Experimental evidence for a causal effect of pair-bond duration on reproductive performance in oystercatchers (Haematopus ostralegus)

Many studies have suggested that reproductive performance improves during the pair-bond, which might explain why individuals remate with the same partner in many species. However, discussion exists about whether the association between reproductive performance and pair-bond duration that is reported in these studies reflects a causal relationship. Usually it is unclear whether a positive association is caused by pairs improving during their pair-bond or by high-quality pairs staying together for longer. Furthermore, reproductive performance often also depends on the age or breeding experience of parents, which all covary with pair-bond duration. A much needed experimental approach is lacking so far. We investigated the effect of pair-bond duration on reproductive performance in a long-lived monogamous bird species based on natural as well as experimental variation. The duration of oystercatcher (Haematopus ostralegus) pair-bonds, which were followed for 21 years, strongly affected reproductive output, even after controlling for effects of age and breeding experience. Pairs improved during their pair-bond, and there were no indications of selective disappearance of low-quality pairs; however, pairs that stayed together for very long performed badly. Experimental removal of one partner showed that the reproductive cost of divorce depended on the pair-bond duration with the old partner. In addition, after remating, the newly formed pairs strongly improved again, independent of the age and breeding experience of the remated pair members. As such, this study provides the first experimental evidence of a causal effect of pair-bond duration on reproductive performanc

Competitive abilities of oystercatchers (Haematopus ostralegus) occupying territories of different quality

Abstract In territorial species, habitat heterogeneity results in some individuals occupying poor quality sites while others occupy high quality sites. Floaters (mature nonbreeders) may accept a low quality territory, because it is the best they can get and defend ('inferior phenotype hypothesis'), or because it is a strategic alternative for a high quality territory in the long run ('queue hypothesis'). Oystercatcher Haematopus ostralegus territories differ consistently in the amount of offspring produced each year and this is related to the distance between the nesting and feeding territories. The inferior phenotype hypothesis was previously rejected on the basis of the absence of morphometric differences (assumed to indicate competitive abilities) among breeders. We investigated social dominance, in the field and in captivity, in relation to the quality of the breeding territory. In the field, birds with high-quality territories won more often compared to those occupying low-quality territories. However, this difference was not apparent in a small dataset of captive birds. These results are discussed in the framework of the long-term fitness prospects of settling in a high or low quality territory

Cost reduction in the cold: Heat generated by terrestrial locomotion partly substitutes for thermoregulation costs in Knot Calidris canutus

To test whether heat generated during locomotion substitutes for the thermoregulation cost, oxygen consumption of four post-absorptive temperate-wintering Knot Calidris canutus was measured at air temperatures of 25 degrees C (thermoneutral) and 10 degrees C (c. 10 degrees below the lower critical temperature) when the birds were at rest at night and during running on a treadmill, After allowing for body mass, the thermoregulation cost at 10 degrees C was significantly lower in active birds compared with birds at rest, At rest, the birds spent, on average, 0.50 watt (W; range, 0.47-0.57 W) on thermoregulation. During exercise, this cost factor averaged 0.33 W (range, 0.25-0.42 W), The average difference in thermoregulation cost was 35% (ranging from 26% to 49% between individuals) and provides an estimate of the amount of substituted heat, A review of nine studies, all restricted to small birds, showed that substitution is a widespread phenomenon, The consequences of such partial substitution for the annual energetics of Knot wintering in the temperate Wadden Sea v tropical west Africa are examined, Compared with a previous additive model, the model which includes substitution (i.e. the use of heat produced during activity) reduces the differences in maintenance metabolism between the two wintering strategies by 17%, from 1.19 W to 0.99 W

Cost reduction in the cold: heat generated by terrestrial locomotion partly substitutes for thermoregulation costs in Knot Calidris canutus

To test whether heat generated during locomotion substitutes for the thermoregulation cost, oxygen consumption of four post-absorptive temperate-wintering Knot Calidris canutus was measured at air temperatures of 25°C (thermoneutral) and 10°C (c. 10° below the lower critical temperature) when the birds were at rest at night and during running on a treadmill. After allowing for body mass, the thermoregulation cost at 10°C was significantly lower in active birds compared with birds at rest. At rest, the birds spent, on average, 0.50 watt (W; range, 0.47-0.57 W) on thermoregulation. During exercise, this cost factor averaged 0.33 W (range, 0.25-0.42 W). The average difference in thermoregulation cost was 35% (ranging from 26% to 49% between individuals) and provides an estimate of the amount of substituted heat. A review of nine studies, all restricted to small birds, showed that substitution is a widespread phenomenon. The consequences of such partial substitution for the annual energetics of Knot wintering in the temperate Wadden Sea v tropical west Africa are examined. Compared with a previous additive model, the model which includes substitution (i.e. the use of heat produced during activity) reduces the differences in maintenance metabolism between the two wintering strategies by 17%, from 1.19 W to 0.99 W.

