Predation risk affects trade-off between nest guarding and foraging in Seychelles warblers

The fitness costs of egg loss for Seychelles warblers (Acrocephalus sechellensis) on Cousin Island are considerable because warblers have a single-egg clutch and no time to lay a successful replacement clutch. On the islands of Cousin and Cousine, with equal densities of Seychelles fodies (Foudia sechellarum), nearly 75% of artificial eggs placed in artificial nests were predated by fodies after 3 days. On Aride Island with no fodies present, loss of artificial eggs was not observed. Female warblers incubate the clutch, and male warblers guard the clutch when females are absent. Deterrence of fodies by male warblers is efficient: loss rate of eggs from unattended warbler nests was seven times as high as from attended nests, and the more nest guarding, the lower the egg loss and the higher the hatching success. Egg loss is independent of the amount of incubation by females. There is no trade-off between incubating and foraging by females. Nest guarding competes with foraging by males, and this trade-off has a more pronounced effect on egg loss when food availability is low The transfer of breeding pairs from Cousin to either Cousine with egg-predating fodies or to Aride without fodies allowed us ro experimentally investigate the presumed trade-off between nest guarding and foraging. On Cousine, individual males spent the same amount of time nest guarding and foraging as on Cousin, and egg loss was similar and inversely related to rime spent nest guarding as on Cousin. Males that guarded their clutch on Cousin did not guard the clutch on Aride bur allocated significantly more time to foraging and gained better body condition. Loss of warbler eggs on Aride was not observed. Time allocation to incubating and for-aging by individual females before and after both translocations remained the same.