FOOD
MICHAELCACHINGBY FEMALE

AMERICAN

KESTRELSIN WINTER

W. COLLOPY

Prey caching is documented for many birdsof prey. A review of some raptor species
which cache food was provided by Mueller( 1974). Additional records of caching among
falconiforms were reported by Roest (1957),
Gullion ( 1965) and Balgooyen (1976). Preycaching has also been noted in many owl
species (Allen 1924, Lockley 1938, Wallace
1948, Mossman 1955, Jansson 1964, Norburg
1964, Grant 1965, Ligon 1968, Catling 1972).
For the purposes of this study, I designatedcaching behavior as any activity involved instoring or retrieving prey. Storage is that
aspect of caching in which a food item isplaced in a cache. Retrieval is that aspectof caching in which a stored food item isremoved from a cache. Typical prey retrieving behavior which failed to produce a food
item is called here an attempt to retrieve.
Most accounts of caching by American Kestrels (Falco sparverius) are based on relatively few observations of wild or captive
birds (Pierce 1937, Tordoff 1955, Rocst 1957).
Stendell and Waian (1968) observed a female kestrel store 17 prey items in one cacheover a 40-day period. Balgooyen (1976)
watched kestrels caching food during thebreeding season. He discussed the advantages of caching food, especially during pcriods when inclement weather is common andwhile raising young. In the laboratory, Mueller(1974) studied the effects of deprivation interval, time of day, and type of food on thecaching behavior of captive kestrels.
The results reported here are from a study
I conducted on the behavior of American
Kestrels wintering in the Arcata Bottoms,
west of Arcata, Humboldt County, California.
I observed kestrels for two winter seasons,from October 1972 to March 1973, and from
September 1973 to March 1974.
STUDY AREA
The Arcata Bottoms, Humboldt
County, California,
is bordered on the south by Humboldt Bay, on thewest by the Pacific Ocean, on the north by the Mad
River, and on the east by the city of Arcata andsurrounding redwood (Sequoia semperuirens) forest.
It consists of extensive pastureland reclaimed frombav tidelands and Darts of the Mad River delta.
Wheeler and Fiarris (1970)
described the vegetation in the pastures, dikes, sloughs and intermittent marshes of the Arcata Bottoms. In the pastures they found primarily orchard grass ( Dactylis[631

velvet grass (Holeus
mollis),fescues
glomerata),( Festuca spp. ), sedges ( Carer spp. ), and rushes(Juncus spp. ). On the dikes the vegetation included
tufted hairgrass (Deschampsia
caespitosa),
coyotebrush ( BacchaTis piZuZuris), blackberry (Rubus uitifolius), mustards ( Cruciferue),and docks ( Rumex
marshes,“pp. ) . In the sloughs and intermittent
stands of one or more of the following were found:cattail( Typha Zatifoliu), arrowgrass ( Triglochinmuritimu), rushes, horsetail (Equisetum spp. ), threesquare bulrush ( S&pus americanus),
and saltgrass( Distichlis spicata ).

MATERIALS

AND METHODSI watched kestrels with a 20~ wide-angle spotting
telescope and 7~ binoculars at distances of 10 to
70 m. For each observation of caching behavior, I
recorded the following: species of prey cached, feeding behavior before storing, location of cache, behavior after storing, time of day, and duration of
the caching sequence.

RESULTS
The male to female sex ratio of kestrels wintering in the Arcata Bottoms was 1:9 (Koplin
1973). Observations of males were relatively
uncommon; therefore, I analyzed data on females only.
I witnessed all aspects of caching behavior
during each winter of study (table 1). The
frequency of caching behavior was estimatedby expressing the total number of cachingacts observed each year as a percent of all
behavioral acts observed. Caching behavior
constituted 1.2% of all behavioral acts observed in 1972-73, and 1.4% in 1973-74. The
grcatcr number of caching observations witnessed in 1973-74, therefore, reflect the increased amount of time spent in the field, notactual increases in the rate of caching behavior.
CACIIE

SITES

I saw 10 kestrels store or retrieve 61 prey items
in 64 caches (three prey items were stored asecond time).
In 14 of these instances, I
observed the storing and subsequent retrievalof a particular prey item. Three of the caches
were short broken posts, 3 were fence posts,
2 were shrubs, and 56 were grass clumps.
Tordoff (1955) reported that a pet kestrel
preferred elevated caches and suggested thattrees may be preferred in the wild. The general absence of trees in the Arcata Bottoms
The Condor 79:63-68,

1977

64

MICHAELW.

