Abstract:Seasonal fluctuations in bird abundance are expected in semi-arid environments, but estimates may be biased if detectability is not considered. In a tropical dry forest in north-eastern Brazil, we evaluated whether bird abundance is highly seasonal, and associated with time-specific variability in detectability. We mark-recaptured birds with mist nets over three field visits (3487 records from 75 species), and used closed-capture models to estimate detectability and abundance in birds divided into three groups (all, residents, insectivores). In the two dry periods, the best models resulted in capture estimates at least three times larger than recapture, and both estimates were twice that of when rains occurred on the day preceding sampling. Abundance varied between dry and wet periods from 4.0 (from 115 ± 34 to 479 ± 144) to 13 times (183 ± 8 to 2463 ± 351). Estimates were 1.5–3.2 times greater in the dry period when behavioural responses of birds were excluded from capture-recapture models. Meanwhile, in the wet period the relative abundance was between 33–76% smaller than best-fit models estimated. This study found variation in avian abundance greater than that observed in other Neotropical dry forests, and indicates that biases may be common when not including detectability.
Studies in the tropics suggest a regional similarity in survival rates of adult birds, however, this literature often overlooks species in semi‐arid tropical environments. Bird survival in seasonally dry environments (e.g. seasonally dry tropical forests – SDTF) may be lower than that in more constantly wet areas (e.g. tropical rainforests – TRF), especially if the birds are negatively affected by seasonal rainfall or food‐limitation. However, survival could be similar across these tropical environments, as the asymmetry between young and adult mortality tends to be high in all tropical areas, and the higher risk of mortality in young animals may favour adult survival (residual reproductive investment), regardless of the local climatic conditions. To fill this knowledge gap, we tested the hypothesis that bird survival is similar between seasonally dry (SDTF) and constantly wet (TRF) Neotropical environments. We estimated the apparent survival of 27 South American bird populations from three SDTF areas, and 39 populations from a TRF. Apparent survival was estimated from CJS models fitted using a Bayesian structure and the resulting variation in survival rates between study areas and with body mass was explored using a Bayesian phylogenetic mixed model. Apparent annual survival of passerines did not differ between areas (geometric mean of survival: SDTF = 0.50, 0.56, 0.64; TRF = 0.58), but body mass was positively associated with survival. The variation in bird survival was partially explained by phylogenetic relationships among species. Our results suggest that bird survival is regionally similar in Neotropical forests, despite the climatic variation. We discuss possible physiological and behavioural mechanisms adopted by birds in SDTF to attenuate effects of environmental seasonality on survival.
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