Red-cockaded Woodpecker (Picoides borealis) Response to Nest
Depredation by an Eastern Rat Snake (Elaphe alleghaniensis)
David K. Delaney, Larry L. Pater, Lawrence D. Carlile, Dirk J. Stevenson,
and Andrew D. Walde
Southeastern Naturalist, Volume 7, Number 4 (2008): 753–759
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2006 NORTHEASTERN NATURALIST 13(1):39–42
Red-cockaded Woodpecker (Picoides borealis) Response to Nest
Depredation by an Eastern Rat Snake (Elaphe alleghaniensis)
David K. Delaney1,*, Larry L. Pater1, Lawrence D. Carlile2, Dirk J. Stevenson2,4,
and Andrew D. Walde3,5
Abstract - We report the depredation of a Picoides borealis (Red-cockaded Woodpecker) nest
by an Elaphe alleghaniensis (Eastern Rat Snake). Of 38 banded woodpecker groups monitored
by video surveillance during 1998–2000, we documented one nest depredation by an Eastern
Rat Snake. Woodpecker visitation to the nest area during the depredation event increased substantially
compared with pre-depredation nesting behavior. Woodpecker visitation to the nest
cavity was minimal during the first six hours after the snake was discovered by the adult woodpeckers.
Visitation levels by woodpeckers remained higher than pre-depredation levels while
the snake remained in the nest cavity. After the snake’s departure from the nest, visitation rates
dropped below pre-depredation levels. Woodpeckers continued to visit the cavity during the day
and roost in the cavity at night for the remaining seven days of surveillance post-depredation.
This same banded woodpecker pair nested in the same cavity in 1999 and fl edged 1 female.
Overall, we observed a low rate of nest depredation by rat snakes during this study; only one
instance was recorded during more than 15,000 hours of video surveillance. Based on the
proclivity of Red-cockaded Woodpeckers to re-nest in previous nest cavities, and the potential
for snakes to re-climb trees based on past successes, resource managers may want to prioritize
placement of snake excluder devices or use of bark-shaving at active cavity trees where snake
depredation has occurred, especially if funding is limited. We urge ornithologists to continue
to incorporate snake research into their avian research to achieve a greater understanding of
predator-prey relationships that impact cavity-nesting birds, especially threatened and endangered
species such as Red-cockaded Woodpeckers.
Elaphe alleghaniensis Holbrook (Eastern Rat Snake; Burbrink et al. 2000) are
proficient tree climbers that are known to depredate nests of many bird species (Fitch
1963, Jackson 1970). These constrictors are thought to regularly climb Picoides
borealis Linnaeus (Red-cockaded Woodpecker) nest trees (Neal et al. 1993), though
rarely have depredation events by rat snakes been documented directly (Jackson
1978, Summerour 1988). Rat snakes may seek out woodpecker nests during the
breeding season, and are reported to climb active cavity trees more often than inactive
cavity trees, especially during the nestling phase (Neal et al. 1993). Rat snakes
use visual and chemosensory stimuli to facilitate foraging (Mullin and Cooper 1998).
Arboreal foraging behavior by rat snakes may be stimulated by repeated use of the
same nest cavities over multiple years by birds (Durner and Gates 1993). Increased
bird activity at nests (e.g., provisioning of young) after the incubation phase (Conner
et al. 1999) provides greater visual cues for foraging snakes (Mullin and Cooper
1998). We are not aware of any studies that have used video surveillance to document
Eastern Rat Snake depredation of active Red-cockaded Woodpecker nests. As
part of a larger project studying the nesting behavior of Red-cockaded Woodpeckers,
we documented a single nest depredation event by an Eastern Rat Snake. This paper
describes the depredation event and characterizes the behavioral responses by banded
Red-cockaded Woodpeckers before, during, and after the nest depredation.
We monitored Red-cockaded Woodpecker nesting behavior between 1998–2000
at Fort Stewart (31.88ºN, 81.57ºW), which is a large Army installation located
in southeastern Georgia 30 km west of Savannah. Video cameras and time-lapse
recorders were used to monitor the nesting behavior of 38 banded Red-cockaded
Woodpecker groups (see Delaney et al. 2002 for more detailed description of study
site and camera surveillance equipment). The size of the video image at cluster 37,
Notes of the Southeastern Nat u ral ist, Issue 7/4, 2008
754 Southeastern Naturalist Notes Vol. 7, No. 4
where we documented the nest depredation by the Eastern Rat Snake, was approximately
90 x 120 cm (height x length), and covered about 90 x 45 cm (height x length)
of the tree bole centered on the natural nest cavity, which was about 4 m high with
an orientation of 218 degrees. Snake excluder devices (SNEDs; Saenz et al. 1999,
Withgott et al. 1995) were not used at any of the active Red-cockaded Woodpecker
cavity trees at Fort Stewart. The use of SNEDs are suggested for use on small fragmented
woodpecker populations, not larger healthy populations (USFWS 2003) like
the one at Fort Stewart. The nest was videotaped between 04:30:00–20:30:00 (EST)
each day, and adult woodpecker attendance was assumed to occur overnight based on
confirmed arrivals at the nest before dark and departures in the morning.
