Although climate change has been implicated in population declines of tropical forest birds, there is a critical lack of data on the mechanisms underlying these declines. Attempts to link climatic factors to variation in adult survival, fecundity, or nest success have been largely inconclusive. Recent community-scale analyses have suggested that tropical birds may be less likely to breed under adverse conditions, but long-term data on individual reproduction are needed to test this hypothesis. Here we leverage 12 years of data on a lowland forest bird, the greater ani (Crotophaga major), to investigate how demographic parameters vary with phase of the El Niño – Southern Oscillation (ENSO), a major driver of climatic conditions in tropical wet forest. The likelihood of breeding and annual reproductive rate both decreased dramatically in El Niño-like years, with only 37.5% of adults attempting breeding in 2015 (a strong El Niño year). For birds that did breed, however, clutch size and daily nest predation rate were unaffected by climate. Of the local climate variables investigated, dry season length and the frequency of high temperatures were most closely associated with reproductive failure. These results indicate that El Niño conditions alter the demography of greater anis by reducing the likelihood of reproduction, a response that may be more widespread than currently recognized. We suggest that reduced reproduction under adverse conditions represents an important and understudied aspect of the life histories of tropical forest birds.

Data were collected from a field site in lowland Panama from 2007-2018 from a wild population of communally breeding greater anis. Individuals were tracked across years and their reproductive output recorded. Response variables in this data set include individual clutch size (number of eggs laid), likelihood of laying (yes/no), and nest predation rate (yes/no and exposure days). Predictors include several climate variables measured at the study site as well as possible biological covariates known to affect reproductive output (number of laying females in the nesting group, location of nest in either emergent vegetation (isolated) or shoreline vegetation). Raw data are presented in these files, but the nest abbreviations have been re-coded to prevent data plagiarism. Nest abbreviations in this data set are internally consistent to allow replication but they differ from the abbreviations used in the field and in other publications. Sample sizes and nest ID varies across analyses because not all nests experience the full nesting cycle (i.e. nesting groups that do not lay eggs are included in the analysis of likelihood of laying, but not in the analysis of clutch size or nest predation. Nests that were depredated prior to the completion of the laying cycle are included in the analysis of likelihood of laying and nest predation, but not in the analysis of clutch size since clutches were not complete). 

Metadata and notes are included in a tab of the Excel file. Explanations and definitions of the variables are given in the text of the manuscript. An additional ReadMe file has been uploaded as well.