COLLOPY

TABLE
1. Number of caching activities by female
American Kestrels wintering in the Arcata Bottoms,
Humboldt County, California in 1972-73 and 197374.

Food stored
Food retrieved
Attempts to retrieve
Yearly totals
Total number ofbehavioral acts
observed
Period of observation( hours )1972-73

1973-74

TOtal

10
9
6
25

36
23
6
65

46
32

2007

4779

6786

130.5

182.6

313.1

::

eliminated this possibility. However, kestrels
did not use any of the numerous utility polesin the area as caches.
Monthly road censuses (44 km long) conducted in the Arcata Bottoms during the fall
and winter of 1972-73 and 1973-74 indicatedthat relatively large numbers of raptors wintered in the area (Collopy 1975). Combining
both seasons, the mean number of kestrels
seen was 15.9 per census. The number ofother raptors seen, primarily Red-tailed Hawks(Buteo jamaicensis), Rough-legged Hawks(B. Zagopus), and Marsh Hawks (Circus
cyaneus), averaged 8.4 individuals per census.
The presence of these raptors, and scavengers such as Common Crows (Corvus brachyrhynchos), Common Ravens (C. corax), gulls( Laws spp. ) , and Turkey Vultures ( Cathartes
uuru ) , increased the probability of losing
stored prey if it was too conspicuous (e.g.,
on utility poles). The use of grass clumps andposts as caches, familiar to kestrels within
their hunting territories, apparently provided
sufficient orientation for them to find stored
prey. These same landmarks probably wereless obvious to other less sedentary raptors
and scavengers, and tended to conceal stored
prey.
PREY SPECIESCACHED

Only vertebrate prey were cached. In 197273, 15 (88.2%) of the prey items cached were
small mammals; of these, 7 (41.2%) werevagrant shrews (Sorex vugrans), 5 (29.4%)
were California meadow voles (Microtus culifornicus), 1 (5.9%) was a harvest mouse(Reithrodontomys megulotis), and 2 (11.8%)
were unidentified. Other prey cached included
a common garter snake (Thumnophis sirtalis)and a weaver finch (Passer spp.). In 197374, 30 (68.2%) of the prey items cached were
frogs and snakes; of these, 6 (13.6%) were
red-legged frogs (Runu aura), 15 (34.1%)

were Pacific treefrogs (Hyluregillu),
6( 13.6%) were unidentified frogs, and 3 (6.8%)
were garter snakes. The 14 (31.8%) otherprey items cached were meadow voles andshrews.
The shift from caching predominately small
mammals in 1972-73 to amphibians and reptiles in 1973-74 reflects the difference in prey
captured. In 1972-73, 29 (90.6%) of thevertebrates captured were small mammals. In
1973-74, 34 (73.9%) of the vertebrates captured were frogs and snakes, while the number of small mammal prey decreased to 12( 26.1% ) . I analyzed the data on capture andstorage of prey to determine if selective storing of particular prey items occurred. Becauseof limited sample size, only data from 197374 could be tested. A Chi-square test indicated that the relative proportion of prey typecached corresponded directly with the relative proportion captured (X” = 3.69, 1 df, P> 0.05). Limited data from 1972-73 suggestthat this relationship also existed the previous
year.
The difference in the proportion of small
mammals vs. amphibians and reptiles captured
during the two winters of study appears correlated with their relative availability during
the two seasons. The absence of frogs fromboth caching and prey captured in 1972-73,
probably reflects a decrease in availability
resulting from the relatively dry, cold winterof 1972-73. The warmer, wetter winter of
1973-74 apparently was conducive to poikilothermic activity, resulting in increased vulnerability to predation. Factors related to theobserved decrease in small mammal abundance during the winter of 1973-74 are lessobvious.
Generally, prey were decapitated before being stored. Of the 17 caches I inspected, I3(76.5%) contained decapitated prey. On one
occasion I found only the inverted pelt of ameadow vole. Stendell and Waian (1968) reported that 14 of 15 (93.3%) small mammals
they found in a cache regularly used by afemale kestrel, were decapitated.
PREY STORINGBEHAVIOR

