1. Food availability has been considered one of the most important factors limiting bird populations, yet the effects of food abundance on non-breeding insectivores has rarely been investigated. We studied the effects of food abundance on the body condition of ovenbirds (Seiurus aurocapillus L.), a sexually monomorphic, ground-foraging, Neotropical–Nearctic migrant warbler during two winters in three habitats on their wintering grounds in Jamaica.
2. Prey biomass decreased from early to late winter in all habitats. Concurrently, ovenbird body mass, corrected for differences in structural size, decreased significantly in five of six habitat–year combinations. Only in second growth scrub habitat in the 1995–96 winter did ovenbirds show no decrease in corrected body mass, and during this period there was no significant decrease in ant biomass, the dominant prey item of ovenbirds in all habitats.
3. Significant positive correlations were found between ovenbirds' rate of feather regrowth and the biomass of ants on their home ranges in early winter, and between overwinter change in ovenbird body mass and the biomass of ants on their home ranges in late winter.
4. The results of both the habitat- and home-range-based analyses suggest a similar threshold of ant biomass (2·5–3·0 mg m−2), below which ovenbirds did not maintain their body mass.
5. The results suggest that late winter rainfall mediates the biomass of prey abundance, which in turn affects the ovenbirds' overwinter body condition.