Parental incubation patterns and the effect of group size in a Neotropical cooperative breeder

In many cooperatively breeding species, young (helpers) from one year help other adults raise offspring the following year. In contrast to helper effects during the nestling or postfledging stages of the avian breeding cycle, potential benefits from helpers during incubation are poorly studied. We analyzed 39 clutches and recorded 6,027 offbouts to document incubation behavior in the White-breasted Thrasher (Ramphocinclus brachyurus), an endangered Neotropical facultative cooperative breeder. Our goal was to test the prediction that cooperation confers benefits during incubation in terms of increased nest attendance and hatching success. We found that 65% of active hours (0500-2100) are spent on the nest, values somewhat lower than average for a tropical passerine with uniparental incubation. Highest incubation constancy was observed at a rare 4-egg clutch, which was attended by putative jointnesters. Excluding this clutch, differences in incubation behavior between pair and cooperative groups were subtle and context dependent. We found temporal variation in incubation behavior, whereby off-bout frequency declined as the breeding season progressed, but more quickly for cooperative groups. Maximum ambient temperature was also an influential abiotic predictor. Across both social group types, incubation constancy declined as temperature increased to similar to 30 degrees C, above which constancy remained high. As expected, we found that the behavior of birds with failed and successful clutches differed. Specifically, failed clutches experienced high early-season constancy, despite temperature data suggesting insufficient warming during that time, and successful cooperative clutches had higher late-season constancy than pairs. Other factors important in avian systems were not predictive of incubation behavior here, and in general, high individual variation for all incubation behaviors swamped most other sources of variation. Results from this work highlight the individuality in incubation behavior and suggest that breeding females in cooperative groups have more flexibility in shifting between incubation demands and maintenance behaviors than do lone pairs.