After killing prey, and feeding on it or not,kestrels flew to a cache. Sites were apparently
selected before flight started, since kestrels
always flew directly to a spot where prey thenwas stored. When kestrels used posts orpoles as caches, they landed on the structureand stuffed the prey remains in a crack at thetop, or wedged it between the post and railing. When storing prey in grass clumps, kes-

FOOD,_,__+_prey

CACHINGBY KESTRELS

65

retrieved__-___unsuc~~~sfulretrievalattempts

TIME

OF

DAY

FIGURE
1. Caching activity of female American Kestrels in relation to time of day in the Arcata Bottoms,
Humboldt County, California, combined for the winters of 1972-73 and 1973-74. Observations are presented
by hourly intervai.

trcls landed next to the clump, transferred theprey from the talons to the beak (prey usually
were carried in the talons during flight, but
occasionally smaller prey were carried in thebeak), walked to the clump, and attempted
to poke the remains between the stems. A
kestrel sometimes made several attempts toposition prey before it remained in the grassclump. Prey were laid prone, as if to takeadvantage of their protective coloration. Balgooyen (1976) also observed this phenomenon. Positioning, and the use of inconspicuous
caches strongly suggest that specific behavior
patterns associated with storing have evolved
to minimize the loss of prey to other raptors
and scavengers. Kestrels always left the cache
immediately after storing prey.
PREY RETRIEVINGBEHAVIOR

Kestrels always perched near the cache before attempting to recover stored food. Whenapproaching a typical cache (a grass clump)
kestrels either landed beside it or hovered
one to three meters above it for three orfour seconds. While hovering, they often
moved about, looking into several adjacentgrass clumps. Once on the ground, kestrels
approached the cache and looked into it. Kes-

trcls commonly searched all around the grassclump for prey remains. If unable to locate
the stored food, they often walked to adjacentgrass clumps to search. When kestrels wereunable to find the cache they occasionallyabandoned the search. However, the mostcommon response after failing to find cachedprey was to return to a nearby perch and resume starching after a few minutes. In aneffort to recover a meadow vole, which I had
previously collected, a kestrel spent overthree minutes on the ground investigating a
number of grass clumps.
When a cache was located, kestrels usually
picked up prey in their beak, transferred it
to their talons, and returned to a feeding
perch. If the prey had slipped too far inside
the clump to be reached with the beak, kestrels often flew to the top of the clump andlanded on it with spread wings. Presumably,
this prevented them from falling between thestems. This behavior enabled kestrels to grasp
the prey with the talons and remove it.
CACHINGRELATION

BEHAVIORTO TIMEIN
OF DAY

Between 8 and 11% of all behavioral datacollected (313 h) was obtained during each

66

MICHAEL

W. COLLOPY

TABLE 2. The behavior of female American Kestrels after caching in relation to feeding behavior
before caching; Arcata Bottoms, Humboldt County,
California, during 1972-73 and 1973-74.
Feedingb;ehw;;

Behavior

after

caching

Inactiveperching

Roosting

5

1”

0

1

7

Prey notfed upon

2

0

0

0

2

Yearly
total

7

1

0

1

9

1973-74
Prey fed
upon

19

lb

2

0

22

Prey notfed upon 12

0

0

0

12

Yearly
total

31

1

2

0

34

38

2

1

43

caching

Continued
hunting

1972-73
Prey fed
upon

Total
a Inactive
b Inactive

for 28 minutes before
for 11 minutes
beforeObserverdeparted2
hunting
hunting

was
was

Total

resumed.
resumed.

