Digging in! burrowing penguins and surface predators

A native Australian corvid, Little Raven Corvus mellori, has emerged as a dominant and problematic predator of an ecologically and economically important seabird, the Little Penguin Eudyptula minor. In 2013, ravens successfully preyed upon clutches/broods of the burrow-nesting penguin, especially those in shallower, more accessible burrows. Individual raven size might mediate which ravens attack penguins; smaller ravens may be able to enter burrows more easily; alternatively, larger ravens may be more capable of overcoming penguin parental defence. Here, we consider the predator-prey interactions to assess whether associations with burrow characteristics persisted and examine whether raven size mediated propensity to prey upon penguin eggs. We compared data from the 2013 and 2015 penguin breeding seasons at Phillip Island (Victoria, Australia) for differences in clutch survival and burrow characteristics. We also examined raven morphometrics to determine if any physical differences existed between known burrow-predators ("culprits") and other birds. During the 2013 breeding season, penguins suffered clutch losses of 61.1% compared with 33.9% in 2015. Burrow characteristics changed between seasons-most noticeably, burrows were deeper in 2015 and ravens no longer discriminated between burrow characteristics. Culprits were heavier and larger. While we could not eliminate inter-annual variability as a possible contributing factor, our results were consistent with rapid adaptation of prey and predator to intense emergent depredation. Penguins may now construct deeper burrows, and ravens evidently no longer select more vulnerable burrows. Larger ravens may be more capable of approaching and attacking penguin burrows.