many limitations to BBS and

CBC data because of inconsistencies between surveys and years. The value of such

counts lies in the sheer volume of data gathered over long time periods. The BBS

data are particularly limited in value for monitoring species like Clark’s nutcracker

that are not actively nesting when the counts are actually being conducted, and that

are nonterritorial and wide-ranging. For example, Mewaldt (1948) found that the

densities of nutcrackers on transects in Montana varied by a factor of eight between

months because of variation in conifer seed availability. Overall, a more reliable

33

A Review of the Literature on Seed Fate in Whitebark Pine and the Life History Traits of Clark’s Nutcracker and Pine Squirrels
method of surveying nutcracker populations is needed before conclusions can be

made on population trends.

Pine Squirrels

Pine squirrels—red and Douglas’ squirrels—are important components of whitebark pine ecosystems (Mattson et al. 2001, McCaughey and Tomback 2001). As

discussed earlier, pine squirrels are efficient cone harvesters and harvest the majority of available whitebark pine seeds in some years and locations (Hutchins and

Lanner 1982). Because of their efficiency, the selective pressures exerted by pine

squirrels on conifers are considerable. In limber and whitebark pine, the presence

of pine squirrels has led to cone traits that hinder avian seed dispersal: the production of sterile scales at the base of the cone, the production of thicker seed coats,

and a reduction in number of seeds per cone (Benkman 1995, Benkman et al. 1984,

Siepielski and Benkman 2007a). Furthermore, pine squirrel harvest activities in a

stand may competitively exclude Clark’s nutcrackers (Benkman et al. 1984,

Siepielski and Benkman 2007a).

Despite their predatory effects, pine squirrels are important components of

whitebark pine communities. Pine squirrels are important prey items for many

predators. They form a major component of the diet of the northern goshawk

throughout its range in North America (Squires and Reynolds 1997 and references

therein). Likewise, American martens, the threatened Canada lynx, great horned

owls, and red-tailed hawks prey on pine squirrels in parts of their ranges (Clark et

al. 1987, Rusch and Reeder 1978, Tumlison 1987). Squirrel middens also are

important components of whitebark pine communities because the pine squirrel’s

cone-storing habits make large quantities of whitebark pine seeds available for

grizzly and black bears (Mattson and Reinhart 1994). This squirrel-related food

source has direct consequences for interactions between grizzly bears and humans

in some regions (Mattson and Reinhart 1997). In years of small whitebark pine

cone crops when squirrel middens are not available, bears migrate to areas where

they are in more contact with humans (Mattson et al. 1992). For diverse reasons,

therefore, a review of the life history traits of the pine squirrel is important when

considering mechanisms of seed dispersal in whitebark pine. 

Morphology—

Pine squirrels are small (200 to 250 g) diurnal tree squirrels (Steele 1999). Both

species are specialized for foraging on conifer seeds. Their small bodies enable

them to forage on the relatively small branches of conifer trees. With the exception

of the nocturnal flying squirrels, pine squirrels are the smallest arboreal squirrels in

Pine squirrels are

efficient cone 

harvesters and 

harvest the majority

of available whitebark pine seeds in

some years and

locations.

General Technical Report PNW-GTR-742

34
North America. They have well-developed jaw musculature for their small size,

which facilitates harvesting of large conifer cones. Sexes are not considered dimorphic, although males are commonly heavier than females (Steele 1998, 1999). Size

varies intraspecifically in pine squirrels. Smaller individuals inhabit forests of

small, soft cones, and larger squirrels predominate in forests of large, heavily

armored cones (Lindsay 1986). 

Red and Douglas’ squirrels are superficially similar in appearance and morphology. However, red squirrels can be distinguished from Douglas’ squirrels in the

field by slight differences in the color of their coats. The red squirrel is generally

more reddish dorsally and white or pale yellow ventrally (fig. 14). The Douglas’

squirrel is gray or olive-brown above and yellowish to deep orange below (Steele

1999). In addition to pelage color, there are differences in size and reproductive

rates of red and Douglas’ squirrels, that aid in survivorship of the two species in

their respective habitats (Smith 1981). The red squirrel has larger jaw muscles, a

larger body, and a reproductive rate that is apparently less influenced by changes in

cone production than that of the Douglas’ squirrel. These factors increase the fitness of the red squirrel in the more open lodgepole and ponderosa pine stands of

the eastern Cascade Range and Rocky Mountains, where the intensity of light in

the forest is higher and the food supply is more consistent between years than in

coastal forests. The opposite factors aid in survival in Douglas’ squirrel in the

35

A Review of the Literature on Seed Fate in Whitebark Pine and the Life History Traits of Clark’s Nutcracker and Pine Squirrels

The pine squirrel’s

cone-storing habits

make large quantities

of whitebark pine

seeds available for

bears.

