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2010 SOUTHEASTERN NATURALIST 9(2):395–402
Partial Predation at Cavity Nests in Southern Pine Forests
Karl E. Miller1,2,* and David L. Leonard, Jr.1,3
Abstract - Although open-cup nesting birds regularly experience partial predation
events, little is known about partial predation for cavity-nesting birds. Here we report
on 12 partial predation events for 5 species of cavity-nesting birds inhabiting
southern pine forests. Snakes, small mammals, and woodpeckers were the primary
predators; many were documented by direct visual observation or video photography.
We documented two types of outcomes from partial predation events: partial failure,
i.e., a single partial predation event followed by successful fledging of >1 young;
and complete failure, i.e., multiple, sequential partial predation events that result in
total nest failure. We propose the “plate too full” and “eat and run” hypotheses to
explain partial nest predation in birds and discuss the characteristics of cavities that
may facilitate this phenomenon.
Despite the importance of nest predation as a research topic in recent decades,
relatively little is known about the causes or circumstances associated
with partial, or incomplete, nest predation. Nest predation traditionally has
been interpreted to be an all-or-nothing phenomenon, where the entire contents
of a nest are taken during a predation event (Nice 1957, Ricklefs 1969).
Consequently, empty nests are typically attributed to a single predation event
caused by a single predator species. In addition, most nest-monitoring studies
may not detect partial losses of clutches or broods because of infrequent
nest inspections. Standard references recommend that nests be checked at
3–4 day intervals (Martin and Geupel 1993, Ralph et al. 1993), and many
studies check nest status weekly or even less frequently (e.g., White and
Seginak 2000). As the interval between nest checks increases, so does the
likelihood for >1 predator to visit the nest in that interval and consume
part, or all, of its contents. In addition, even if they are observed, partial
clutch and brood loss may not be reported because nest success is often the
demographic metric of greatest interest (but see Roy Nielson et al. 2008).
Moreover, many nests cannot easily be inspected because of their height or
their location in cavities. Thus, observers frequently make inferences about
nest status without actually inspecting the contents.
Recent use of video photography at songbird nests in various habitats has
revealed that open-cup nesting species may regularly experience partial or
1Department of Wildlife Ecology and Conservation, University of Florida, PO Box
110430, Gainesville, fl32611. 2Current address - Florida Fish and Wildlife Conservation
Commission Fish and Wildlife Research Institute, 1105 SW Williston Road,
Gainesville, fl32601. 3Current address - Hawaii Department of Land and Natural
Resources, Division of Forestry and Wildlife, 1151 Punchbowl Street, Room 325,
Honolulu, HI 96813. *Corresponding author - Karl.Miller@MyFWC.com.
396 Southeastern Naturalist Vol. 9, No. 2
incomplete predation (Antworth 2005, Carter et al. 2007, Pietz and Granfors
2000, Robinson and Robinson 2001, Small 2005, Stake and Cimprich 2003,
Stake et al. 2004, Thompson et al. 1999). In contrast, there is little available
evidence of partial predation in cavity-nesting species. This phenomenon
may be more difficult to detect in cavity-nesting species because tree cavities
are often difficult to inspect and monitor.
Here, we report observations of partial predation at 12 nests of 5 cavitynesting
bird species in the pine forests of northern Florida and southwestern
Georgia. We document two types of outcomes from partial predation events:
partial nest failure and complete nest failure. Finally, we propose hypotheses
that may explain partial nest predation in birds and discuss the characteristics
of cavities that may facilitate this phenomenon.
Observations were collected at 4 study areas in southwestern Georgia and
northern Florida: a mosaic of Pinus elliottii Engelm. (Slash Pine) plantations
(even-aged, 30–35 years old) and P. palustris Mill. (Longleaf Pine) sandhill
forests (uneven-aged, with scattered ≥100 year-old trees) at Camp Blanding
Training Site in Clay County, fl; a remnant old-growth Longleaf Pine
clayhill forest (Wade Tract; uneven-aged, with scattered 300–500 year old
trees) in Thomas County, GA; second-growth Longleaf Pine clayhill forest
(uneven-aged, with scattered trees ≥150 years old) at Pebble Hill Plantation
in Grady County, GA; and old field pine (P. echinata Mill. [Short-leaf Pine]
and P. taeda L. [Loblolly Pine]) clayhill forest (uneven-aged, with scattered
trees ≥100 years old) at Tall Timbers Research Station in Leon County,
fl. Some of the study sites at Camp Blanding were provisioned with nest
boxes designed to attract Myiarchus crinitus L. (Great Crested Flycatchers)
and other passerines (Miller 2000, 2002). The Wade Tract and Tall Timbers
sites were provisioned with nest boxes designed to attract Sitta carolinensis
Latham (White-breasted Nuthatches; Leonard 2005).
