Altruism and infidelity among warblers

In cooperatively breeding vertebrates, indirect fitness benefits1,2 are maximized by subordinates who choose to help their own closely related kin after accurately assessing their relatedness to the group's offspring3,4,5. Here we show that in the cooperatively breeding Seychelles warbler (Acrocephalus sechellensis), female subordinates help to raise new nestlings by providing them with food only when the offspring are being raised by parents who also fed the subordinates themselves when they were young3. These helper females use the continued presence of the primary female, rather than of the primary male, as their provisioning cue — presumably because female infidelity is rife in this species6, making their relatedness to the father less reliable.