Foraging behaviour and time allocation of chick‐rearing Razorbills Alca torda at Græsholmen, central Baltic Sea

Abstract

A new type of bird‐borne data logger, which stores data from flight and depth sensors at pre‐set intervals, was used to investigate the foraging pattern and diving behaviour of chick‐rearing Razorbills Alca torda breeding on the islet of Græsholmen (central Baltic Sea, Denmark). The instruments recorded all relevant events in the 35 foraging trips accomplished by six different individual birds; a seventh bird, equipped with a direction recorder, provided information on directional preferences exhibited on seven different trips. A foraging trip usually consisted of a number of flights interrupted by a small number of dives probably performed to explore the site for prey availability. True foraging occurred during the last stops in the outbound trip, after which the bird returned to the nest with a single flight or a sequence of a few flights. The duration of nocturnal trips was significantly longer than diurnal trips, possibly due to a preference for the familiar marine environment shown by birds when they are not ‘on duty’ in the nest. The majority of dives consisted of V‐shaped dives in which birds descend to a maximum depth and then start ascending. A clear diurnal variation of the dive depth, which never exceeded 43 m, was recorded. Our results show that the new types of recording devices can be used to answer basic questions about the foraging strategies of marine birds.

Seabird foraging behaviour indicates prey type

“…The V-pelagic and U-benthic dichotomy had been deduced previously because pelagic foragers generally have V-shaped dives (Benvenuti et al 2001, Schreer et al 2001, Kuroki et al 2003, whereas benthic foragers generally have U-shaped dives (Rodary et al 2000, Schreer et al 2001, Gazo et al 2006. Nonetheless, species with strongly stratified epipelagic prey also show U-shaped dive patterns (Chappell et al 1993), and some species show both dive patterns when foraging.…”

Diving behavior of blue-footed boobies Sula nebouxii in northern Peru in relation to sex, body size and prey type

“…The loggers (29 g, 23 × 19 × 97 mm) accounted for < 2% of a bird's body mass (below the 5% limit recommended by the Ornithological Council Guidelines: www.nmnh.si.edu/BIRDNET/GuideToUse). These devices carried a depth meter and a flight sensor manufactured by the Istituto di Elaborazione dell'Informazione, CNR, Pisa, Italy, which have been field tested in other seabirds such as the thick-billed murre Uria lomvia , the razorbill Alca torda (Benvenuti et al 2001), and the northern gannet (Garthe et al 2000). The depth meter had an operative range of 0 to 70 m and 1 m resolution.…”

“…The depth meters were used to determine dive depths, dive duration, timing of dives and number of dives per trip. The flight sensor was a modified microphone with a membrane activated by body movements and wing flapping, so the duration of different at-sea activities such as resting on the water, traveling (including plunging time), and diving could be measured by recognizing changes in the intensity and frequency of signals (Benvenuti et al 2001, Lewis et al 2002. The memory capacity of the dataloggers was 128 kB; the flight sensor was set to record data every 4 s, whereas the depth sensor collected data at its highest resolution of 2 s intervals.…”

Preliminary results on the foraging ecology of Balearic shearwaters (<i>Puffinus mauretanicus</i>) from bird-borne data loggers

“…Actually, we cannot rule out the possibility that the devices had an effect on the behaviour of the marked birds. However, the following facts make us confident that the data reported by our instruments are not drastically different from natural behaviour: 1) the weight of our devices was about 4-5% of the birds' body mass, a percentage which should not significantly alter their behaviour (see rerefences in Benvenuti, 1993;Benvenuti et al, 2001); 2) the foraging behaviour of the birds with the heaviest type of logger (equipped with a depth meter) was similar to that of the two birds tagged with a lighter type (equipped with a water switch); 3) the weight of the devices, with respect to the birds' body mass was similar to that in other successful studies of marine bird foraging ecology (Croll et al, 1992;Benvenuti et al, 1998; and 4) the low recapture rate of tagged birds (50%) was plausibly not due to nest abandoning after the experimental manipulation, but to raw sea conditions, which prevented us from performing routine recapture attempts.…”