Foraging behaviour of thick-billed murres breeding in different sectors of the North Water polynya: an inter-colony comparison

Abstract

The North Water polynya is an area of open water that occurs year round between northwest Greenland and Canada. Oceanographic conditions differ between the western ('latent heat' polynya type) and extreme eastern ('sensible heat' polynya type) sectors of the polynya, and the effects of this variation on prey availability and foraging conditions for high trophic-level predators are unknown. Thick-billed murres Uria lomvia breed on both sides of the polynya, and we conducted inter-colony comparisons of their foraging ecology. We measured time allocation and foraging efforts of chick-rearing adults using electronic data-loggers which recorded dive profiles and flight activity. Murres on the western side of the North Water polynya foraged at relatively shallow depths and rarely (4.4% of dives) explored depths beyond 70 m. In contrast, murres on the eastern side searched for prey at > 70 m in 23% of all dives, and spent a greater share of each trip actively diving. The Canadian birds made foraging trips of longer duration than the Greenland murres, but they also spent more time 'resting' at the sea surface. Rest time at sea was apparently the only time buffer available for increasing foraging effort. The Greenland birds had little room for increasing foraging effort, while the Canadian birds had spare capacity for additional work. Maximum potential foraging ranges were equal at the 2 colonies (75% within 50 km), and the murres had access to approximately equally-sized areas of open sea. We estimate that the density of foraging birds at sea within the 50 km of colonies would be 6.5 times higher on the Canadian side due to the large breeding colony at Coburg Island. Intra-specific competition for food should increase with increasing colony size because the predators may reduce food resources within their foraging range. Nevertheless, the Canadian birds worked less than Greenland murres, indicating a relatively good food availability in the western part of the polynya. However, the higher foraging effort by Greenland murres paid off in a higher chick growth rate, so it remains unclear why the Canadian birds did not also make use of their spare capacity to increase foraging effort. Since high-level zooplankton/fish stock interactions are seldom part of oceanographic studies in the Arctic, seabird foraging behaviour and breeding ecology serve as two of the few indicators of possible local variation within the polynya ecosystem.

Seabird foraging behaviour indicates prey type

“…We converted flight time into distance assuming 75 km h -1 flight speed, ignoring variation in flight velocity with wind speed, load and other factors (Elliott & Gaston 2005). This calculation assumed that murres returned in a straight line from their foraging destination, as predicted from theoretical considerations for single-prey loaders and shown for murres from colonies in Iceland (Benvenuti et al 1998) and Greenland (Falk et al 2000(Falk et al , 2002. We assumed that the maximum depth of the last dive prior to returning to the colony represented the depth at which the prey item was collected because for all prey types final dives were on average shorter in duration, but not consistently shallower than the average for the entire final dive bout, suggesting that the final dive was aborted once a prey item was captured.…”

“…During 1999-2000, we used TDRs identical to those described by Falk et al (2000Falk et al ( , 2002 and Benvenuti et al (1998). The 'Benvenuti' TDRs (length: 80 mm;width: 23 mm (tip) to 30 mm (base); depth: 13 to 18.5 mm) housed a pressure sensor and 2 motion recorders.…”

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

“…The instruments used in our tests are modified versions of the logger designed by researchers at the Istituto di Elaborazione dell'Informazione, C.N.R., Pisa, Italy (Dall'Antonia et al, 1993), and used in tests on the homing strategies of pigeons and of Cory's shearwaters (Calonectris diomedea) (Papi et al, 1991;Dall'Antonia et al, 1995, 1999Bonadonna et al, 1997Bonadonna et al, , 2000Holland et al, 2000) and on the foraging strategies of marine birds (Benvenuti et al, 1998Baccetti et al, 2000;Dall'Antonia et al, 2000Falk et al, 2000Falk et al, , 2001Falk et al, , 2002Garthe et al, 2000).…”

Seasonal changes in diets of seabirds in the North Water Polynya: a multiple-indicator approach

“…These small subsurface feeders dive up to 35 m (Falk et al 2000). Thick-billed murres Uria lomvia (hereafter 'murres'; estimated population 350 000 pairs; Falk & Kampp 1997) are large subsurface divers that regularly dive to depths over 70 m (Falk et al 2002). Black-legged kittiwakes Rissa tridactyla (hereafter 'kittiwakes') are surface feeders that also make shallow dives (estimated population 30 000 pairs; McLaren 1982).…”