Natural History and Reproductive Biology of the Communally Breeding Greater Ani (Crotophaga major) at Gatún Lake, Panama

Abstract

ABSTRACT.-The Greater Ani (Crotophaga major) is the least well-known of the communally breeding crotophagine cuckoos, although it is locally abundant in Panama and northern South America. We present substantial new life history information from 87 breeding groups of Greater Anis at Gatún Lake, Panama, and the first description of their conspicuous, highly stereotyped communal displays. Breeding groups were composed of two to five socially monogamous pairs; no pairs nested singly. Seven groups also included an unpaired individual, which in three cases was confirmed to be a 1-year-old male from the previous year's nest. Groups of two and three pairs were most common (accounting for 75 and 20% of groups, respectively); groups containing more than three pairs were rare and their nests were abandoned before incubation began. Eggs were large (,17% of adult body mass) and varied greatly in size (19-37 g). Egg and nestling development were exceptionally rapid: eggs were incubated for 11-12 days and nestlings were capable of leaving the nest after 5 days, although adults continued to feed fledglings for several weeks. On average, each female laid 4.3 6 0.9 eggs; three-pair groups had larger overall clutch sizes than did two-pair groups. The first 2-3 eggs to be laid were usually ejected from the nest by other group members, and number of ejected eggs increased with group size. Thirty-seven nests (43%) fledged at least one young successfully; snakes (Pseustes, Spilotes, Boa) and white-faced capuchin monkeys (Cebus capuchinus) were identified as nest predators.

Social living without kin discrimination: experimental evidence from a communally breeding bird

“…Approximately 60 % of nesting units consist of two breeding pairs, 30 % of three pairs, 8 % of four or more pairs, and 2 % of lone pairs (Riehl 2011;CR 2006CR -2014. Approximately 15-20 % of nesting groups also contain at least one unpaired Bhelper,^typically a non-breeding male from a previous nesting attempt that has delayed dispersal (Riehl and Jara 2009). For this study, we did not include groups that contained non-breeding helpers so as to restrict the experiments to nesting groups where all of the adult group members were reproductive.…”

“…For this study, we did not include groups that contained non-breeding helpers so as to restrict the experiments to nesting groups where all of the adult group members were reproductive. The habitat at this study site is tropical lowland forest bordering Gatún Lake; anis build large, open-cup stick nests along the water's edge, in tree branches overhanging the water or in emergent vegetation close to the shoreline (Riehl and Jara 2009). Nesting is restricted to the rainy season, typically late June-late September.…”

“…8 months after fledging; approx. 85 % of offspring then disperse and the remainder stay with their natal groups as non-breeding helpers for up to 2 years (Riehl and Jara 2009;Riehl 2011).…”

“…In order to collect at least 12 h of data per nest, total observation periods varied from 2 to 4 days across nests due to weather (rain events), limits on camera battery life, and logistics. Details of camera placement and protocols followed Riehl and Jara (2009). A pilot test in which a human observer simultaneously recorded food deliveries to nests indicated that cameras captured only ∼90 % of food deliveries, since some visits were too rapid to be fully captured by the camera (CR unpubl.…”

“…Each pair typically contributes three to four eggs to the shared clutch, and the final clutch size ranges from 6 to 15 eggs depending on the number of adults in the nesting group. Starvation is responsible for approximately 8-13 % of nestling mortality (Riehl and Jara 2009). Therefore, adult anis could theoretically increase their reproductive fitness by preferentially feeding their own offspring in the mixed clutch, since each adult is closely related to some of the nestlings and unrelated to the rest.…”