Partial predation events were documented incidentally during the course
of other studies on the nesting ecology of cavity-nesting birds (Leonard 2005,
2009; Miller 2000, 2002). We used standard methods (Martin and Geupel
1993) to search for nests. When a nest was located, it was monitored at 2–4 d
intervals. Nests located <4 m above ground were accessed with a stepladder,
and the contents checked with a light and mirror. Nests in cavities >4 m high
were monitored with a camera probe mounted on a telescoping fiberglass pole
(TreeTop II, Sandpiper Technologies, Inc., Manteca, CA), or by climbing
with Swedish sectional climbing or extension ladders. Activity at a portion
of nuthatch nests (162 hours) was monitored with a Canon XL-1 digital video
camcorder with a 300-mm zoom lens. We visited all nest territories within 1 d
after the expected date of fledging to confirm the presence of fledglings and rechecked
territories 1–3 times if fledglings were not located on the initial visit.
2010 K.E. Miller and D.L. Leonard, Jr. 397
We defined partial predation as any event in which a partial clutch or brood
is taken by a predator. We distinguished between two types of outcomes from
partial predation events: partial failure, i.e., a single partial predation event followed
by successful fledging of ≥1 young; and complete failure, i.e., multiple,
sequential partial predation events resulting in total nest failure.
We documented partial predation events on 12 nests of 5 species (Melanerpes
carolinensis L. [Red-bellied Woodpecker], Great Crested Flycatcher,
Poecile carolinensis Sennett [Carolina Chickadee], Baeolophus bicolor
L. [Tufted Titmouse], and White-breasted Nuthatch) during 1997–2003
(Table 1). Five of these nesting attempts were partial failures (i.e., fledging
of ≥1 young), and 7 were complete failures. Details relevant to interpretation
of these events are presented below.
Direct observations and video recordings, combined with evidence left
at the nest, were used to determine that snakes, small mammals, and woodpeckers
were responsible for predation events.
We documented one case of partial predation on eggs (2 of 4 eggs were
removed from a Great Crested Flycatcher nest), where the adults continued
to attend the nest and successfully fledge 2 young.
We documented one case of partial predation on nestlings (1 of 4 nestlings
was removed from a White-breasted Nuthatch nest), where the adults
continued to attend the nest and successfully fledge 3 young. In this instance,
a M. erythrocephalus L. (Red-headed Woodpecker) was filmed pulling nest
Table 1. Cavity nests at which partial predation events were documented in northern Florida and
southwestern Georgia. Stage = stage depredated. # = number of fledglings.
Nesting species Cavity location StageA Nest predator #
Great Crested Flycatcher Nest box E Unknown 2
White-breasted Nuthatch Nest box N Woodpecker 3
Red-bellied Woodpecker Dead pine N Snake 1B
White-breasted Nuthatch Dead pine N Snake 1B
White-breasted Nuthatch Live pineD N Snake 1C
Red-bellied Woodpecker Dead oak E Unknown -
Great Crested Flycatcher Nest box E Flying squirrel -
Great Crested Flycatcher Nest box E Unknown -
Carolina Chickadee Dead pine E Unknown -
Tufted Titmouse Nest box E Flying squirrel -
Great Crested Flycatcher Nest box N Snake -
White-breasted Nuthatch Live pine4 N Snake -
AE = egg, N = nestling.
BFledged prematurely, survived <1 d.
CFledged prematurely, survived throughout subsequent monitoring.
DNest in an abandoned Picoides borealis Vieillot (Red-cockaded Woodpecker) cavity.
398 Southeastern Naturalist Vol. 9, No. 2
material and a nestling from a nest box that contained four 20-day-old nestlings.
The woodpecker could fit only its head inside the narrow entrance
hole; it tried unsuccessfully to remove additional nestlings for 3 minutes,
and then flew off when the adult nuthatch returned with food. Three days
later, the remaining nuthatch nestlings were observed out of the nest being
fed by their parents.
We documented three cases of partial predation on nestlings where the
remainder of each brood fledged prematurely (Table 1). In the first case, a ca.
1-m Pantherophis alleghaniensis Say (Eastern Ratsnake) was observed in a
Red-bellied Woodpecker cavity in a pine snag and a freshly dead nestling was
found on the ground below. The adult woodpeckers brought food to the cavity
several times, but each time entered only partially and then flew off with the
food. Four days earlier, the cavity contained two 19-day-old nestlings. Fledging
occurs at 28 days in this population (K.E. Miller, unpubl. data).