Figure 14—A red squirrel foraging on spruce cones. Anchorage, Alaska. 

Donna Dewhurst, U.S. Fish and Wildlife Service
coastal forests of the Pacific coast, where stands are dense and dark and cone crops

fluctuate more drastically (Smith 1981). 

Distribution— 

The range of the red squirrel extends from the Appalachian Mountains of northern

Georgia, into the boreal forests to the Arctic Circle, south to the border of Mexico,

and westward to the Rocky Mountains and eastern slopes of the Cascade Range

and into the Rocky Mountains and south to the border of Mexico (fig. 15). It is

absent from the Great Plains, the southeastern lowlands, and the Pacific coast of

North America (Steele 1998). The red squirrel is most commonly found in coniferous forests but may take up residence in mixed stands, especially in the eastern

portions of its range (Layne 1954, Steele 1998). 

The range of the Douglas’ squirrel is limited to coniferous forests of the

Pacific coast, from coastal British Columbia, through the Cascade Range of

Washington and Oregon, and south to central California (Steele 1999) (fig. 16).

The ranges of red and Douglas’ squirrels are allopatric except for two areas in 

the Pacific Northwest (Johnson and Cassidy 1997, Steele 1999). A small zone of

General Technical Report PNW-GTR-742

36

Figure 15—Range of red squirrel (Patterson et al. 2005) in brown. Range of whitebark pine (U.S.

Geological Survey 1999) overlaid in black crosshatch.
sympatry occurs in eastern Oregon and in the Cascade Mountains of southern

British Columbia and northern Washington. Both species may coexist in forests of

Engelmann spruce, Pacific silver fir, subalpine fir, and mountain hemlock (Smith

1981) in the transitional forest types in the Cascade Range. Past reports of

hybridization in these areas were based on observed differences in pelage color. It

is now believed that the two species do not hybridize, and observed pelage differences are the result of converging morphological traits (Lindsay 1986).

Systematics— 

The family Sciuridae is the second largest family in the order Rodentia, with 276

recognized species worldwide. Family Sciuridae is diverse and includes members

of about 50 genera, including pine squirrels, chipmunks, marmots, ground squirrels, flying squirrels, and other tree squirrels (Duff and Lawson 2004). Pine squirrels have a deep reddish pelage and morphological adaptations to a conifer seed

diet, which distinguishes them from other tree squirrels. They are endemic to North

America, where there are 25 recognized subspecies of red squirrel and three subspecies of Douglas’ squirrel. 

37

A Review of the Literature on Seed Fate in Whitebark Pine and the Life History Traits of Clark’s Nutcracker and Pine Squirrels

Figure 16—Range of Douglas’ squirrel (Patterson et al. 2005) in brown. Range of whitebark pine

(U.S. Geological Survey 1999) overlaid in black crosshatch.
Diet— 

Pine squirrels are conifer-seed-eating specialists (Ferron et al. 1986). Densities of

pine squirrels in stands differ over time in response to seed availability (Carey

1991, Flyger and Gates 1982, Klenner and Krebs 1991, Smith 1970, Yeager 1937).

In the Pacific Northwest, pine squirrels rely on the seeds of western hemlock,

Douglas-fir, grand fir, Sitka spruce, and western redcedar west of the Cascade crest.

East of the crest and in the intermountain West, they forage most extensively on

lodgepole pine, ponderosa pine, Engelmann spruce, subalpine fir, and Douglas-fir

(Smith 1968a, 1970). They prefer cones containing seeds with the highest caloric

value (Smith 1970). 

