In the tropics, passerines are expected to have survival rates higher than those in temperate zones, though their post-fledging survival has rarely been quantified. Furthermore, because of increased care by adults, cooperative breeding species should have even higher rates of juvenile survival, but few data are available on such species also. Over three breeding seasons we examined factors relating to post-fledging survival of the cooperatively breeding Puff-throated Bulbul (Alophoixus pallidus) in an evergreen forest in northeastern Thailand. These factors included the presence of helpers, fledgling age, sex, body mass prior to fledging, date of fledging within a year, and variation by year. The probability of surviving the 8-week period of dependency was 0.61 ± 0.09 (SE), higher than most reports from the temperate zone. Survival probability was strongly associated with age though not with the presence of helpers. Weekly survival was lowest during the first week post-fledging (0.73 ± 0.08), while average weekly survival during the 7 subsequent weeks was constant at 0.97 ± 0.01. The other factors tested appeared to have little or no effect on survival. Understanding factors influencing post-fledgling survivorship provides insights to this limiting life-history stage, which is typically characterized by high mortality and affects population dynamics. The survivorship of fledglings we report adds to the body of evidence that the post-fledgling survivorship of tropical passerines is generally higher than that of temperate-zone species. However, fledglings from cooperative breeders may not experience significantly higher survivorship than those from noncooperatively breeding birds in the same system.
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Vol. 111 • No. 4