Effect of El Nino and La Nina on abundance of frugivorous and nectarivorous terrestrial birds in three tropical forests

El Nino and La Nina climatic oscillations have dramatic effects on population dynamics and community structure of different animals. For marine birds, El Nino phenomenon drastically increases their mortality and reduces their reproductive success. In terrestrial ecosystems, the lack of long-term longitudinal data limits our understanding of the impact of El Nino and La Nina on bird populations and communities. We analyzed changes in abundance of frugivorous (large, medium, and small) and nectarivorous birds on three tropical forest types (lowland, premontane and montane) during El Nino, La Nina and neutral years using data from 16 Christmas' Bird Counts in Costa Rica. Abundance of large and medium size frugivores increased during La Nina events, and the abundance of nectarivores during El Nino in the lowland forest, but neither of these events affected the abundance of small frugivores. In the montane forest, abundance of all four bird groups increased during El Nino, but decreased during La Nina events. Abundance of large, and small frugivores increased in the premontane forest during La Nina events, but other bird groups were not affected by La Nina. The abundance of small frugivores also increased during El Nino. We hypothesize that changes in abundance of frugivores and nectarivores during La Nina and El Nino events is probably correlated with fluctuations in food resources as it has been reported for other terrestrial tropical ecosystems.