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E.A. Forys, D. Hopkins, P. Ingham, M. Miller, and L. Gluckman
22001155 SOUTHEASTERN NATURALIST 1V4o(4l.) :1643,5 N–6o4. 04
Do Effigies Deter Fish Crows Hunting in a Black Skimmer
Colony Mid-season?
Elizabeth A. Forys1,*, David Hopkins2, Paul Ingham2, Maggie Miller1,
and Loren Gluckman1
Abstract - Rynchops niger (Black Skimmer) is a colonial-nesting seabird that nests on open,
sandy or gravel beaches. In densely populated Pinellas County, FL, Black Skimmer colonies
often occur on municipal beaches where there is relatively little mammalian predation, but
intense predation by Corvus ossifragus (Fish Crow) and Leucophaeus atricilla (Laughing
Gull). In 2013, a previously successful Black Skimmer colony experienced high egg-loss
due to Fish Crow predation. To determine if effigies would decrease egg depredation in the
middle of the breeding season, we monitored the number of crow-foraging passes over the
colony and number of eggs taken before and after we placed 6 crow effigies among the nests.
During 36 hours of monitoring, we observed 83 foraging passes and 28 eggs being taken
by Fish Crows. Neither foraging passes nor eggs taken decreased after the placement of the
effigies, and the Black Skimmers abandoned their nests. Future research should focus on
other methods to limit nest predation after the onset of the breeding season.
Introduction
Rynchops niger L. (Black Skimmer) is a colonial seabird that nests on open,
sandy or gravel beaches along the Baja peninsula and the Atlantic and Gulf coasts of
the US and Mexico (Gochfeld and Burger 1994). Nests consist of 1–4 eggs laid in a
shallow scrape, and both sexes incubate the eggs and care for the young (Gochfeld
and Burger 1994). Incubation lasts for 21–25 days (Erwin 1977), but egg-laying
may extend over several days to weeks as new birds arrive. Black Skimmers only
leave the colony for short periods to defend their nest or forage, primarily in the
evening (Yancey and Forys 2010). Egg predation by terrestrial predators such as
Procyon lotor L. (Raccoon), Canis latrans Say (Coyote), and Rattus spp. (rats);
and avian predators such as Larus sp. and Leucophaeus sp. (gulls) and Corvus sp.
(crows) has increasingly lowered Black Skimmer productivity in human-dominated
landscapes (Gochfeld and Burger 1994, O’Connell and Beck 2003).
In Pinellas County, located on the west coast of Florida, Black Skimmer colonies
often occur on municipal beaches lined by hotels and condominiums. Municipal
beaches have relatively few Raccoons and Coyotes, but high numbers of crows and
gulls (Forys et al. 2009). In 2005, 2010, and 2011, a colony of Black Skimmers
of 72–250 pairs nested successfully on Sand Key (27.95258°N, 82.83300°W) and
produced 22–72 fledges. In 2012, 172 pairs attempted to nest near the original colony
site, but experienced heavy egg predation by Corvus ossifragus Wilson (Fish
1Environmental Studies Discipline, Eckerd College, St. Petersburg, FL 33711. 2Clearwater
Audubon Society, 1255 South Druid Road, Belleair, FL 33756. *Corresponding author -
forysea@eckerd.edu.
Manuscript Editor: Michael Steinberg
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2015 Vol. 14, No. 4
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Crow). After complete egg-loss due to predation, the Black Skimmers abandoned
their nests and attempted to establish a new colony 1 km away where they experienced
a similar level of predation. After an early tropical storm (Tropical Storm
Debby), they stopped nesting for the season. In 2013, 62 pairs of Black Skimmers
attempted to nest about 1 km from the most recent 2012 colony location and again
lost most of their eggs to Fish Crows. All of these colonies were larger than the average
Black Skimmer colony size reported for Florida, which is >50 nests (Florida
Fish and Wildlife Conservation Commission 2014).
Lethal crow-control is ultimately the most effective short-term strategy to
reduce nest predation, but there are ethical and practical concerns about corvid
control in an urban setting, and crows from surrounding areas might quickly move
in after other flocks are removed if the crows are territorial (Goodrich and Buskirk
1995). Illness-induced aversion to egg eating (conditioned-taste aversion) has been
somewhat successful at decreasing corvid depredation of seabird eggs, but requires
the purchase of restricted-use chemicals and is most effective when done before the
onset of nesting (Avery et al. 1995, Nicolaus et al. 1983).
