Diel vertical migration of prey and light availability constrain foraging in an Arctic seabird

Studies of seabird-prey interactions often focus on biotic factors, such as prey abundance, seabird biomechanics and competition. In contrast, we examined the influence of abiotic factors, particularly weather, light and tide, on the diving behaviour of thick-billed murre (Uria lomvia) foraging in the Canadian Low Arctic. We found little evidence that tide and weather influenced dive behaviour. As visual predators, light availability limits foraging opportunities; however, prey often surface at night so there may be a trade-off between increased food availability and reduced foraging ability during low-light conditions. Our data lent support to both ideas, as dive depth increased with light availability and the proportion of vertically migrating schooling prey was highest during sunup and sundown. There was no difference in dive depth between sexes outside the period of sundown; males, which forage at night, dove shallower than females in the late afternoon, which we suggest is because they specialize on shallow prey often caught at night. Apparently, adaptation for higher oxygen stores or lower oxygen consumption in deeper-diving females overrode any adaptation for improved vision in night-specialist males. We concluded that light availability interacted with prey vertical migration to impact underwater foraging abilities of breath-hold divers.