UC Santa Cruz

Cooperative breeding and conspecific brood parasitism can both be favored by ecological saturation of breeding territories or nest sites. Here, we develop a model that links these alternative reproductive tactics by focusing on nonnesting females (S) that either breed cooperatively with a nesting female (N) or parasitize a third, outside host female (H). We find that cooperative breeding is more likely to evolve with increasing relatedness of cooperating females (S or N) to the outside host female (H) and with increasing costs to the hosts for receiving parasitic eggs. Conversely, cooperation is less likely with increasing kinship between the two potentially cooperative nesters (S and N). This is because even the nesting female gains higher inclusive fitness as long as the number of parasitic eggs (of her otherwise potentially cooperating partner) is sufficiently high. We find the relationship between kinship and reproductive skew within cooperative nests can be either positive or negative depending on the fecundity of parasites versus nesting females. We also find that either of the cooperatively nesting females is more likely to tolerate a smaller fraction of group reproduction as kinship with the host female increases and as the host reproduces more (relative to the parasite) in outside nests. Finally, our model predicts that, as the outside option of conspecific brood parasitism becomes more profitable, helping behavior (zero reproduction by one female) is less likely to evolve in cooperatively breeding groups.