In late spring and summer, conifer seed is not ripe enough for consumption,

and a variety of fresh foods may be consumed. Fungi are important food items—

more than 45 species of fungi are consumed by squirrels in the Pacific Northwest

(Smith 1968a). Secondary food sources are numerous and include tree buds and

flowers, pine cambium and phloem, fleshy fruits, insects, and songbird eggs and

nestlings (Ferron et al. 1986, Flyger and Gates 1982, Smith 1968a). Pine squirrels

also forage extensively on conifer pollen cones in summer if other foods are

unavailable, although large quantities of pollen cones may be needed because they

have relatively low caloric value. Squirrels commonly remove the branch containing pollen cones when foraging, thereby precluding cone production on such

branches in the future. Pine squirrels thus can cause extensive tree and stand damage while foraging.

Squirrels subsist on stored conifer seed in winter and supplement their diet

opportunistically as food stores are depleted. In winter and spring, this comes at a

great loss to living trees. Squirrels strip bark and forage on cambium, phloem,

buds, and shoots (Smith 1968b). They preferentially consume bark of pines infected by rusts (Peridermium and Cronartium) because infected bark has a high sugar

content (Hoff 1992, Smith 1968a). Extensive stand damage in years of food shortage may occur because of tree girdling and the destruction of new tree growth

(Sullivan and Sullivan 1982, Sullivan and Vyse 1987, Sullivan et al. 1993). 

Territoriality— 

Pine squirrels aggressively defend their caches against all potential competitors,

including conspecifics, chipmunks, and Clark’s nutcrackers (Carey 1991, Hutchins

and Lanner 1982, Smith 1968a, Torick 1995). Such aggressive resource defense is

imperative for individual survivorship because squirrel larders sustain individuals

throughout the winter and early spring (Smith 1968b). The cone larders are often

centrally located in a circular-shaped territory, making food defense as economical

General Technical Report PNW-GTR-742

38
as possible (Hatt 1943, Smith 1968a). Pine squirrels establish territorial boundaries

around their food stores by aggressive chases and territorial calls. Territories are

defended year-round.

Both males and females defend territories and are equally intolerant of intruders (Smith 1968a). Where the ranges of Douglas’ and red squirrels overlap, the two

species defend territories interspecifically (Smith 1981). Territorial defense is particularly heated in autumn when cones are being harvested and there is increased

competition from juveniles. Territory overlap or breakdown may occur in some

populations when defensible food is depleted, such as in late winter and spring

(Smith 1968a, 1968b). Pine squirrels may leave their territories and migrate more

than 5 km under such circumstances (Rusch and Reeder 1978).

As long as a squirrel has sufficient food stores and is capable of defending its

cone-bearing trees and caches, its territory is its permanent, year-round residence

(Buchanan et al. 1990, Kemp and Keith 1970, Smith 1968a, Sullivan 1990,

Sullivan and Klenner 1992). Many species of conifer in western North America

produce large crops of cones at intervals of 3 to 5 years, and this periodicity is synchronized within populations (Krugman and Jenkinson 1974). Successive years

with no cone crop result in high mortality in pine squirrels and emigration of the

remaining population (Smith 1968b). Because seeds are the primary food source of

squirrels, squirrels inhabiting homogenous stands are at a disadvantage because

there are few alternative seed sources in consecutive years of low cone production.

Stands of pure whitebark pine stands may therefore be devoid of squirrels in most

years (Hatt 1943, Mattson and Reinhart 1997). 

Breeding biology— 

As in most rodents, pine squirrels do not pair and are promiscuous. Territoriality in

both sexes prevents interaction except for a single day in which a female is in

estrous and permits males into her territory. Several males will fight for dominance

throughout the day of a female’s estrous, and dominance might change among several males over the course of this day (Koford 1982, Smith 1968a). Females control

the timing of copulation and therefore can select the male with which she will

mate. Gestation is approximately 30 days. 

Pine squirrels may breed in spring (February–May) or late summer

(July–September). They typically bear one litter each year, although there are

instances of females producing two litters in one year, apparently in response to

surpluses of food (Ferron et al. 1986, Koford 1982). Conversely, females in poorquality habitat or following years of poor cone production may forego reproduction

for a year (Kemp and Keith 1970, Smith 1968a). A single litter may be born in late

39

A Review of the Literature on Seed Fate in Whitebark Pine and the Life History Traits of Clark’s Nutcracker and Pine Squirrels
summer in years following cone crop failure but in which the current year’s cone

crop is moderate or heavy. Such late summer breeding provides females with a

large supply of fresh conifer seed during lactation.

