Foraging behaviour and habitat use of a short-ranging seabird, the crested tern

Abstract

We used satellite tracking technology on the crested tern Sterna bergii, a seabird weighing <400 g. GPS units weighing <22 g were deployed on adult terns brooding young chicks. Individuals typically commuted to foraging grounds <40 km from the colony where their travel speeds slowed to ≤10 km h–1, presumably as prey encounter rates increased. Individuals undertook trips up to 4 h 17 min in length and 118 km in distance, and trip duration was positively correlated with the maximum distance and total distance traveled. Foraging behaviour, examined in relation to habitat characteristics (benthic habitat type, depth, sea surface temperature [SST], chlorophyll a [chl a]), was typically associated with warm (19 to 21°C), shallow (<20 m depth) waters that were relatively high in chl a (>0.5 mg m–3). The most well-supported model (generalised linear mixed model) of foraging behaviour indicated a positive relationship between time spent at sea, distance travelled and chl a, suggesting individuals spent relatively more time foraging at greater distances from the colony in zones of higher primary production. The timing and location of crested tern breeding may be linked to the 2-fold increase in primary production near Troubridge Island over the austral summer. Individual differences in the length (distance and duration) of foraging trips may reflect either prior knowledge of where prey aggregations exist, distinctions in individual niche use driven by the types or sizes of prey available, and/or alternate behavioural states (self feeding and provisioning). The restricted foraging range of crested terns while breeding may make them sensitive to competition with fisheries that operate within their foraging range.