Nocturnal foraging by great skuas Stercorarius skua: implications for conservation of storm-petrel populations

Abstract

At St Kilda, Outer Hebrides, a large colony of great skuas Stercorarius skua feed extensively on one of the largest colonies of Leach's storm-petrels Oceanodroma leucorhoa in Europe, but little is known about the dynamics of this predator-prey system. Recently published population estimates of storm-petrels make it possible to estimate the impact of skua predation for the first time. Although skuas in the southern hemisphere catch petrels attending breeding colonies at night, it is not known whether congeners in the northern hemisphere also forage during the hours of darkness. We found (using radio-transmitters) that skuas regularly forage at night and (using light intensifying equipment) observed them catching storm-petrels at night. However, skuas also foraged during daylight hours, and it is unknown whether they might also catch storm-petrels at sea. Data on diet composition reveals that the proportion of storm-petrels in skua diet declined between 1996 and 1997, but remained constant thereafter. Although a large proportion of the storm-petrel prey is likely to consist of non-breeders, numbers consumed suggest that breeders and an unknown quantity of transients may also been eaten. The numbers of storm-petrels eaten are not sustainable and may result in substantial long-term population declines. Under current conditions, maintenance of large populations of both Leach's storm-petrels and great skuas at St Kilda appears to be mutually exclusive.

Examining the development of individual recognition in a burrow-nesting procellariiform, the Leach's storm-petrel

“…Priddel and Carlile, 1995;Votier et al, 2006). Moreover, many species disperse to other islands to breed, suggesting that developing or retaining a memory for the scent of the home burrow or colony would not be required for homing in pre-reproductive individuals.…”

Probable predation of Leach’s Storm‐petrelOceanodroma leucorhoaeggs by St Kilda Field MiceApodemus sylvaticus hirtensis

“…This coincided with heavy predation of adult Storm-petrels by a large population of Great Skuas Stercorarius skua (Votier et al 2006), estimated to consume approximately 15 000 adults each year (Phillips et al 1999). It is unclear whether this level of predation will continue but declines in alternative food sources may lead to increased reliance on seabirds as prey (Votier et al 2004).…”

“…Therefore, our findings most likely represent predation or scavenging and as the only predatory terrestrial mammal on Hirta, this is almost certainly by field mice. Skuas are known predators of adult Storm-petrels on St Kilda (Phillips et al 1999, Votier et al 2006) and although corvids have been observed excavating Stormpetrel burrows (Huntingdon et al 1996), such behaviour American Crow (C.brachyrhynchos) (?) * n (for fledging success in years 1991-1993) = 160; BP -breeding pairs; AOS -apparently occupied sites, (?)…”

Applied conservation services of the evolutionary theory

“…However, if the predator is only a facultative predator and an opportunistic feeder and scavenger, doubts can arise, especially because such predators are commonly abundant due to human activities and some of their prey can be endangered species (Oro et al 2009). Large seabirds such as gulls and skuas are facultative predator of long-lived smaller seabirds, many of them endangered (Oro et al 2005(Oro et al , 2009Stenhouse et al 2000;Votier et al 2006). Faced with this situation wildlife managers have to decide whether this interaction has to be allowed to occur (with or without modulation) or whether, on the contrary, needs to be extirpated or at least alleviated.…”