Most litters are born in spring, and young are weaned and independent 2

months after birth. The timing of spring breeding enables the young of the year to

establish territories in late summer prior to cone harvesting. Although still dependent on their mother, young squirrels explore the area within 2.6 km of their natal

territory in search of suitable habitat in which to establish their own territory

(Kemp and Keith 1970, Larsen and Boutin 1994). Females may assist their young

by expanding their territory prior to juvenile dispersal and subsequently giving up

parts of their territory to their young or dispersing themselves (Larsen and Boutin

1994, Price 1992, Price and Boutin 1993). Juveniles that inherit their mother’s territory show low predator-induced mortality in autumn, compared to dispersing

juveniles, but they have lower winter survival rates (Larsen and Boutin 1994).

These juveniles may face heavier intraspecific competition, which directly affects

survivorship as juvenile survivorship is closely correlated with cone productivity

and the amount of food an individual is able to store (Halvorson and Engeman

1983, Larsen and Boutin 1994). 

Cone harvesting and storage behaviors in pine squirrels appear to be inherent.

Juveniles will participate in the cone harvest in the year of their birth, although typically they are unable to secure larders large enough to enable breeding the following year. Most pine squirrels first breed at 2 years of age (Hatt 1943, Kemp and

Keith 1970, Rusch and Reeder 1978). 

Winter survival— 

Pine squirrels do not hibernate and do not undergo torpor of any kind. They remain

active throughout the winter although they may seek shelter for days at a time during major weather events. A high body temperature and fat deposits enable pine

squirrels to survive extreme cold (Steele 1998). Their ability to survive the winter

also depends on the insulating qualities of their nests and the amount of food they

have stored. 

Nest site selection is critical for winter survival and effective thermoregulation

(Smith 1968a). Although natural cavities appear to be preferred where available,

pine squirrels most commonly build vegetative nests (composed of grasses, mosses,

bark, and leaves) in the canopies of trees. Squirrels often place nests in large-diameter trees with many interlacing branches, which provide canopy escape routes

(Rothwell 1979, Vahle and Patton 1983). Because squirrels subsist on stored cones

General Technical Report PNW-GTR-742

40
during the winter months, most nests are built within 30 m of middens (Rothwell

1979; Smith 1968a, 1968b; Vahle and Patton 1983).

Conclusion

Clark’s nutcracker, pine squirrels, and scatter-hoarding rodents are the primary 

vertebrate species that influence whitebark pine seed fate and ultimately affect the

ability of whitebark pine populations to regenerate. Clark’s nutcracker is wellknown for its mutualistic role as a seed disperser for whitebark pine, and it also

acts as a seed predator in many circumstances. Pine squirrels are highly effective

seed predators. Scatter-hoarding rodents are confirmed seed predators. They are

confirmed seed dispersers for many species of pine, and although their role has not

been studied in whitebark pine, they likely disperse seeds in some populations. 

However, seed dispersers can be transformed into seed predators, and mutualistic partnerships can become exploitative interactions as a result of fluctuations in

cone productivity on a landscape scale, predator abundance, and inter- and intraspecific competition. The roles of these granivores vary both spatially and temporally—roles vary spatially in relation to the distribution of whitebark pine trees and

alternative food sources on the landscape, and they vary temporally in response to

demographic changes within their populations.

These variations affect whitebark pine seed fate in complex ways. Spatially,

not all whitebark pine trees are equally likely to have their seeds dispersed in a

manner that will enable germination. Trees in pure stands and isolated trees in open

areas will have the greatest proportion of their seeds harvested by Clark’s nutcrackers (Christensen and Whitham 1991, Hutchins and Lanner 1982, Reinhart and

Mattson 1990) and fewer of their seeds will be lost to pine squirrel predation.

Conversely, whitebark pine in mixed stands would have a greater proportion of

their cones harvested by pine squirrels, and the likelihood of regeneration for such

individuals is lower. Whitebark pine regeneration is also influenced by temporal

changes in nutcracker and rodent populations. All of these animals show plasticity

in foraging behavior and have evolved to survive periods of low whitebark pine

seed production. Short-term and small-scale changes in food availability will affect

species differently than long-term, widespread, and simultaneous changes in seed

production.

