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

The fitness costs of egg loss for Seychelles warblers (Acrocephalussechellensis)on Cousin Island are considerable because warblers have asingle-eggclutch and no time to lay a successful replacement clutch. Ontheislands of Cousin and Cousine, with equal densities of Seychellesfodies(Foudia sechellarum), nearly 75% of artificial eggs placedinartificial nests were predated by fodies after 3 days. OnAride Island with nofodies present, loss of artificial eggswas not observed. Female warblersincubate the clutch, and malewarblers guard the clutch when females areabsent. Deterrenceof fodies by male warblers is efficient: loss rate of eggsfromunattended warbler nests was seven times as high as from attendednests,and the more nest guarding, the lower the egg loss andthe higher the hatchingsuccess. Egg loss is independent ofthe amount of incubation by females. Thereis no trade-off betweenincubating and foraging by females. Nest guardingcompetes withforaging by males, and this trade-off has a more pronouncedeffecton egg loss when food availability is low. The transfer of breedingpairsfrom Cousin to either Cousine with egg-predating fodies or toAridewithout fodies allowed us to experimentally investigatethe presumed trade-offbetween nest guarding and foraging. OnCousine, individual males spent thesame amount of time nestguarding and foraging as on Cousin, and egg loss wassimilarand inversely related to time spent nest guarding as on Cousin.Malesthat guarded their clutch on Cousin did not guard theclutch on Aride butallocated significantly more time to foragingand gained better bodycondition. Loss of warbler eggs on Aridewas not observed. Time allocation toincubating and foragingby individual females before and after bothtranslocations remainedthe same.