segment from OS:00 to 17:O0. Since
field observations were distributed more orless equally throughout the day, the possibilityof biasing the temporal distribution of caching activities was minimized. Therefore, I consider figure 1 a representative sample ofthe time of day in which kestrels stored andretrieved prey. Trends in caching behavior
are apparent.
Kestrels tended to store prey throughout
the day and to retrieve late in the day. Observations of prey storing were scatteredthroughout most of the day, with 8 to 17%of all observations occurring during eachhourly interval from 09:OO to 16:O0. However,most prey retrieval observations (78.1%) occurred after 15:O0. This pattern was consistent through both winters, even though thetypes of prey cached were different.
hourlyEFFICIENCYOF PREY RETRIEVAL

Since field observations were evenly distributed throughout the day, I assumed that there

was no difference in the probability of observing either prey storing or retrieving bchavior. Based on this assumption, I treatedthe caching data as a representative sampleof the relative frequency of prey storing andretrieving. Using the number of prey items I
saw stored and retrieved, I calculated a minimum recovery efficiency of 70%. This estimate suggests that 30% of food stored bykestrels was not recovered. Observations of

unsuccessful attempts to retrieve prey (13.3%
of all caching efforts) indicate that kestrels
often were unable to find previously stored
food. Presumably, the pirating of stored preyby other raptors and scavengers, and the inability of kestrels to relocate caches contributed to this loss.
BEHAVIOR BEFORE AND
AFTER STORING PREY
The behavior of kestrels after storing prey
was stratified in relation to presence or absence of feeding prior to storage (table 2).

For both winters combined, 67.4% of the prey
items cached were partially consumed before
storing, while 32.6% were stored without priorfeeding. After storing prey, kestrels continued
hunting 88.4% of the time. Kestrels continued
hunting in all cases where prey were stored
without prior feeding. Other behavior after
storing included inactive perching (4.7%) androosting (4.7%).
Observations of kestrels feeding prior tocaching suggest that they usually satiate themselves before storing prey. Kestrels have acrop capacity of approximately 12 g, and eat amaximum of 8 to 12 g per meal (J. R. Koplin
pers. comm.). This explains why kestrels often were unable to fully consume large prey,
especially snakes and voles.
Other observations suggest that kestrels killand store prey when they are not hungry. In
February 1974, I watched a kestrel capture
and partially consume a treefrog. After storing the remains in a grass clump, she continued hunting and within 17 min captured
another frog. This frog was quickly decapitated and then cached.
DISCUSSION
My findings suggest that caching behavior in
kestrels evolved as a behavioral mechanism
to maximize food intake and to dampen fluctuations in prey availability (see Balgooyen1976). The estimated 70% success rate inrecovering stored prey illustrates the important role caching behavior can play in storing
temporary food surpluses. By hunting immediately after capturing prey, kestrels increase
the chance of obtaining food which might not
be available later. If this extra food is captured and stored, it can be retrieved later
when prey are scarce.
The tendency to store prey throughout theday, but retrieve it in the late afternoon may
dampen daily variation in food availability forthe diurnal-hunting kestrel. Assuming thatvertebrate prey have limited vulnerability

