Benefits and Costs to Plain-Fronted Thornbirds (Phacellodomus rufifrons) of Interactions with Avian Nest Associates

Abstract

AnSTRACT.--From 1990 through 1993, I examined the interactions among a group of bird species in central Venezuela that use the multichambered stick nests of the Plain-fronted Thornbird (Phacellodomus rufifrons). Eleven species besides thornbirds, referred to as associates, used thornbird nests as nest sites and, in some cases, roost sites. During the 18-month study, 130 incidents of associates nesting in thornbird nests were recorded. More than 90% of thornbird territories observed for at least two breeding seasons (n = 33) had at least one associate nesting attempt. Four species accounted for over 86% of the attempts by associates: Cattle Tyrants (Machetornis rixosus), Stripe-backed Wrens (Campylorhynchus nuchalis), Saffron Finches (Sicalis fiaveola), and Troupials (Icterus icterus). Sometimes thornbird and associate nesting attempts overlapped, taking place within the same nest in different chambers, or in different thornbird nests within a territory. Nine of 27 (33%) thornbird nesting attempts at the beginning of the 1991 breeding season overlapped associate attempts, and 5 of 26 (19%) overlapped in 1992. Nesting with associates involved benefits and costs for the thornbirds. In 1991, thornbird nesting attempts that overlapped Troupial nesting attempts were no more likely to succeed (33% successful, n = 6) than attempts with no associate overlap (28% successful, n = 18). Thornbird attempts that overlapped those of other species, however, were significantly more likely to succeed than attempts with no overlap (100% successful, n = 3). In 1992, patterns were similar (0% successful with Troupials, n = 2; 10% successful with no associates, n = 21; 33% successful with other associates, n = 3), although high thornbird nest mortality and small sample sizes precluded statistical analysis. Thornbird nesting attempts that overlapped two associate nesting attempts (n = 7, 1991 and 1992 combined) had a high success rate (71%). Rainfall in the month before fledging was positively associated with the number of fledglings produced in 1991, but not in 1992. Nest-guarding behavior by associates and enhanced mobbing of predators (demonstrated in predator-presentation experiments) were likely mechanisms of enhanced reproductive success when thornbirds nested with associates. Thornbirds often were treated with aggression by associates, and circumstantial evidence indicated that Troupials sometimes preyed on thornbird eggs and nestlings. Valuable nest sites may lead coexisting species to converge, rather than diverge, in their use of this resource. Received 20 February 1995, accepted 26 July 1995.