Many birds roost communally during at least part of their annual cycle, suggesting that for themthe advantages of living in a group outweigh the disadvantages. However, perch sites within a roost may vary inquality because of differences in degree of exposure to the elements, predators, and fecal droppings. Individualsshould select perches in the roost that minimize costs while enabling them to experience the benefits of communalroosting. We studied communally roosting Turkey Vultures (Cathartes aura) in northeastern Iowa (USA) fromlate August to mid-October, when hatching-year (HY) birds had joined the roost and were distinguishable fromafter-hatching-year (AHY) birds. On 82 d during our 4-yr study (2004–2007), we noted the age class and perchposition of vultures on two communication towers used as a preroost site. Perches used by vultures were classifiedas top-level (with no perches above them) or lower-level (with other perches above them). Top-level perches werepreferred by Turkey Vultures. Of 1713 birds recorded, 71 were on top-level perches, even though only 39 ofavailable perches were top-level. Vultures did not use lower perches if top perches on that tower were unoccupied.The percentage of birds using lower perches increased as the number of vultures present increased, suggestingthat top-level perches were occupied first. AHY birds used top-level perches more often than expected and HYbirds used top-level perches less often than expected, implying that age-related dominance affected perch selection.On 61 of 82 d (74), top-level perches of both towers were occupied and, on 8 d (10), only top perches onone tower were occupied. However, on 13 d (16), both top-level and lower-level perches were occupied on onetower while no vultures perched on the other tower, suggesting that social attraction to other vultures can overridea general preference for top-level perches. Thus, our results provide evidence that social attraction, age-relateddominance, and preference for higher perches are proximate factors influencing perch selection in communallyroosting Turkey Vultures. Ultimate factors that may be responsible for Turkey Vultures preferring higher roostingperches are reduced risk of predation, less exposure to fecal droppings that might reduce their plumage quality, andbetter visual information for locating food sources.
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