Living with strangers: direct benefits favour non-kin cooperation in a communally nesting bird

Abstract

The greater ani (Crotophaga major), a Neotropical cuckoo, exhibits an unusual breeding system in which several socially monogamous pairs lay eggs in a single nest and contribute care to the communal clutch. Cooperative nesting is costly-females compete for reproduction by ejecting each other's eggs-but the potential direct or indirect fitness benefits that might accrue to group members have not been identified. In this study, I used molecular genotyping to quantify patterns of genetic relatedness and individual reproductive success within social groups in a single colour-banded population. Microsatellite analysis of 122 individuals in 49 groups revealed that group members are not genetic relatives. Group size was strongly correlated with individual reproductive success: solitary pairs were extremely rare and never successful, and nests attended by two pairs were significantly more likely to be depredated than were nests attended by three pairs. Egg loss, a consequence of reproductive competition, was greater in large groups and disproportionately affected females that initiated laying. However, early-laying females compensated for egg losses by laying larger clutches, and female group members switched positions in the laying order across nesting attempts. The greater ani, therefore, appears to be one of the few species in which cooperative breeding among unrelated individuals is favoured by direct, shared benefits that outweigh the substantial costs of reproductive competition.

Figure 1.

Percentage (±95% CI) of successful nests (black bar, left axis) and average individual reproductive success (white bar, right axis) with respect to group size of communally breeding greater anis.

Figure 2.

Percentage (±95% CI) of greater ani breeding groups that nest in shore or emergent vegetation. White bars, two-pair groups; black bars, three-pair groups.

Figure 3.

Six measures of reproductive fitness (mean ± s.e.) for female greater anis in communal breeding groups with respect to group size (two-pair, white diamonds, or three-pair, black diamonds) and position in the laying order (first-laying, A; second-laying, B or third-laying, C). (a) Total number of eggs laid, (b) number of eggs lost to ejection, (c) number of eggs incubated in the final clutch, (d) number of eggs that survived to hatching, (e) number of nestlings that survived to fledging and (f) number of fledglings produced per egg laid. Asterisks indicate statistically significant (p < 0.05) differences between females within groups of the same size. Comparisons between groups of different sizes, and details of statistical tests, are given in the text.