As whitebark pine populations continue to decline owing to white pine blister

rust, mountain pine beetle attacks, fire suppression and, in the future, global warming, it will become increasingly important for successful whitebark pine restoration

to consider the complex and variable roles that vertebrates play in whitebark pine

Clark’s nutcracker,

pine squirrels, and

scatter-hoarding

rodents are the 

primary vertebrate

species that influence whitebark pine

seed fate and ultimately affect the 

ability of whitebark

pine populations to

regenerate.

41

A Review of the Literature on Seed Fate in Whitebark Pine and the Life History Traits of Clark’s Nutcracker and Pine Squirrels

Successful restoration and management

of whitebark pine

depends on recognizing the complexities

of interactions

between multiple 

vertebrate species

and whitebark pine.
seed fate. There are many gaps in our knowledge of the interactions among Clark’s

nutcracker, pine squirrels, scatter-hoarding rodents, and whitebark pine. This paper

provides a review of the current literature to inform managers who are interested in

restoring and managing whitebark pine ecosystems.

Glossary

allopatric–Occurring in separate, nonoverlapping geographic areas. This term 

is often used to describe populations of related organisms unable to crossbreed

because of geographic separation.

cache–In relation to seed dispersal by animals, refers to a discrete site selected by 

an animal for temporary cone or seed storage. Caches may contain one or many

cones or seeds depending on the species of animal storing and the species plant

being stored. 

corvid–A bird in the family Corvidae, a family of passerine birds that contains the 

crows, ravens, rooks, jackdaws, jays, magpies, and nutcrackers. Collectively its

members are called corvids, and there are more than 120 species. 

granivore–An animal that selectively eats seeds.

indehiscent–A fruit or cone that does not split open to scatter its seeds when 

mature.

keystone–A species that has a disproportionate effect on ecosystem function 

relative to its abundance or biomass. 

larder-hoard–Method of food storage by animals in which items are concentrated

in one or few caches. Larders are visited multiple times.

masting–An event that results when plants within a population synchronize their 

reproductive activity. Generally, masting describes not only abundance but also

paucity of seed production. However, a “mast year” usually means a year in which

plants produce a significant amount of seed. It is thought that masting occurs as an

evolutionarily stable strategy through which plants are able to influence the size of

animal populations that predate on the seed.

midden–An accumulation of cone debris that collects beneath the preferred feeding

perches of squirrels. Squirrels may use middens for food storage. 

molt–The gradual shedding and replacement of the feather coat in birds. 

General Technical Report PNW-GTR-742

42
mutualism–An interaction between two or more species where both species derive 

benefit. Mutualisms can be lifelong interactions involving close physical and biochemical contact (known as symbiosis) such as those between plants and mycorrhizal fungi; they can also be briefer, nonsymbiotic interactions, such as those

between flowering plants and pollinators or seed dispersers. Mutualisms may be

facultative (optional) or obligatory.

passerine–A bird of the order Passeriformes also known as perching birds or song

birds. Passerines are characterized by feet with three toes directed forward without

any webbing or joining, and one toe directed backward. More than half of all

species of bird are passerines.

population irruption–A density-dependent form of migration that is independent 

of predictable seasons, tides, or similar geographic events. In species of seed-eating

birds, irruptions are population-wide responses to the absence of seed crops and

result in individuals occurring outside of their defined range. 

primary seed dispersal–Transport of seeds away from the parent plant to the site 

of first deposition.

scatter-hoarding–A method of food storage by animals in which items are cached 

in many locations throughout an individual’s home range. Unlike larders, scatterhoards are usually visited only once for caching and once for retrieval.

secondary seed dispersal–Movement of seeds from the site of initial deposition to 

all subsequent sites prior to seed germination.

songbird–A bird of the order Passeriformes in which the vocal organ is structurally 

more complex and more muscular than in other birds. Songbirds consequently have

the most varied and intricate vocalizations among birds. 

sympatric–Occupying the same or overlapping geographic areas without interbreeding. This term is used to describe populations of closely related species. 

Acknowledgments

The authors are particularly grateful to H.E. Hutchins, M.G. Raphael, D.F.

Tomback, and S.B. Vander Wall for detailed reviews and constructive comments on

this publication. We also appreciate thoughtful reviews by T. DeSpain, D. Doede,

B. Howell, and K.A. Sullivan on earlier drafts of the manuscript. Mary Carr, Forest

Service Publishing Arts, provided editorial support.

