Fruit abundance may fine-tune timing of reproduction of the Chilean Elaenia (Elaenia chilensis), a long-distance migratory bird in South America

Timing of reproduction has a great impact on the breeding success of birds because a mismatch with the moment when environmental conditions are warm or when food is most plentiful can reduce nestling survival and increase the energetic cost of parental care. Consequently, birds synchronize gonadal maturation with the most favourable environmental conditions, using photoperiod changes throughout the year as an initial proximal cue. Additionally, non-photic cues, such as temperature and food abundance, may be necessary to fine-tune reproductive timing. However, the influence of non-photic cues on finely tuning reproductive timing is not yet fully understood for migratory birds. Here, we evaluate how much non-photic cues influence the reproductive timing of the Chilean Elaenia Elaenia chilensis, a long-distance migrant that reproduces in the Andean-Patagonian Forest. We assessed associations of mean temperature, ripe fruit and arthropod abundances with the number of nests in the laying period, and also with the number of nests with hatchlings. In both analyses we used cross-correlations and partial least squares path modelling. Mean temperature was not consistently associated with the breeding phenology of Chilean Elaenias. The increase in number of nests in the laying period was preceded by the increase in caterpillar abundance and coincided with the increase in ripe fruit abundance. The timing of nests with hatchlings matched with the timing of highest ripe fruit abundance. Both types of food could contribute to the beginning of reproduction of birds and be used as proximal cues by Chilean Elaenias. Ripe fruits would also be beneficial for nestling growth because parents feed them with fruits, and might potentially play an ultimate role in reproduction. Because ripe fruit abundance was related to the egg-laying and hatching stages, it may be the main cue used by Chilean Elaenias to fine-tune reproductive timing. These findings allow advances in our understanding of the importance of non-photic cues in the reproductive phenology of migratory birds and also generalize our knowledge among regions and taxa, as most studies on this topic focus on the Northern Hemisphere.