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Do Effigies Deter Fish Crows Hunting in a Black Skimmer Colony Mid-season?
Elizabeth A. Forys, David Hopkins, Paul Ingham, Maggie Miller, and Loren Gluckman

Southeastern Naturalist, Volume 14, Issue 4 (2015): 635–640

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Southeastern Naturalist 635 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 Southeastern Naturalist E.A. Forys, D. Hopkins, P. Ingham, M. Miller, and L. Gluckman 2015 Vol. 14, No. 4 636 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, Southeastern Naturalist 637 E.A. Forys, D. Hopkins, P. Ingham, M. Miller, and L. Gluckman 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. Southeastern Naturalist E.A. Forys, D. Hopkins, P. Ingham, M. Miller, and L. Gluckman 2015 Vol. 14, No. 4 638 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 . Southeastern Naturalist 639 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. Literature Cited Avery, M.L., M.A. Pavelka, D.L. Bergman, D.G., Decker, C.E., Knittle, and G.M. Linz. 1995. Aversive conditioning to reduce Raven predation on California Least Tern eggs. Colonial Waterbirds 18:131–138. Avery, M.L., E.A. Tillman, and J.S. Humphrey. 2008. Effigies for dispersing urban crow roosts. Proceedings of the Vertebrate Pest Conference 23:84–87. Caffrey, C. 1994. California Least Tern breeding survey, 1993 season. The Regents of the University of California, Los Angeles, CA. Final report to California Department of Fish and Game, Wildlife Management Division, Nongame Bird and Mammal Section Rep. 94-07, Sacramento, CA. 39 pp. Erwin, R.M. 1977. Black Skimmer breeding ecology and behavior. The Auk 94:709–717. Southeastern Naturalist E.A. Forys, D. Hopkins, P. Ingham, M. Miller, and L. Gluckman 2015 Vol. 14, No. 4 640 Florida Fish and Wildlife Conservation Commission. 2014. Florida shorebird database: Annual report – 2013. Available online at http://flshorebirdalliance.org/media/27007/ FSD_AnnualReport2013_FINAL.pdf. Accessed 14 March 2015. Forys, E.A., A.A. Ormsby, and M. Borboen. 2009. Can education help make Florida municipal beaches more suitable for beach-nesting birds? Florida Fish and Wildlife Conservation Commission. Tallahassee, FL. Gabriel, P.O., and R.T. Golightly. 2014. Aversive conditioning of Steller’s Jays to improve Marbled Murrelet nest survival. The Journal of Wildlife Management 78:894–903. Gochfeld, M., and J. Burger. 1994. Black Skimmer (Rynchops niger). No. 82, In A. Poole and F. Gill (Eds.). The Birds of North America. Cornell Laboratory of Ornithology, Ithaca, NY. Available online at http://bna.birds.cornell.edu/bna/species/108/articles/ introduction. Goodrich, J.M., and S.W. Buskirk. 1995. 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Gull predation limits nesting success of terns and skimmers on the Virginia barrier islands. Journal of Field Ornithology 74:66–73. Peterson, S.A., and M.A. Colwell. 2014. Experimental evidence that scare tactics and effigies reduce corvid occurrence. Northwestern Naturalist.95:103–112. R Core Team. 2014. R: A language and environment for statistical computing v. 2.14.3. R Foundation for Statistical Computing, Vienna, Austria. Avalable online at http://www.Rproject. org/. Accessed 17 March 2014. Voigts, D.K. 1999. Observations of a colony of roof-nesting Least Terns, 1988–1997. Florida Field Naturalist 27:103–108. Yancey, M.R., and E.A. Forys. 2010. Black Skimmers feed when light levels are low. Waterbirds 33:556–559.