FOOD

CACHINGBY KESTRELS

67

they are satiated. They also are able to rethroughout the day, kestrels hunting at all
times can increase food consumption by stor- trieve several stored prey in succession. Theseing prey whenever an excessive amount is observations suggest that the normal behavior
captured. Late in the day, kestrels have less of kestrels studied by Mueller was inhibitedby the artificial, laboratory conditions.
time to capture the amount of prey required
The evolutionary significance of cachingfor maintenance through the night. Consequently, they tend to consume more of the behavior must be viewed in relation to its effect on the reproduction of kestrels which
prey they capture and to recover stored prey.
cache food. Clearly, those kestrels who are
My observations agree with
Mueller’smost efficient in procuring food for their(1974) data on the caching activities of capyoung, especially during periods of temporary
tive kestrels. He found no apparent correlafood shortage, will be most successful at raistion between storing prey and the time of day.
However, retrieving behavior did appear to ing young. Caching behavior enables kestrels
to increase the amount of food they can procorrelate with time of day. He reported that“the data suggest a circadian rhythm in the vide for young, especially during periods ofshortage. The caching of food by kestrels
tendency to retrieve stored food; a rhythm
during the winter, when prey availability is
which peaks in the late afternoon or evening.”
unpredictable, probably also reduces overMueller (1974) also discussed the efficiency
winter mortality.
of captive kestrels in retrieving prey. He
stated that his kestrels attempted to retrieve
prey on 65% of the opportunities. For an SI JMMARY
attempt to be considered successful, he reCaching behavior by the American Kestrel isquired the kestrels to return directly to thedescribed. In 1972-73, I recorded 25 instances
spot where they previously stored food. Also, of prey caching; of these 10 (40.0%) involved
the time kestrels were allowed to find thefood storage and 9 (36.0%) involved foodcache was limited to five minutes after reretrieval. I observed 6 (24.0%) unsuccessful
introduction to the feeding area. It is difficult
attempts to retrieve prey in 1972-73. In 1973to interpret these results since kestrels in the74, I recorded 65 instances of prey caching,wild probably lose cached prey to otherof which 36 (55.4%) involved storage and 23
animals and forget the location of some caches. (35.4%)
involved retrieval.
I observed 6
The time limitation imposed by Mueller prob(9.2%) unsuccessful attempts to retrieve prey
ably reduced the number of successful rein 1973-74. I witnessed the storage of a single
trieval attempts. Kestrels also had limited
prey item and its subsequent retrieval twice inarea in which to store and retrieve food. This
1972-73, and 12 times in 1973-74.
probably increased the likelihood of successSmall mammals, birds, snakes, and frogs
ful retrieval attempts. These factors affectwere the prey items cached. In 1972-73, 15
caching efficiency in different ways and prob(88.2%) of the prey cached were small mamably confound the results sufficiently to limit
mals, and in 1973-74, 30 (68.2%) were snakes
their applicability to kestrels in the wild.
and frogs.
After storing food, kestrels normally hunted
Prey was cached predominantly in grass
and killed prey, regardless of feeding behavclumps (87.5%)) but other sites occasionally
ior before storing (table 2). These observawere used; these included, short broken posts
tions differ from the laboratory findings of(4.7%))
fence posts (4.7%))
and shrubs
Mueller( 1973 ) , in which the predatory(3.1%).
behavior of kestrels was directly correlated
Kestrels stored prey throughout the day.
with the length of time between feedings.
Between 8 and 17% of all storing observations
Mueller also stated that kestrels never killedoccurred during each hourly interval fromanother mouse until the mouse already cached
09:OO to 16:O0. Most attempts at retrieval
was entirely eaten. On three occasions in(25; 78.1%) occurred after 15:O0.
March 1974, I observed a kestrel sequenMy findings indicate that kestrels continue
tially store four frogs in less than five hours. hunting immediately after storing prey, reIn each case, at least two frogs were notgardless of feeding behavior beforehand. Kesfed upon, or were only decapitated, betrels stockpiled surplus prey items in caches,
fore being stored. In January, 1973, I also and retrieved several stored prey in successaw a kestrel retrieve and eat two shrews sion.
from different caches within 58 minutes.
No hunting occurred between the first and
ACKNOWLEDGMENTS
second retrieval. Apparently, kestrels may
I thank James R. Koplin for his invaluableaid
continue to hunt and stockpile prey after
throughout this study, and Erick Campbell, Howard

68

MICHAELW.

COLLOPY

Levenson, Larry Norris, and Jeff Froke for assistancein related fieldwork.
Thanks also go to Dale McCullough for reviewing the manuscript.

LITERATURE

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