Pre-depredation woodpecker behavior. Adult woodpecker’s regularly entered
and exited the active nest cavity at cluster 37 prior to the depredation event. The pair
also regularly worked resin wells above and below the nest cavity which appeared
to be well maintained. We recorded 186.7 hours of nesting behavior at cluster 37
between the 5th day of incubation (29 May 1998) and the 6th day of the nestling phase
when the snake entered and depredated the nest (10 June 1998). The mean number
of nesting bouts (period of time that an adult attends the nest between entrances and
exits from the cavity) per hour increased from incubation (2.13 ± 0.12; range = 1–4
nesting bouts) through the brooding phase (5.12 ± 0.28; range = 1–14 nesting bouts)
until the rat snake arrived.
Depredation event. We used a Tree Top Peeper TM (Sandpiper Technologies, Inc.,
Manteca, CA) to ascertain the reproductive status of the woodpecker’s nest at cluster
37 the morning of 10 June 1998. The attending adult fl ushed off the nest in response
to the peeper pole, but returned within 8 minutes. The nest contained 2 eggs and 2
nestlings approximately 6 days old. The adult Red-cockaded Woodpeckers made repeated
prey deliveries to the nestlings after the nest status inspection and before the
rat snake arrived about 3 hours later. At 11:18:02, an Eastern Rat Snake entered the
video field of view approximately 1 m above the nest cavity. The snake climbed down
the tree and lunged its body into the nest cavity at 11:19:16. The nest only contained
the eggs and nestlings when the snake entered; the attending adult had exited the
nest at 11:17:37. The rat snake stayed in the nest cavity for nearly 32 hours, though
the snake was observed extending its head outside the cavity a number of times over
the course of its stay. The rat snake exited the cavity and climbed down the tree on
11 June between 18:51:37–19:15:57. Due to the late departure of the snake from the
nest, no woodpeckers were observed near the nest cavity that evening. We did not
observe any direct aggressive interactions between the adult Red-cockaded Woodpeckers
and the Eastern Rat Snake.
Woodpecker behavior during the depredation event. We identified six behaviors
in association with the nest cavity and tree during depredation (i.e., snake present;
behaviors 1–5 observed) and post-depredation periods (i.e., snake absent; behaviors
1–6 observed). Woodpeckers were observed to either: (1) perch on the lip of the nest
cavity, (2) perch on the nest tree in proximity to the nest, (3) fl utter in front of the
nest cavity, (4) fl y-by the nest, (5) maintain resin wells while the rat snake was in the
nest cavity, or (6) enter the nest cavity.
An adult Red-cockaded Woodpecker first arrived at the depredated nest cavity at
11:20:56 on 10 June about 2 minutes after the snake had entered the cavity, landing
on the cavity lip for 1 second before fl ying off, apparently in response to the snake’s
presence. After the rat snake had arrived, the mean number of woodpecker visitations
to the cavity per hour increased substantially (11.38 ± 2.16, range = 1–27 visitations;
Fig. 1) compared to pre-depredation visitation rates. Woodpeckers initially
responded to the snake’s presence by fl uttering in front of or fl ying by the nest cavity
2008 Southeastern Naturalist Notes 755
(Fig. 1). After about 6 hours, woodpeckers perched at the cavity lip. Woodpecker
visitations to the nest stayed relatively high compared to pre-depredation conditions
during the remainder of the snake’s stay in the nest (Fig. 1). There was a decrease in
the mean number of visitations (4.95 ± 0.85, range = 1–12 visitations) and an increase
in the diversity of woodpecker behaviors observed after the snake departed from the
nest cavity (Fig. 2). We did not observe adult Red-cockaded Woodpeckers interact
directly with the rat snake before, during, or after the depredation event. Woodpecker
behaviors associated with the cavity tree (i.e., resin well maintenance, perching on
the lip and tree) occurred over a longer duration (range = 1 sec to ≥1 min) than activity
not directly occurring on the nest tree, such as fl uttering or fl y-by behavior which
occurred over ≤5 sec. durations. Woodpeckers did not enter the nest cavity while the
rat snake was present (Figs. 1 and 2).
