The diet of a nocturnal pelagic predator, the Bulwer's petrel, across the lunar cycle.

The lunar cycle is believed to strongly influence the vertical distribution of many oceanic taxa, with implications for the foraging behaviour of nocturnal marine predators. Most studies to date testing lunar effects on foraging have focused on predator activity at-sea, with some birds and marine mammals demonstrating contrasting behavioural patterns, depending on the lunar-phase. However, to date no study has focused on how the lunar cycle might actually affect predator-prey interactions in the upper layers of the ocean. Here, we tested whether the diet of the predominantly nocturnal pelagic predator, the Bulwer's petrel (Bulweria bulwerii) would change throughout the lunar cycle, using molecular analysis to augment detection and taxonomic resolution of prey collected from stomach-contents. We found no evidence of dietary shifts in species composition or diversity, with Bulwer's petrel always consuming a wide range of mesopelagic species. Other co-variables potentially affecting light availability at-sea, such as percentage of cloud cover, did not confound our results. Moreover, many of the species found are thought not to reach the sea-surface. Our findings reveal that nocturnal predators are probably more specialized than previously assumed, irrespective of ambient-light, but also reveal deficiencies in our current understanding of species vertical distribution and predation-dynamics at-sea.

Estimation of mercury background values in sediment and biota of the Bijagós archipelago, Guinea-Bissau.

This work evaluates the mercury (Hg) contamination status (sediments and biota) of the Bijagós archipelago, off the coast of Guinea-Bissau. Sediments exhibited very low concentrations (<1-12ngg(-1)), pointing to negligible sources of anthropogenic Hg in the region. Nevertheless, Hg is well correlated to the fine fraction, aluminium, and loss on ignition, indicating the effect of grain size and organic matter content on the presence of Hg in sediments. Mercury in the bivalves Tagelus adansoni and Senilia senilis did not vary considerably among sites, ranging within narrow intervals (0.09-0.12 and 0.12-0.14µgg(-1) (dry weight), respectively). Divergent substrate preferences/feeding tactics may justify slight differences between species. The value 11ngg(-1) is proposed as the sediment background concentration for this West-African coastal region, and concentrations within the interval 8-10ngg(-1) (wet weight) may be considered as reference range for S. senilis and T. adansoni in future monitoring studies.

Seasonal sexual segregation in two Thalassarche albatross species: competitive exclusion, reproductive role specialization or foraging niche divergence?

Sexual segregation by micro- or macrohabitat is common in birds, and usually attributed to size-mediated dominance and exclusion of females by larger males, trophic niche divergence or reproductive role specialization. Our study of black-browed albatrosses, Thalassarche melanophrys, and grey-headed albatrosses, T. chrysostoma, revealed an exceptional degree of sexual segregation during incubation, with largely mutually exclusive core foraging ranges for each sex in both species. Spatial segregation was not apparent during brood-guard or post-guard chick rearing, when adults are constrained to feed close to colonies, providing no evidence for dominance-related competitive exclusion at the macrohabitat level. A comprehensive morphometric comparison indicated considerable species and sexual dimorphism in wing area and wing loading that corresponded, both within and between species, to broad-scale habitat preferences relating to wind strength. We suggest that seasonal sexual segregation in these two species is attributable to niche divergence mediated by differences in flight performance. Such sexual segregation may also have implications for conservation in relation to sex-specific overlap with commercial fisheries.

Territorial intrusions and copulation behaviour in the great skua, Catharacta skua

Birds of many species intrude on to territories in order to obtain extra-pair copulations, and frequent within-pair copulations are thought to be a response to cuckoldry where mate guarding is not possible. Great skuas are colonial birds in which females are left alone in the breeding territory while males forage for the pair. Opportunities for cuckoldry are therefore numerous, and it could be predicted that sperm competition should be intense in this species. We tested several hypotheses that attempt to explain territorial intrusions by female great skuas. Few intrusions coincided with the main fertile period and extra-pair copulations were almost never solicited, and therefore the sperm competition hypothesis was rejected. Only 0.9% of the copulations observed (N=339) were extra-pair. Thus opportunity for cuckoldry is a very poor predictor of the intensity of sperm competition, in spite of the relevance given to this factor in the literature. Of three extra-pair copulations observed, two involved unpaired territorial males. This suggests that genetic benefits were not the aim of unfaithful females. All three were preceded by courtship feeding, while only 26% of within-pair copulations followed successful food begging. This statistically significant difference constitutes evidence for a trade of copulations for food in a monogamous bird. Evidence is presented supporting hypotheses that females intrude on to territories in order to induce males to give away some food, and to become familiarized with potential partners for future breeding seasons.Copyright 1997 The Association for the Study of Animal Behaviour1997The Association for the Study of Animal Behaviour

Partnerships and mechanisms of divorce in the great skua.

There are two main theories attempting to explain divorce in birds as an adaptive decision for at least one of the members of the pair. The 'incompatibility hypothesis' states that birds separate because they are unsuccessful breeders as a pair, although their individual characteristics are not negative per se. Divorce comes about as an initiative of both individuals to search for a more compatible mate. The 'better option hypothesis' suggests that divorce is usually initiated by one of the birds in search of a 'higher quality' partner (or territory). The ages of partners of great skuas, Catharacta skuabreeding on Foula (Shetland) were poorly correlated (r48=0.28), 22% of the partners differing in age by 10 years or more. Mate fidelity in this population was high, with an annual divorce rate of only 6.4% (N=20/311). Death was responsible for three times more interruptions of partnerships than divorces. Three types of divorce were detected. (1) In six cases, females deserted their partners, leaving them temporarily unpaired. (2) One female was recorded being forced to desert her partner by a usurper. (3) One female deserted her partner after they lost their breeding territory to another pair. Mate changes had a direct cost, because new pairs (of experienced birds) laid later in the season and reared fewer chicks. After mate loss, 26% (N=61) of the individuals did not breed, while faithful pairs always laid eggs (N=222). Males, but not females, had a higher probability of remaining unpaired after being divorced than after becoming widowers. There was no difference between the sexes in the probability of re-pairing after a mate died. The results suggest that divorces that are initiated by birds within the pair are better explained by the 'better option hypothesis' than the 'incompatibility hypothesis'.Copyright 1997 The Association for the Study of Animal Behaviour