43

A Review of the Literature on Seed Fate in Whitebark Pine and the Life History Traits of Clark’s Nutcracker and Pine Squirrels
English Equivalents

When you know: Multiply by: To find:

Centimeters (cm) 0.39 Inches 

Meters (m) 3.28 Feet 

Kilometers (km) .62 Miles 

Square kilometers (km2) .386 Square miles 

Square kilometers (km2) 247.1 Acres 

Hectares (ha) 2.47 Acres 

Milligrams (mg) .015432 Gram

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A Review of the Literature on Seed Fate in Whitebark Pine and the Life History Traits of Clark’s Nutcracker and Pine Squirrels

Appendix 1: 

Common and Scientific Names

Common name Scientific namea

Plants (trees and shrubs):

Beeches Fagus spp.

Bitterbrush Purshia tridentata (Pursh) DC.

Bush chinquapin Chrysolepis sempervirens (Kellogg) Hjelmqvist

Digger pine Pinus sabiniana Dougl. ex Dougl.

Douglas-fir Pseudotsuga menziesii (Mirb.) Franco

Engelmann spruce Picea engelmannii Parry ex Engelm.

Foxtail pine Pinus balfouriana Grev. & Balf.

Giant sequoia Sequoiadendron giganteum (Lindl.) Buchh.

Grand fir Abies grandis (Dougl. ex D. Don) Lindl.

Great Basin bristlecone pine Pinus longaeva D.K. Bailey

Jeffery pine Pinus jeffreyi Grev. & Balf

Knobcone pine Pinus attenuata Lemmon

Korean stone pine Pinus koraiensis Siebold et Zucc.

Limber pine Pinus flexilis James

Lodgepole pine Pinus contorta Dougl. ex Loud.

Monterey pine Pinus radiata D. Don

Mountain hemlock Tsuga mertensiana (Bong.) Carr.

Alderleaf mountain mahogany Cercocarpus montanus Raf.

Noble fir Abies procera Rehd.

Oaks Quercus spp.

Pacific silver fir Abies amabilis Dougl. ex Loud.

Colorado pinyon pine Pinus edulis Engelm.

Ponderosa pine Pinus ponderosa P.& C. Lawson 

Red fir Abies magnifica A. Murr.

Rocky Mountain bristlecone pine Pinus aristata Engelm.

Siberian stone pine Pinus sibirica Du Tour

Single-leaf pinyon pine Pinus monophylla Torr. & Frém.

Sitka spruce Picea sitchensis (Bong.) Carr.

Southwestern white pine Pinus strobiformis Engelm.

Subalpine fir Abies lasiocarpa (Hook.) Nutt.

Subalpine larch Larix lyallii Parl.

Sugar pine Pinus lambertiana Dougl.

Swiss stone pine Pinus cembra L.

Western hemlock Tsuga heterophylla (Raf.) Sarg.

Western juniper Juniperus occidentalis Hook.

Western larch Larix occidentalis Nutt.

Western redcedar Thuja plicata Donn ex D. Don

Western white pine Pinus monticola Dougl. ex D. Don

White fir Abies concolor (Gord. & Glend.) Lindl. ex Hildebr.

White spruce Picea glauca (Moench) Voss

Whitebark pine Pinus albicaulis Engelm.
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Common name Scientific name

Birds:

Blue jay Cyanocitta cristata

Cassin’s finch Carpodacus cassinii

Clark’s nutcracker Nucifraga columbiana

Common raven Corvus corax

Crows Corvus spp.

Great horned owl Bubo virginianus

Mountain chickadee Poecile gambeli

Northern goshawk Accipter gentiles

Pine grosbeak Pinicola enucleator

Pine siskin Carduelis pinus

Pinyon jay Gymnorhinus cyanocephalus

Red crossbill Loxia curvirostra

Red-breasted nuthatch Sitta canadensis

Red-tailed hawk Buteo jamaicensis

Scrub jay Aphelocoma coerulescens

Spotted nutcracker Nucifraga caryocatactes

Steller’s jay Cyanocitta stelleri

Western scrub jay Aphelocoma californica

White-breasted nuthatch Sitta carolinensis

White-winged crossbill Loxia leucoptera

Williamson’s sapsucker Sphyrapicus thyroideus

Woodpeckers Picoides spp.