Post-depredation woodpecker behavior. We recorded 78.5 hours of post-depredation
woodpecker behavior at cluster 37 from 10–13 June and 14–17 June 1998.
Woodpeckers returned to the proximity of the cavity tree at 05:15:46 on June 12 the
morning after the snake departed. Woodpeckers entered the nest cavity for the first
time at 07:49:53 on June 12. Woodpeckers then began to occasionally perch on the lip
of the cavity for a few seconds at a time or enter the cavity for periods of <3 minutes.
On the evening of June 12, an adult Red-cockaded Woodpecker was observed night
roosting in the cavity. Adult woodpecker activity levels in proximity to the cavity
were still relatively high even up to 68 hours after the nest failed (Fig. 2). Activity
levels decreased substantially after 99 hours post-depredation (mean = 1.94 ± 0.31
visitations per hr, range = 1–6). Activity may have dropped off sooner than that; we
Figure 1. Number and type of nest-area visits by Red-cockaded Woodpeckers per hour (hours:
0–32) while the rat snake was present in the nest cavity at cluster 37 on Fort Stewart, GA,
1998. The asterisk signifies that only 13 minutes of video data were recorded directly after the
snake entered the cavity at 11:18:02 on 10 June 1998 before the tape ended at 11:33:58, and the
next tape started at 13:47:04. The rat snake left the cavity during the 32nd hour block between
18:51:37–19:15:57 on 11 June 1998.
756 Southeastern Naturalist Notes Vol. 7, No. 4
did not have video coverage between hours 76–98 post-depredation. Woodpeckers
continued to visit the cavity during the day and roost there at night for the remaining
seven days of surveillance post-disturbance. We do not know exactly when the rat
snake ate the eggs and young while in the nest cavity due to the depth and orientation
of the cavity opening relative to the camera angle, but found the nest empty when
we checked nest status on 19 June. This same banded woodpecker pair nested in the
same cavity in 1999 and fl edged 1 female.
A variety of species, such as rat snakes (Jackson 1978), Glaucomys volans L.
(Southern Flying Squirrel; Laves and Loeb 1999), and Melanerpes carolinus L. (Redbellied
Woodpecker; D. Delaney et al., unpubl. data; Hazler et al. 2004; Kappes 1997)
are known to depredate or kleptoparasitize the nests of Red-cockaded Woodpecker.
Researchers have commented on the importance of incorporating snake behavioral
research into avian work (Weatherhead and Blouin-Demers 2004) and describing
snake depredation of bird nests and associated response behaviors (Stickel 1962). The
persistent use of specific cavities by birds may promote repeated visitations by snakes
(Durner and Gates 1993). Snakes may re-climb trees due to past successes and may investigate
multiple cavities during arboreal trips to improve foraging success (Jackson
1977). Evidence suggests that Red-cockaded Woodpeckers evolved the behavior of
opening and maintaining resin wells at active cavities (Jackson and Thompson 1971) as
a way to deter predators, such as snakes (Rudolph et al. 1990, Steirly 1957). However,
few researchers have experimentally tested the effectiveness of this behavior (Jackson
1974, Mullin and Cooper 2002). Rat snakes avoid exposure to tree resin by climbing
around active resin wells or avoiding sap-laden trees (Jackson 1976). Successful
climbs by rat snakes are thought to be infrequent due to the impact of resin barriers
(Jackson 1974, Rudolph et al. 1990), and documented cases of snake depredation are
often associated with compromised resin barriers (Jackson 1978, Rudolph et al. 1990).
The exact manner of cavity entrance by the snake during our observation could not be
Figure 2. Number and type of nest-area visits by Red-cockaded Woodpeckers per hour (hours:
33–75) post nest depredation at cluster 37 on Fort Stewart, GA, 1998.
2008 Southeastern Naturalist Notes 757
positively ascertained due to the field of view of the video, which was about 65% of the
tree’s circumference. Therefore, the snake may have gained access to the cavity on the
backside of the tree or on the tree trunk above the cavity. The snake may have climbed
up vegetation adjacent to the cavity tree, and gained access to the nest from behind or
above the nest cavity, as has been documented previously for rat snakes (Dennis 1971).
