Predation

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Published on December 30, 2007

Author: Beverly_Hunk

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Slide1:  Nest Predation in Suburbia: Effects of Predator Guards, Effects on Bluebirds Sarah Hatfield, Christina Rockwell, Alicia Bever, Mark Stanback Department of Biology, Davidson College, Davidson, NC Abstract 1845 nests 1183 succ 661 fail Max avail to egg preds 1845 Max avail to chik preds 1490 Egg pred 235/1845 Chik pred 160/1490 Mam chick 78 / 1490 Mam egg 114/1845 Snake chick 65/1490 Snake egg 115/1845 We examined nest success and clutch size in eastern bluebirds (Sialia sialis) nesting in “safe” and “unsafe” cavities on golf courses near Davidson, NC. From 1999-2001, all nestboxes were tree-mounted, allowing us to assess rates of depredation on vulnerable nests. Depredation by mammals and snakes was roughly equivalent, as was depredation at the egg and chick stages. In 2002, half of the nestboxes on each course were transferred to metal poles with stovepipe-style predator guards. Predator guards had a dramatic effect on nest success. Combining all years, vulnerable nests suffered a depredation rate of over 25% (vs. 3% for safe boxes). Predator guards also had a significant effect on clutch size, with females laying smaller clutches in vulnerable nestboxes (independent of nest date effects). This represents the first experimental demonstration that birds can adjust their clutch size in response to perceived vulnerability. Methods Photo: Carole Franklin Conclusions Snakes and mammals have similar impacts Eggs and chicks suffer similar depredation rates Predator guards dramatically reduce depredation. Nest vulnerability had a significant effect on clutch size, suggesting that bluebirds invest more when success is likely. Golf courses nationwide are being encouraged to implement nestbox programs for cavity nesting birds such as bluebirds. Although participating courses often report substantial occupancy rates, little is known about the effect of nest depredation on such programs. This is problematic in that suburban golf courses provide excellent habitat for a variety of bluebird nest predators, including raccoons, cats, o’possums, and snakes. Because nest predation constitutes one of the most powerful selective pressures on avian life history traits, we wanted to assess the impact of nest predation on bluebirds breeding on suburban golf courses. We initially monitored nest predation over several years, then provided half the boxes on each course with predator guards. This allowed us to not only document the nature of nest depredation and the efficacy of predator guards, but also test (for 2002-2003) whether bluebirds respond to the availability of safer nest sites by adjusting their reproductive investment. Since the spring of 1999, the Avian Ecology Lab at Davidson College has monitored approximately 250 identical tree-mounted nestboxes on seven local golf courses. In early 2002, half the nestboxes on each course were transferred to 1.75m aluminum poles with ERVA stovepipe-style predator guards. In each year, all nestboxes were checked weekly from mid-March through early August for nesting activity and evidence of predation. We did not consider nest usurpation by house sparrows, house wrens, or flying squirrels to be depredation. Introduction Acknowledgements: For assistance in the field we thank Morgan Check. For access and carts, we thank the Birkdale, Mallard Head, NorthStone, Peninsula, Point, River Run, and Skybrook golf clubs. Funding was provided by a NSF REU Site Grant, an Associated Colleges of the South Grant, a Robert Stone Environmental Grant, and a Davidson College Faculty Research Grant. Cause of Depredation Predator Cases (1999-2004) Snake 170 Mammal 175 Timing of Depredation Nesting Stage Cases (1999-2004) Egg 197 Chick 155 Fisher Exact Test P < 0.0001 Predator guards reduced depredation dramatically 1430 311 Source df SS F P Safety 1 2.49 6.48 0.011 Date 2 93.10 121.16 <0.001 Year 1 9.59 24.96 <0.001 Model 11 1 22.67 29.03 <0.001 R2 = 0.23, n = 1060 Clutch size was larger in safe nests

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