THE EFFECTS OF FOOD, NEST PREDATION AND WEATHER ON THE TIMING OF BREEDING IN TROPICAL HOUSE WRENS

I tested three hypotheses that could explain variation in the timing o f breeding in populations of House Wrens (Troglodytes aedon) at four s ites in Costa Rica. The sites were located at 200-1,500 m elevation on both sides and on top of the central mountain range, and had climates differing in temperature, the severity of the dry season, and total r ainfall. For the first hypothesis, that breeding is timed to coincide with peaks in food availability, I monitored the monthly abundance of arthropod prey and wren clutch initiations at the four sites. Cross-co rrelation analysis showed that at three sites, wrens initiated clutche s several months prior to when prey levels were high. Indeed, breeding began when prey levels were at their annual low. At the fourth site, prey levels varied little throughout the year and House Wrens nested n early year round. These results indicate that sufficient food to produ ce eggs or feed nestlings may have been available throughout the year. Breeding appeared to be timed so that juvenile dispersal and molt occ urred when food was most plentiful. The second hypothesis, that breedi ng is timed to avoid seasons when nest predation is high, was not supp orted because the rate of nest predation did not vary temporally. The third hypothesis, that breeding is timed to avoid climatic events that can increase the physiological costs of reproduction, was not support ed at the three lower elevation sites. Clutch initiation at the highes t site, however, did not commence until the early dry season wind and mist subsided. The termination of breeding was not correlated with cli matic changes at any of the sites. Thus reproduction in tropical House Wrens seems generally to be timed to facilitate post-breeding activit ies, not activities associated with nesting itself.