In the second case, a ca. 1.5-m P. spiloides Duméril, Bibron, & Duméril
(Gray Ratsnake) was filmed entering a White-breasted Nuthatch nest cavity
containing four 21-day-old nestlings. When we returned to retrieve the camera,
the snake was climbing out of the cavity; 3 lumps were clearly visible
in its body, and 1 nestling was found on the ground below the cavity. Subsequent
searches of the area failed to locate the prematurely fledged nuthatch.
We could not determine if the bird had been attacked by the snake before it
fledged, because the camera had run out of film prior to the nestling’s escape.
Fledging occurs at 22 d in this population (D.L. Leonard, unpubl. data).
In the third case, a White-breasted Nuthatch nest cavity contained a
ca. 1 m P. gutatta L. (Red Cornsnake), and no nestlings, and a 21-day-old
nuthatch was observed on a nearby tree. The parents were provisioning the
prematurely fledged nuthatch and also bringing food to the nest cavity but
not entering it. Two days earlier, the cavity had contained at least 3 nestlings.
This fledgling was subsequently observed with its parents.
Repeated partial predation events resulted in eventual loss of the entire
clutch at 5 nests (Table 1). At 1 Red-bellied Woodpecker nest, 2 Great Crested
Flycatcher nests, and 1 Tufted Titmouse nest, clutches were gradually
removed or destroyed over periods of 3–9 d. Two of these nests contained
Glaucomys volans L. (Southern Flying Squirrels) and their nest material
(Tillandsia usneoides L. [Spanish Moss]) on the day the last egg was found
destroyed. In addition, the titmouse nest showed signs of physical disturbance
(i.e., nest lining pulled out of the nest cup and the entire structure tilted
at a 45° angle from the nest box walls) on repeated occasions that indicated
≥2 sequential predation events.
Similarly, repeated partial predation events resulted in eventual loss of
an entire clutch at a Carolina Chickadee nest in a pine stump. Consecutive
nest inspections at 3–4 day intervals revealed 1, 1, 2, 2, 0, and 0 eggs. On the
penultimate visit, the cavity entrance had been enlarged and the cavity was
empty. On the last visit, the cavity contained egg shells, indicating that at
2010 K.E. Miller and D.L. Leonard, Jr. 399
least 1 additional egg had been laid and subsequently depredated. It is likely
that additional eggs were laid and depredated (i.e., >2 sequential predation
events) because 7 days elapsed between the first observation of 1 egg and the
first observation of 2 eggs and the typical clutch size is 4–6 for this species
(Miller 2000, Mostrom et al. 2002).
We also documented repeated partial predation events on nestlings that
resulted in eventual loss of the entire brood at 2 nests (Table 1). In the first
case, a small (ca. 30 cm in length) Eastern Cornsnake was observed under a
Great Crested Flycatcher nest in the corner of the nest box during the egglaying
period. We monitored the nest during the next several weeks, but no
predation was documented until 4 of the 5 nestlings disappeared just before
they were 11 d old. The remaining nestling was gone the following morning,
and no fledglings were observed in subsequent searches. Fledging occurs at
15 d in this population (K.E. Miller, unpubl. data).
In the other case, we observed a White-breasted Nuthatch nest with two
14-day-old nestlings and a Gray Ratsnake (ca. 1.5 m in length). Four days
earlier, the cavity had contained 4 nestlings. We monitored the nest throughout
the day and found that adult nuthatches continued to feed the nestlings despite
the presence of the snake in the cavity. The following day, the cavity contained
the snake and only 1 nestling. Two days later, the snake was gone and the cavity
was empty. No fledglings were observed in subsequent searches.
We documented partial predation events throughout the nesting cycle.
We attributed observations of partial clutch loss to predation because of the
gradual and incremental losses over time coincident with physical damage
to the nest site and observations of predators and their nesting material. We
attributed partial brood losses to predation because of direct observations
and video recordings. Partial brood loss can sometimes result from starvation
(i.e., brood reduction; Mock 1994), but brood reduction was rare in the
species that we studied. For example, only a single case of brood reduction
was documented in White-breasted Nuthatches during 3 breeding seasons
Our study was not designed to measure the frequency of partial predation
because we were not able to employ cameras at all nests. It is probable that we
missed some partial predation events because many cavity nests, especially
those of woodpeckers, titmice, and chickadees, were difficult or impossible to
inspect because they were located high off the ground in unstable dead wood
or inside cavities with narrow openings that would not accommodate a camera
probe. Complete inspection of all nest contents was uniformly possible
only for Great Crested Flycatcher nests in nest boxes, and they experienced
partial predation fairly frequently (4 of 32 nests; 12.5%; K.E. Miller, unpubl.
data). We suggest that the lack of information on partial predation in cavitynesting
birds is an artifact of the difficulty of monitoring cavity nests. Partial
predation is often reported in open-cup nesting species studied with cameras
400 Southeastern Naturalist Vol. 9, No. 2
(Antworth 2005, Pietz and Granfors 2005, Stake et al. 2004) and accounted for
60% of all predation events in one study (Carter et al. 2007).