Mammals:

American marten Martes americana

Black bear Ursus americanus

Canada lynx Felix lynx

Chipmunks Tamias spp.

Deer mice Peromyscus spp.

Douglas’ squirrel Tamiasciurus douglasii

Flying squirrels Glaucomys spp.

Golden-mantled ground squirrels Spermophilus spp.

Grizzly bear Ursus arctos

Ground squirrels Ammnospermophilus spp., Spermophilus spp.

Kangaroo rats Dipodomys spp.

Least chipmunk Tamias minimus

Marmots Marmota spp.

Tree squirrels Sciurus spp.

Pocket mice Perognathus spp.

Red squirrel Tamiasciurus hudsonicus

Voles Microtus spp., Myodes spp.

Yellow pine chipmunk Tamias amoenus
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A Review of the Literature on Seed Fate in Whitebark Pine and the Life History Traits of Clark’s Nutcracker and Pine Squirrels

Insects and diseases:

Cone beetles Conophthorus spp.

Cone worms Dioryctria spp., Eucosma spp.

Limb rust Peridermium spp.

Mountain pine beetle Dendroctonus ponderosae Hopkins

White pine blister rust Cronartium ribicola Fisch.

a Scientific names and authorities of plants are from U.S. Department of Agriculture, Natural 

Resources Conservation Service (2007).
General Technical Report PNW-GTR-742

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Appendix 2

Table 9—Stomach contents of 426 Clark’s nutcrackers from western Montana expressed as volume (percent) 

and frequency (percent) by months

Volume (and frequency) as percentages

Num- Ponderosa Whitebark Douglas- Misc. MamYear Month ber pine pine fir plants Arthropods mals

1946 Oct 6 48 (100) 5 (17) 45 (67) 2 (50)

Nov 5 2 (80) 98 (100) T (20)

Dec 8 1 (38) 99 (100) T (12)

1947 Jan 7 55 (72) 3 (14) 42 (43) T (29)

Feb 10 37 (100) 28 (60) 34 (50) 1 (10) T (70)

Mar 25 24 (68) 5 (32) 69 (80) T (16) 1 (44) 1 (44)

Apr 16 46 (88) 9 (25) 36 (69) 1 (6) 8 (62)

May 22 31 (64) 29 (77) 14 (36) T (5) 25 (96) 1 (5)

Jun 8 16 (25) 46 (88) 38 (100)

Jul 9 2 (10) 62 (100) T (33) 36 (100)

Aug 13 84 (100) 1 (8) 14 (100) 1 (8)

Sep 6 87 (100) 13 (33)

Oct 13 55 (93) 13 (46) 32 (85)

Nov 16 56 (94) 38 (88) 4 (44) 2 (19)

Dec 18 47 (95) 33 (89) 4 (33) 16 (28)

1948 Jan 11 48 (100) 52 (91) T (9) T (27)

Feb 12 40 (100) 56 (100) 4 (42)

Mar 12 51 (100) 37 (100) 1 (8) 9 (83) 2 (8)

Apr 17 39 (94) 23 (59) 8 (47) 22 (71) 8 (24)

May 12 30 (83) 38 (92) T (8) 26 (100) 6 (17)

Jun 4 6 (50) 51 (75) 30 (100) 13 (25)

Jul 20 1 (5) 19 (70) T (5) 76 (100) 4 (5)

Aug 0

Sep 3 95 (100) 5 (67) T (33)

Oct 0

Nov 41 96 (100) T (5) 2 (32) 2 (5)

Dec 0

1949 Jan 21 91 (100) T (5) T (5) 9 (38)

Feb 24 94 (100) 3 (37) 3 (17)

Mar 24 86 (100) 4 (46) 10 (33)

Apr 23 85 (100) 15 (91)

May 20 82 (100) T (5) 18 (90)

Weighted summationa 52.4 (80) 19.3 (42) 11.2 (15) 0.7 (6) 13.3 (59) 3.1 (12)

a Volume percent = Swv = Σ (N1V)/N2 when = Swv = weighted summation of volumes, N1 = number that month, V = percent of volume 

that month, and N2 = 426 samples. Frequency percent, = Swf = Σ (N1F)/N2, when Sw

v = weighted summation of frequencies, and F = 

percent frequency that month

Source: Reproduced from Giuntoli and Mewalt (1978).
Pacific Northwest Research Station

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