Behav Ecol Sociobiol (2004) 56:81–88 DOI 10.1007/s00265-004-0762-0
ORIGINAL ARTICLE
Sue Lewis · Keith C. Hamer · Luisa Money · Richard Griffiths · Sarah Wanless · Thomas N. Sherratt
Brood neglect and contingent foraging behavior in a pelagic seabird Received: 6 October 2003 / Revised: 11 December 2003 / Accepted: 15 January 2004 / Published online: 13 February 2004 Springer-Verlag 2004
Abstract Among species where there is a risk to leaving offspring unattended, parents usually take alternating shifts guarding their young. However, they may occasionally exhibit brood neglect by leaving their offspring unattended at the nest. To investigate this phenomenon further, we examined the foraging behavior of the northern gannet (Morus bassanus) during chick-rearing. This species has a prolonged nestling period (13 weeks) and the single chick is usually guarded by one or other of its parents, because unattended chicks are frequently attacked by conspecifics. We tested the prediction that the foraging behavior of adults when they left their offspring alone at the nest (unattended trips) would differ in character to when adults left offspring with their partner (attended trips). Brood neglect typically occurred after a Communicated by C. Brown S. Lewis ()) · L. Money · S. Wanless NERC Centre for Hydrology and Ecology, Banchory Research Station, Hill of Brathens, Banchory, Aberdeenshire, AB31 4BW, UK e-mail:
[email protected] Tel.: +44-1330-826314 Fax: +44-1330-823303 K. C. Hamer · T. N. Sherratt School of Biological and Biomedical Sciences, University of Durham, Durham, DH1 3LE, UK K. C. Hamer Ecology and Evolution Group, School of Biology, University of Leeds, Leeds, LS2 9JT, UK R. Griffiths Ornithology Group, Division of Environmental and Evolutionary Biology, University of Glasgow, Glasgow, G12 8QQ, UK T. N. Sherratt Department of Biology, Carleton University, 1125 Colonel By Drive, Ottawa, Ontario, K1S 5B6, Canada
longer-than-average attendance period at the nest. Unattended trips were, on average, about half the duration of attended trips, and therefore much closer to the colony. There was also a difference in departure direction between attended and unattended trips, with unattended trips tending to be northeast of the colony. Chicks were fed by parents after both attended and unattended trips, but the frequency and the duration of unattended trips increased as chicks grew older whereas the duration of attended trips decreased as chicks grew. These results indicate that parents may be making a trade-off between risk of attack to their offspring and food-intake rate, and that the solution to this trade-off is dependent on chick age. Keywords Dual foraging strategy · Gannet · Morus bassanus · Nest attendance · State-dependent foraging
Introduction The foraging decisions of individuals may be influenced by their current state (McNamara and Houston 1996; Clark and Mangel 2000). For instance, individuals may decide to include relatively unprofitable prey items in their diet if they are hungry (Houston and McNamara 1985; Sherratt 2003) or may choose to forage in areas of high predation risk if their energy reserves are low (Milinski and Keller 1978; Houston et al. 1988; Lima and Dill 1990; Pettersson and Bronmark 1993). Many birds face a similar dilemma during the breeding season, because they need to balance time spent foraging against time spent incubating eggs or guarding chicks. When one parent attends the offspring and the other is responsible for providing food, or when both parents systematically alternate their activities between foraging and attending offspring, the time taken for a parent to return from a foraging trip can have important consequences for the behavior and physiological state of the attending partner (Szekely et al. 1996). For instance, in birds of prey, the male hunts for food for himself and his mate whereas the
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female incubates the clutch, but the female may leave eggs unattended to forage for herself if the male provides insufficient food (Newton 1986). Such behavior can have dramatic consequences for the offspring, particularly when eggs or chicks experience high predation risk or thermal stress if left unattended (Croxall and Ricketts 1983; Hunter 1984; Reid et al. 2000; Nelson 2002). Foraging decisions, such as trip duration, are dependent on an individual’s own body condition (Weimerskirch et al. 1997a, 1997b; Weimerskirch 1998), as well as that of its partner (Tveraa et al. 1997, 1998) and, in some cases, its chick (Weimerskirch et al. 2000). However, to our knowledge, the prediction that the foraging behavior of adults when leaving their offspring alone is different in character to that during trips where the offspring are not left alone has never been tested. The northern gannet (Morus bassanus) is an ideal model species to examine state-dependent foraging strategies associated with brood neglect, because although breeding partners typically alternate periods of attending their single chick at the nest with foraging at sea, they do occasionally leave their chick unattended during the 13week nestling period (Nelson 2002). Leaving offspring unattended may have serious consequences for the chick and therefore the reproductive success of the parent: gannets nest at high densities and adults are highly aggressive to their neighbors, including their neighbors’ chicks. Furthermore, prospecting birds often attempt to usurp nests without an adult present, and can cause serious injury to unattended chicks (Nelson 2002). In 2 consecutive years, we examined the durations and departure directions of foraging trips (which together give an estimate of foraging location, see Hamer et al. 2000) and the changes in body masses of individuals following foraging trips. We compared trips in which the foraging parent left the chick attended by its partner (attended trips) with trips in which individuals left their offspring alone (unattended trips). To elucidate the motivation to leave the chick unattended, we examined the influence of attendance duration, departure mass and chick age (a proxy for food requirement at the nest) on the incidence of non-attendance. We also examined whether or not chick age and adult arrival mass were related to the length of time a bird stayed at the nest before leaving the chick unattended (Tveraa et al. 1998). In addition, we recorded how often a bird making an unattended trip returned before or after its partner and, thus, how long the chick was actually left alone at the nest. Finally, we examined how periods of non-attendance influenced the co-ordination of foraging trips and attendance periods between the two members of a pair.
SE Scotland (5660 N, 2360 W; ca. 40,000 breeding pairs). During 2001, we radio-tracked individuals to record foraging-trip duration, nest-attendance duration and foraging-trip departure direction. In 2002, electronic weighing platforms were used to record the body masses of adults arriving at the nest (arrival mass) and immediately prior to leaving on a foraging trip (departure mass), as well as foraging-trip duration and nest-attendance duration. Chicks were aged individually in weekly categories in both 2001 and 2002 from observations in the field, using age-specific plumage characteristics (Nelson 2002). Foraging behavior and nest attendance (2001) Both members of 14 pairs of adult gannets with young chicks (age range 2–3 weeks) were caught at the nest using an extendible roach pole. A VHF radio transmitter (Biotrack) weighing 20 g (