Both lethal control and use of emetic eggs require permits. In 2013, after Fish
Crows began taking eggs from the Black Skimmer colony at Sand Key, we contacted
the USDA Wildlife Services Office (Bartow, FL) for assistance. They suggested
we try using effigies to deter the crows while we awaited our pe rmits.
Although much of the research is anecdotal, studies have shown that crow effigies
(dead or artificial crows arranged to mimic dead crows) decreased corvid depredation
at California Sternula antillarum Lesson (Least Tern) colonies (Caffrey 1994). Entire
roosts of Fish Crows and Corvus brachyrhynchos Brehm (American Crow, Avery et
al. 2008) moved when effigies were combined with other scare tactics. Peterson and
Colwell (2014) found that effigies were successful in reducing the number of American
Crows visiting baits on beaches where Charadrius nivosus (Cassin) (Snowy
Plovers) nested, although the reduction was temporary. In all of these previous studies,
effigies were placed before many of the eggs were laid, discouraging the crows
before they had multiple successful experiences eating the eggs.
Often, managers do not expect problematic crow predation. By the time that a
significant number of eggs have been taken, it is impractical to use crow-management.
Thus, the purpose of our study was to determine if placing crow effigies in
a Black Skimmer colony would decrease the amount of egg depredation by Fish
Crows in the middle of the breeding season, after crows had positive experiences
eating eggs.
Methods
We conducted our study at the 72-pair Black Skimmer colony located on Sand
Key, Pinellas County, FL. The beach was 150 m wide and bounded by a seawall with
condominiums to the east. We monitored the colony at a distance of 10 m from the
outer boundary from sunrise, until all eggs were taken (~6–8 hours/day) from 30
May–3 June 2013. We assume that our presence did not disturb the Black Skimmers
because there were hundreds of other people on the beach and on any given day,
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2015 Vol. 14, No. 4
50–100 of whom were sitting closer to the colony than our observation point. On the
first 2 days of data collection, we placed no effigies in the colony. On the 3rd day of
data collection, we installed 6 artificial crow effigies throughout the colony. Avery et
al. (2008) used 1–5 effigies to move crow roosts, but we used 1 additional effigy to
make sure that they were visible throughout the colony. Similar to the Peterson and
Colwell (2014) study, we twisted the necks of the crow effigies to make them appear
dead. We hung 3 of these effigies from 1.5-m high curved plant-holders, and placed
the other 3 effigies on the ground. We purchased 4 of the effigies from the same company
used in the Avery et al. (2008) study, however the company subsequently closed
and we purchased the last 2 effigies from a store that sold realistic feather-covered
crows (http://www.wildlifetreasures.com/); these crows looked similar in size and
form to the crows used in the Avery et al. (2008) study.
We calculated the number of times crows flew low passes over the colony where
they appeared to be looking for eggs and the number of eggs taken each hour. To
determine if Fish Crow foraging and number of eggs taken was influenced by
the effigies, we used generalized linear models (GLM) with Poisson distribution
because we collected count data (McCullagh and Nelder 1989). Analyses were
conducted in R statistical software (R Core Team 2014). We also included in our
analysis time since sunrise because Black Skimmers lay most of their eggs before
1000 h (Erwin 1977). We screened for over-dispersion by comparing the residual
deviance to the degrees of freedom.
Results
We observed the colony for 11 hours before and 25 hours after installing the
effigies. Fish Crows foraged over the colony 83 times and took 28 eggs. Most of
the Fish Crows foraged alone, but we also noted groups of up to 3 birds foraging
together. The largest number of Fish Crows seen in the area at one time was 6. After
we placed the effigies, several crows flew to where they were placed and examined
them, but quickly resumed their previous activities.
Nineteen of the eggs were taken after the effigies were placed, including 1 egg
that was taken <10 minutes after we placed the effigies. We estimated the number of
Black Skimmers on eggs, but it was difficult to obtain accurate egg-counts/nest during
the day. However, by the end of each day there was at least one major disturbance
that scared the birds off their nests and revealed that all of the eggs were gone.