We documented that a rat snake inhabited a tree cavity for an extended period
after depredating a nest, which has been documented once by Jackson (1977) who
reported continuous use of a Colaptes auratus L. (Northern Flicker) nest by a rat
snake over a nine-day period. Snakes appear to reduce their movements after feeding
and may actually seek out secluded, safe, and thermally favorable microhabitats
to rest and digest meals (Bontrager et al. 2006). Thermal stability of tree cavities
may produce favorable conditions for snakes to increase their body temperature
following ingestion of prey. This behavior, postprandial thermophily, has been reported
in snakes (Blouin-Demers and Weatherhead 2001), and is thought to optimize
physiological performance, such as increased digestive efficiency, in some reptiles
(Espinoza and Tracy 1997, Tattersall et al. 2004, Toledo et al. 2003).
Depredation by rat snakes appears to occur primarily during diurnal and crepuscular
hours (Weatherhead and Charland 1985), though Hensley and Smith (1986)
documented rat snake depredation of bluebird nests at night. The ability of rat
snakes to forage at different times of day, and their willingness to stay in cavities for
extended periods of time, may reduce exposure to diurnal predators and lessen their
interactions with the previous cavity occupants. During our observed depredation
event, the rat snake departed from the cavity during crepuscular hours when diurnal
predators and adult woodpeckers were inactive. This behavior potentially reduced
the snake’s exposure to attack during its descent from the cavity tree. Presence of
adult birds at the nest may deter depredation by rat snakes, but nests may still be vulnerable
during periods when adults are not attending the nest. We did not observe any
aggressive behavior by Red-cockaded Woodpeckers toward the rat snake during the
depredation event. Larger woodpecker species such as Red-bellied and Dryocopus
pileatus L. (Pileated Woodpeckers) are known to defend their nests against predators
and cavity kleptoparasites (Boone 1963, Kappes 1997, Nolan 1959).
A week after the nest failed due to snake depredation, woodpeckers still used the
cavity as a roost and continued to maintain resin wells above and below the cavity.
Continued visitation by birds to failed nests has been documented to a limited degree
(Hensley and Smith 1986). This behavior is not surprising when we consider the high
degree of interspecific competition that Red-cockaded Woodpeckers experience for
roosting and nesting cavities (Carrie et al. 1998), and how the availability of suitable
cavities for nesting may limit reproductive success and lead to the decline of cavitynesting
birds (USFWS 2003).
Overall, we observed a low rate of nest depredation by rat snakes (and by other
predators) during this study. We believe that resin-well maintenance by Red-cockaded
Woodpeckers at their nest cavities reduces the risk of predation of their nests
by snakes compared to the predation of nests of other cavity-nestng birds that do no
deploy such protective measures. Based on the proclivity of Red-cockaded Woodpeckers
to re-nest in previous nest cavities, and the potential for snakes to re-climb
trees based on past successes, resource managers may want to prioritize placement
of SNEDs or use of bark-shaving at active cavity trees where snake depredation has
occurred, especially if funding is not available for placement of SNEDs on all cavity
trees. Our understanding of predator-prey relationships between cavity nesting
birds and arboreal predators such as rat snakes is limited. We urge ornithologists
to continue to incorporate snake research into their avian work to achieve a greater
758 Southeastern Naturalist Notes Vol. 7, No. 4
understanding of factors that impact cavity-nesting birds (Weatherhead and Blouin-
Demers 2004), especially threatened and endangered birds like the Red-cockaded
Woodpecker (Saenz et al. 1999).
Acknowledgments. This study was supported by US Army Forces Command and
the Fort Stewart Army Installation with funding from the Strategic Environmental
Research and Development Program, under Conservation Project No. CS-1083,
and the Construction Engineering Research Laboratory, which is part of the Engineer
Research and Development Center for the US Army Corps of Engineers. The
Environmental Division at Fort Stewart provided logistical support and conducted
woodpecker nest surveys. We thank T. Brewton, H. Erickson, M. Fay, M. Huffman,
M. Klich, S. Kovac, B. Platt, and A. Rinker for assisting with data collection; A.
Cone, B. MacAllister, L. Nguyen, and C. Smith for reviewing videotapes; and T.
Grubb and T. Brewton for their assistance in placing video cameras. We also thank
D. Richardson and one anonymous reviewer for comments that improved the paper.
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1US Army Construction Engineering Research Laboratory, PO Box 9005, Champaign, IL 61826.
2Environmental Division, 1177 Frank Cochran Drive, Fort Stewart, GA 31314. 37686 SVL Box,
13233 Sea Gull Drive, Victorville, CA 92395. 4Current address - Project Orianne, Indigo Snake
Initiative, 414 Club Drive, Hinesville, GA 31313. 5Current address - 8000 San Gregorio Road,
Atascadero, CA 93422. *Corresponding author - David.Delaney@erdc.usace.army.mil.