Activity patterns and foraging behaviour of Audouin' s gulls in the Ebro Delta, NW Mediterranean

Daniel Oro,Santi Mañosa,Xavier Ruiz

doi:10.3989/scimar.2004.68n4605

Abstract

SUMMARY: Twenty Audouin’s gulls, Larus audouinii, breeding in the Ebro Delta (NW Mediterranean) were radio-tracked in 1998 to study their foraging behaviour and activity patterns. Some detrimental effects of tagging on the breeding success of the birds were detected, especially when both members of the pair were tagged. The results were actually constrained by the low number of locations due to natural breeding failure and failure in tag emission, as well as the adverse effect of tagging. However, through a combination of aircraft surveys at sea and a fixed station for automatic tracking of the presence of the birds at the colony, novel individual-based information of home ranges and activity patterns was obtained. Trawler fishing activity seemed to influence both the foraging range and habitat use: while trawlers operated, gulls overlapped their fishing grounds with vessels, probably to scavenge on discards. Very few locations were obtained during a trawling moratorium period, although they were all recorded in coastal bays and terrestrial habitats. During the trawling activity period, gulls ranged over a minimum convex polygon area of 2900 km2. Gulls were tracked up to 40 km from the colony, but some individuals were observed beyond 150 km while still breeding. Arrivals and departures from the colony were in accordance with the trawling timetable. However, most birds also showed some nocturnal foraging activity, probably linked to active fishing of clupeoids (following diel migrations) or to the exploitation of purse-seine fishing activity. Foraging trips lasted on average 15 hours: males performed significantly shorter trips than females, which spent more time outside the colony. The proportion of nocturnal time involved in the foraging trips was the same for males and females, but whilst all males initiated their trips both during the day and at night, some females only initiated their trips during the day. Hatching success was found to be related to foraging effort by males. Gulls spent on average ca. 38% of their time budget outside the nesting territory, representing the time devoted mainly to flying, foraging and other activities.

Highlights

Audouin’s gull, Larus audouinii, is an ecological outlier among the Laridae, especially with regard to its foraging ecology
Very few locations were obtained during a trawling moratorium period, they were all recorded in coastal bays and terrestrial habitats
During the trawling activity period, gulls ranged over a minimum convex polygon area of 2900 km2

Summary

Introduction

Audouin’s gull, Larus audouinii, is an ecological outlier among the Laridae, especially with regard to its foraging ecology. The species is a foraging specialist in feeding on shoals of clupeoids (mainly sardine Sardina pilchardus and anchovy Engraulis encrasicolus) at night, taking advantage of their diel vertical migration cycles. These prey can represent more than 85%. Of the biomass intake of Audouin’s gulls (Oro et al., 1997; Pedrocchi et al, 2002) This very narrow foraging niche width and specialised diet is very rare among gulls (which usually show a very generalist diet and opportunistic foraging behaviour) and was one of the main factors proposed to explain the reduced distribution of the species and its small total population size (de Juana and Varela, 1993). The species is one of the few endemic seabird species of the Mediterranean region and during the

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