Site attachment of floaters predicts success in territory acquisition

In many territorial species, a fraction of all mature individuals are classified as floaters, and little is known about how these animals eventually acquire a breeding territory of their own. We observed intrusion behavior of floaters, subsequently removed breeding birds, and then observed floaters as they were filling these vacancies in an oystercatcher, Haematopus ostralegus, population. Birds familiar with the area and its inhabitants filled 80% of the experimentally created vacancies. These could be either neighbors or floaters with a former breeding history nearby, but they were mainly floaters with an intrusion record nearby. Floaters obtaining experimentally vacated territories intruded significantly nearer to this territory before removal compared with floaters not obtaining the vacancy. In general, vacancies that were not occupied by intruding floaters tended to be located in areas where less intruding floaters were seen prior to removal. We show quantitatively that only birds familiar with a site succeed in establishing a territory at that site, suggesting that local information is essential for territory acquisition. We propose that the main aim of intrusion behavior may be the collection of such information. Copyright 2004.floaters; habitat selection; Haematopus ostralegus; intrusion behavior; local dominance; prospecting; queuing; territoriality

Fitness consequences of divorce in the oystercatcher, Haematopus ostralegus

We investigated the fitness consequences of divorce in oystercatchers. We made a distinction between two types of divorce: in desertions the disruption of the pair bond is initiated by one of the pair members, and in usurpations by a conspecific individual. Survival and reproduction prospects for oystercatchers are largely determined by their social status (nonbreeder or breeding bird at a site of a specific quality). Changes in social status in relation to divorce showed that birds taking the initiative to leave their mate increased in fitness, relative to birds that were forced to leave their partner. Status of individuals that remained in their territory after divorce was unaffected if their mate was expelled, but declined if their mate deserted. Survival after divorce was significantly lower for birds that were expelled than for those deserting. Divorce rate, and especially desertion rate, was higher among occupants of low- than high-quality territories. In general, divorce rate increased following elevated mortality. In high-quality territories usurpations increased with increasing breeder mortality, but at low-quality territories this relation was absent. Desertion rates were similarly related to mortality in both territory types. Divorce participants thus differed strongly in their fitness prospects, depending on the type of divorce, the role played in the divorce and the quality of the territory where divorce took place. Studies that do not observe the birds during divorce cannot determine the type of divorce and the role played by the individuals, and this may lead to misleading conclusions on the costs and benefits of divorce. (C) 2003 Published by Elsevier Ltd on behalf of The Association for the Study of Animal Behaviour

The benefit of large broods in barnacle geese:a study using natural and experimental manipulations

1. In precocial birds, where the young feed themselves, the costs and benefits of brood size are still poorly understood. An experimental manipulation of brood size was employed to examine the effects of brood size on both parents and young in a wild population of barnacle geese [Branta leucopsis (Bechstein)] during brood-rearing on Svalbard. 2. Social dominance of the family unit, the amount of vigilance behaviour of the parents, the growth of the goslings in the family unit and an index of body condition for female parents during moult were all positively correlated with brood size. 3. When brood size changed as a result of natural events (i.e. predation or adoption) or experimental manipulation, rates of dominance, parental vigilance, gosling growth and female parent condition changed in a similar direction to the observed relation between the variable and brood size in unchanged broods. 4. After fledging, the fast-growing goslings in large broods survived better during autumn migration, while there was no apparent net cost in survival or next-year breeding for the parents. 5. Via a direct effect of brood size on dominance of the family unit, large broods were beneficial for both parent and young in a situation where there was strong intraspecific competition for the available food resources. 6. This study provides a clear demonstration of a causal relationship between brood size and various components of both gosling and adult fitness and is of direct relevance to the phenomenon of adoption and the evolution of brood size in this species