Adjustment of paternal care in response to experimental variation in extra-pair mating opportunity in male zebra finches

Extra-pair paternity is prevalent in socially monogamous bird species with biparental care. Male extra-pair matings may coincide with paternal care provisioned to within-pair offspring. This may lead to a trade-off between obtaining additional fertilizations and securing offspring growth and survival through paternal care. In this pre-registered study, we manipulated the social environment of zebra finches to investigate how males trade off extra-pair mating versus providing paternal care. We compared paternal care provision between a social environment where two pairs bred together in a cage-resulting in an opportunity for extra-pair mating (Double-pair group)-to one with single breeding pairs (Single-pair group). We additionally measured plasma testosterone and corticosterone to identify hormonal correlates of male behavioral responses. To further increase the opportunity for extra-pair mating for the focal males from the Double-pair group we removed the male of the non-focal pair during chick rearing by the focal males. Contrary to our predictions, Double-pair males incubated more than Single-pair males. While we found substantial extra-pair paternity in the Double-pair group after removal of the non-focal males, male brood provisioning, chick growth and survival, and testosterone levels were not affected by the experimental increase in male extra-pair mating opportunity. Corticosterone levels were higher in Single-pair males between the first and replacement clutch. Refuting common assumptions, our findings provide no experimental evidence for a trade-off between extra-pair mating and paternal care. Males were able to pursue extra-pair fertilizations while they simultaneously secured the growth and survival of their within-pair offspring.