Controlling for hours since sunrise, effigies did not significantly influence the
number of foraging passes over the colony (Z = 0.271 P = 0.113), although there
were actually more foraging passes when the effigies were present (Fig. 1). Effigies
did not influence the number of eggs taken each hour ( Z = 1.262, P = 0.883).
The number of hours since sunrise did not significantly influence the number
of foraging passes by the Fish Crows over the colony (Z = -1.061, P = 0.229).
However, hours since sunrise significantly influenced the number of eggs taken
(Z =-2.721, P = 0.006). Fish Crows were less likely to take eggs later in the day,
presumably when there were fewer eggs. On 3 June, the last day of the study, the
Black Skimmers abandoned the colony and did not renest during the season.
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Discussion
Fish Crows did not appear to be deterred by the placement of effigies in the
Black Skimmer colony. The Fish Crows appeared to be spending most of their day
at the colony, were successful getting eggs from the colony, and did not seem to be
easily deterred from foraging in this area. We observed that aggressive behavior by
Fish Crows towards Black Skimmers increased over time, including one instance
of a crow dragging a Black Skimmer off its egg. Black Skimmers attempted to
mob crows, but crows quickly came back to take eggs. Research on Fish Crows is
relatively sparse, but studies have indicated that they are highly intelligent animals
that are not easily deterred by mobbing Least Terns (Voigts 1999). They have even
been observed to employ tool use to move Laughing Gulls off their nests to take
their eggs (Montevecchi 1978).
Ours is the first published study that measured Fish Crow egg depredation before
and after placement of an effigy. Avery et al. (2008) found that effigies were effective
in displacing roosts of crows, but they did not look at the efficacy of effigies
in altering foraging habits. Caffrey’s research (1994) was antectodal and involved
using actual dead crow carcasses hung on a perimeter fence prior to nesting.
Peterson and Colwell’s (2014) study was experimental and observed fewer corvids
on baited beaches with effigies than baited beaches without effigies, but the
assumption was that the effect was due to availability of an alternative food source
Figure 1. Number of times crows flew over the colony (passes) and number of eggs they
took before and after the effigies were placed in the colony .
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E.A. Forys, D. Hopkins, P. Ingham, M. Miller, and L. Gluckman
2015 Vol. 14, No. 4
rather than the presence of effigies. At our study site, the nearest seabird colony
without effigies was >15 km from the Sand Key colony .
It is important to note that all of the studies we cite that demonstrated success
with effigies were done before crows had eaten many eggs. Effigies might be a successful
deterrent if used before the onset of the breeding season. It also should be
noted that our research is based on a single colony on an urbanized beach. Future
research should be done at multiple Black Skimmer colonies at which effigies are
placed before the breeding season. In addition, data are needed to determine if the
use of additional scare tactics (provided these tactics don’t scare the Black Skimmers)
increases the power of effigies to deter crows.
Another important line of research would be to look at methods that could be
used in the middle of the season, after crows have begun eating eggs; perhaps
effigies combined with the sound of dying crows or other scare tactics would be
effective. Gorenzel and Salmon (1993) were able to disperse roosting American
Crows using sound alone.
Our results indicated that there were only a small number of Fish Crows involved
in the egg predation because the number of crows in the area consistently
ranged from 1 to 6. One of the Fish Crows had a crooked leg that made it easy
to identify. This bird was present every day of the study, suggesting that the Fish
Crows we observed were likely permanent summer residents at this beach. We
suggest that that early-season use of conditioned-taste aversion might be another
research avenue to pursue (Avery et al. 1995, Gabriel and Golightly 2014). As crow
abundance in urban areas continues to increase (Marzluff and Neatherlin 2006) and
seabird habitat decreases, managers will increasingly need to develop strategies to
reduce crow depredation of eggs.
Acknowledgments
Clearwater Harbormaster B. Morris and Clearwater Mayor G. Cretekos generously
granted permission to conduct this unusual study on a busy municipal beach. N. Douglass
provided support and helped implement the study. M. Van Wezel, I. Szwarc, and M. Miller
assisted with data collection. M. Steinberg and 2 anonymous reviewers provided helpful
suggestions for the manuscript.
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