Why does partial predation occur? One likely explanation, which we
term the “plate too full” hypothesis, is that predators either cannot handle
or cannot consume all the contents at once because of the relative size of
predator and prey. For example, Mephitis mephitis Schreber (Striped Skunk)
predation on waterfowl nests was influenced by clutch size: small clutches
were more likely to be consumed entirely than large clutches (Lariviere and
Messier 1997). Video recordings of grassland nests found that predators
responsible for partial, repeated predations over >1 d period were primarily
small rodents or Molothrus ater Boddaert (Brown-headed Cowbirds) and not
larger mammalian predators (Pietz and Granfors 2000). In Florida, a small
(56 cm in length) Eastern Ratsnake removed Great Crested Flycatcher nestlings
from a nest box one at a time over a >2 d period (Taylor and Kershner
1991). In Panama, a snake 80 cm in length was observed consuming only
1 egg from a Tinamus major Gmelin (Great Tinamou) nest (Robinson and
Robinson 2001); after spending >3 h to engulf the first egg, it tried to swallow
a second egg but eventually gave up and left the nest.
Another explanation, which we term the “eat and run” hypothesis, is that
parental defense, or the anticipation of it, interrupts or hurries the predator,
causing it to leave before consuming all nest contents. This scenario likely
represents a tradeoff between the predator’s hunger and its perception of
risk. Carter et al. (2007) describe a Coluber constrictor L. (Southern Black
Racer) being chased by Aphelocoma coerulescens Bosc (Florida Scrub-jay)
from a nest before it finished swallowing the first nestling. Robinson and
Robinson (2001) observed 2 vireos chasing away a toucan after it took 2
of 3 nestlings; the toucan came back 3 d later and took the third. These hypotheses
are overlapping, and it is likely that interactions among predator
size and aggressiveness, prey size, parental vigilance, and parental defense
determine whether the entire contents of a nest are consumed.
We also note that the “eat and run” hypothesis may not adequately explain
the behavior of snakes at cavity nests. Snakes may benefit by remaining
inside a cavity and consuming nestlings gradually over several days, as we
observed, when they would otherwise likely be mobbed or attacked when
leaving the cavity. Climbing ratsnakes are mobbed by a variety of cavity
nesting birds in Florida (Leonard 2009, Miller 2000), and woodpeckers are
capable of injuring or dislodging ratsnakes from trees (Casey et al. 2005).
Even if some mobbing species do not strike a snake directly, their mobbing
can alert raptors to the presence of a snake (Withgott and Amlaner 1996).
Cavities provide snakes safety from the attacks of birds (Withgott and
Amlaner 1996) and also provide them with an environment conducive to
digestion (Bontrager et al. 2006).
Partial predation can lead to an underestimate of the frequency of nest predation,
particularly if investigators consider partial nest failures as successful
nests. Moreover, some partial predation events stimulate premature fledging,
2010 K.E. Miller and D.L. Leonard, Jr. 401
which may put fledglings at greater risk; in our study, 2 of 3 prematurely
fledged nestlings failed to survive the first day outside the nest. We agree with
Small (2005) that nests should only be considered successful by investigators
when fledglings are observed near the nest after the presumed fledgling date.
Disparate results for partial predation in birds may be an artifact of different
monitoring intensities (Weidinger 2007) or could reflect local (Roy Nielsen
and Gates 2007) or geographic (Leonard 2005, Miller 2000) differences in the
composition of predator communities. Partial predation in cavity nesting birds
warrants further study, particularly in light of the high rates of nest predation
on cavity nesting-birds in the pine forests of the southeastern coastal plain of
the United States (Leonard 2005, Miller 2000).
K.E. Miller’s research was supported by funding from the Florida Fish and
Wildlife Conservation Commission (FWC), the Florida Army National Guard, the
University of Florida (UF), the North American Bluebird Society, a Sigma Xi Grantin-
Aid of Research, and a Student Research Grant from Sandpiper Technologies, Inc.
G. Jones, M. Williams, and A. van Doorn assisted with data collection, and FWC and
UF provided administrative support. D.L. Leonard’s research was supported by the
staff at Tall Timbers Research Station and UF. Mr. and Mrs. Jeptha Wade graciously
provided access to the Wade Tract, and P. Doherty, K. Maute, and R. Ripley assisted
with data collection. We thank M. Reetz, E. Stolen, and three anonymous reviewers
for their comments on earlier drafts of the manuscript.
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