The choice of the appropriate time to reproduce is important for breeding and survival in birds, since reproduction demands a great amount of energetic investment. Resident species have a better capacity for perceiving environmental changes through the year and, thus, show greater flexibility in the choice of the appropriate time to breed. On the other hand, migratory species probably have a short time interval to perceive environmental changes in the breeding areas, since they stay in these areas only part of the year. The study of mechanisms that determine the breeding period of birds allows a better understanding of how species are able to cope with annual variations in environmental conditions for proper breeding. The present study had as its main objective the evaluation of which factors determine the breeding period of Elaenia chiriquensis, a migratory passerine of the Cerrado (savanna) of Central Brazil. I evaluated: a) how migration dates interfere in the period of E. chiriquensis occurrence and breeding in the breeding area, b) how climatic factors may be used to adjust the breeding period, and c) if the breeding period may be adjusted relative to increased risk of nest predation. I conducted the study at “Estação Ecológica de Águas Emendadas” (ESECAE), Distrito Federal, Brazil, in a 100 ha (1 km x 1 km) plot. I collected data by banding birds (January 2003 - December 2007), through point counts (from August to December 2005 to 2007) and through nest searches and monitoring (from August to December 2003 to 2007). Climatic data were obtained from a climatologic station (EMBRAPA Cerrados) located 11 km in a straight line from the grid. The reproductive window of E. chiriquensis was fixed and was constrained by its migratory period (which is fixed) and not by environmental factors directly related to the breeding habitat. Despite the small flexibility in the reproductive window, the breeding starting date, and to a lesser extent, finishing date varied somewhat in the five years of study. Thus, E. chiriquensis had the ability to partially adjust its breeding period in response to environmental changes in the breeding site. The beginning of the breeding period of E. chiriquensis was strongly correlated with a lower daily temperature range and with an increase in relative air humidity. The end of egg-laying was more fixed than the initiation of egg- laying, and no climatic factor analyzed explained this. Possibly, the end of breeding is related to endogenous factors, or to long-term signals. Despite being the most important cause of nest failure in E. chiriquensis nest predation apparently does not exert selective pressure in the adjustment of the species breeding period. On the contrary, predators showed a functional response to E. chiriquensis nest density. Also, since predators changed their predation strategies among years, it is unlikely that E. chiriquensis has a selective response to risk of nest predation.
