Dads on duty: First account of nest sitting in barnacle ganders

In most Anseriformes (ducks, geese and swans) only females are known to incubate. The few exceptions where males have been described to incubate regularly are several species of swans (Cygninae) and whistling ducks (Dendrocygninae). Here we describe, never investigated quantitatively in any goose species, indicidents of male nest sitting in barnacle geese (Branta leucopsis) as a form of paternal care of nest attendance. Based on pictures from wildlife cameras we identified males, which sat on their nests when their mates took incubation recesses. Wildlife cameras were placed at nests of which either the male or female was fitted with a GPS neck collar in the year prior to this study. To attach transmitters, some geese were caught while defending their nests, thus we may have unintentionally selected bolder males which defended their nests more aggressively and were easier to catch. Nest sitting occurred relatively frequent, i.e. in 6/15 individuals. Our results show that males with collars were more likely to nest sit, but this does not deflect from the fact this behaviour is novel in geese. Nest sitting may even be more common as collared birds are more easily recognised, but also uncollared males have been observed to nest sit. We discuss several possible functions of this behaviour, i.e. against raiding of nests by aerial predators, thermal control of nest temperature, and intraspecific brood parasitism. At this time we cannot demonstrate a possible function, as chances of successful hatching were not increased in nests of males, which were nest sitting and we lack sample size for more in depth analyses. Lastly, we discuss whether male incubation is misleading in the waterfowl literature, as it is truly justified for only two species, the black swan and black-bellied whistling duck.