NEST PREDATION AND DIFFERENTIAL INSULAR EXTINCTION AMONG SELECTED FOREST BIRDS OF CENTRAL PANAMA

Barro Colorado Island (BCI) is a 75-yr-old tropical forest isolate in Gatun Lake in the Republic of Panama, where depredation of understory bird nests is hypothesized to be more intense than in nearby mainland forest. Eight of 12 bird species in the terrestrial insectivore guild, with historical breeding records from BCI, are now extinct on the island. I tested the hypothesis that interspecific variation in nest design and placement underlies differential avian extinction from BCI. Selecting five species from the terrestrial insectivore guild, two that are extinct and three that persist on BCI, I constructed 200 hand-made mimics of their nests (40 each of the five types) using species-specific nest materials collected in local forest. Artificial nests, containing Coturnix quail eggs as bait, were placed on two island and two mainland plots for representative incubation periods (15 d) and checked at 3-d intervals for predation. Experiments were repeated in February, late April, and June of 1988 to assess seasonal shifts in predation pressure in relation to timing of breeding in understory birds, and temporal constancy of interspecific differences in nest loss rates. BCI nest losses were higher than on the mainland in two of the three experiments and predation intensity varied seasonally. Peak losses occurred in April-May (late in the dry season), immediately before the onset of breeding in understory insectivorous birds, suggesting nest predation may influence the timing of nesting activities. Two nest types designed to mimic the nests of species now extinct from BCI suffered lower predation on BCI and on the mainland than three nest types of species persisting on BCI. However, ratios of island to mainland predation rates were disproportionately higher for BCI-extinct nest types than for BCI-persistent nests in two of the three samples. Results suggest that over evolutionary time species-specific rates of nest loss may determine clutch replacement capability, thereby causing variation in tolerance of inflated insular nest predation. In this way, interspecific variation in nest design and placement among terrestrial insectivorous birds may underlie the pattern of differential extinction from Barro Colorado Island.