David M. Bryant

Testosterone influences basal metabolic rate in male house sparrows: a new cost of dominance signalling?

Sexually selected signals of individual dominance have profound effects on access to resources, mate choice and gene flow. However, why such signals should honestly reflect individual quality is poorly understood. Many such signals are known to develop under the influence of testosterone. We conducted an experiment in male house sparrows in which testosterone was manipulated independently during two periods: before the onset of the breeding season and prior to the autumn moult. We then measured the effects of these manipulations on basal metabolic rate and on the size of the chest bib, a sexually selected signal. The results demonstrate that testosterone simultaneously affects both signal development and basal metabolic rate in the house sparrow (Passer domesticus). This evidence, therefore, supports a novel conclusion: that testosterone–dependent signals act as honest indicators of male quality possibly because only high–quality individuals can sustain the energetic costs associated with signal development.

Reproductive Costs in the House Martin (Delichon urbica)

(1) The aim of the study was to identify costs of reproduction in house martins and to make an assessment of the value of life-history theories and qualitative differences between individuals for interpreting intraspecific variation in reproductive output. (2) Breeding statistics for house martins in central Scotland were similar to southeastern England (except laying was 8 days later and II Y. fewer had a second clutch). There was a correlation between arrival date at the colony and the start of breeding for each individual. Females invariably arrived after their mates. (3) Older males were heavier, tended to pair with older, double-brooded females, laid earlier and reared more young. (4) Older females bred earlier and reared more young but were not heavier. (5) Adults lost weight when food was scarce and also during the most demanding (middle) phase of nesting growth. Laying anomalies (a stop to laying due to food shortages) probably led to significant weight losses in females. (6) Laying interruptions wasted time, reduced the chance of a second brood and marginally lowered the size of the first. (7) First brood fledglings survived equally over winter, irrespective of brood size and (less convincingly) parent age. (8) Older parents tended to be more successful in bringing clutches to independence. (9) Mortality of adult house martins occurred mainly outside the nesting period and averaged 57%. The most important finding of the study was that double-brooded females survived less well (especially if they bred early) than single-brooded ones. This did not apply to males and there was no age-specific change in mortality for double brooded birds of either sex. (10) Food supply limits the start of breeding and can account for some of the changes in output through the season. For a full explanation of intraspecific differences in output within first and second broods however, it is useful to invoke qualitative differences between individuals. They may be considered as proximate mechanisms whereby the attributes of individuals (females in particular) (i.e. weight, size and ability) interact with resources leading to a reproductive pattern which is optimal for each individual. The advantage of early breeding for males was much greater than for females (as indicated by the reproductive value of different breeding patterns) in which the benefits of early laying and double broodedness were largely offset by the mortality costs experienced by female parents.

Climate change and constraints on breeding.

Although climate change apparently affects the breeding patterns of many animals, the wider implications for breeding success are unclear. Here we describe an energy trade-off between reproduction and maintenance that occurs during cold weather in great tits (Parus major L.), pointing to a thermal constraint on the timing of egg laying. Our observations indicate that the fine-scale pattern of climate change could be critical to the reproduction of some species and underlies previously unexplained variation in the breeding success of other temperate birds.

EFFECT OF WIND ON FIELD METABOLIC RATES OF BREEDING NORTHERN FULMARS

The field metabolic rate (FMR) of nesting Northern Fulmars (Fulmarus glacialis) was measured using the doubly labeled water technique. Although some labeled Northern Fulmars showed marked differences in behavior compared to controls. FMR did not change correspondingly. At—sea FMR averaged 1444 kJ/d, equivalent to 4.5 X basal metabolic rate (BMR). As a multiple of BMR it was independent of sex, wing length, body mass, mass change, and duration of the period at sea, but was strongly dependent on wind speed, being higher during slack winds. There was also a trend for higher wingbeat frequency at winds to low for sustained dynamic soaring. The high energetic cost of flapping flight during windless conditions may explain patterns of nest attendance: in particular, why nest site attendance by Northern Fulmars in winter is greater during calm weather. Lack of wind might limit the breeding range of this species and other Procellariiformes with high flapping—flight costs.

Energy expenditure in wild birds

Allometric scaling of energy metabolism is one of the most familiar and robust models in biology (Brody, 1945; Kleiber, 1975; Calder, 1984), although its cause remains contentious (West et al. 1997). Energy requirements of animals are often calculated assuming metabolism scales as a 0.67 or 0.75 power of body mass (Peters, 1983; Brown & Maurer, 1986). Yet, the practices and assumptions of this approach will often be inadequate if the goal is precise prediction of energy expenditure or requirements at the species level. This is because organismal and environmental influences on avian metabolism are ubiquitous, both regarding their effect on basal energy requirements (Aschoff & Pohl, 1970; Bennett & Harvey, 1987) and, more pertinently in the present context, on expenditure of energy by free-living individuals (Walsberg, 1983; Nagy, 1987; Bryant & Tatner, 1991). It is important, therefore, to identify factors, additional to those simply allied with body mass, which account for this variation. Identification of significant categorical factors affecting energy expenditure has been a main concern to date. A study of interspecies variation in mammalian energetics, for example, showed that desert dwellers had generally low field (free-living) metabolic rates but identified no other significant factors (Nagy, 1994). Amongst birds, desert dwellers and sea-birds stand apart from others, with respectively low and high field metabolic rates (Nagy, 1987). Also, some studies have indicated that aerial-feeding birds, including hummingbirds, have a relatively high field metabolism (Walsberg, 1983). Nevertheless, very few reliable predictor variables of wide applicability are available to explain interspecies variation in energy expenditure, either for wild birds or mammals. The present study has the general aim of accounting for interspecies variation in energy expenditure of birds. More specifically, it confirms that metabolic scaling with body mass (W) remains pre-eminent in explaining energy expenditure amongst wild birds, even when W only varies across about an order of magnitude. The main aim, however, is to identify and evaluate other factors which predict variation in energy expenditure amongst free-living birds.

Central place foraging by swallows (Hirundinidae): The question of load size

Abstract Some predictions of Orians & Pearsons (1979) models for central place foragers (CPF) were tested with three species of swellows (Hirundinidae). House martins ( Delichon urbica ) and sand martins ( Riparia riparia ) brought larger food loads to the nest mainly when foraging distances were great, whereas swallows ( Hirundo rustica ) gathered large loads when food was plentiful. For all three species the outcome conformed qualitatively with the predictions of the CPF models. Overall, house martins were the most sensitive to travel time effects, but in a quantitative test the predicted load size was 20–40% less than the observed size for a range of realistic travel times. Additional models are presented which emphasize the significance of foraging techniques and foraging costs for optimal load size in multiple prey loaders.

Effects of prey density and site character on estuary usage by overwintering waders (Charadrii)

Abstract The usage by wading birds of 14 sites on the Forth Estuary in winter was examined in relation to the density of their invertebrate prey and the area, configuration and exposure sequence of the intertidal sites. Six common waders were studied over the whole estuary and three species in greater detail at the largest single site. Significant associations were shown in five species for feeding-hours km −2 and numbers km −2 with the density of at least one of their main prey; including oystercatchers Haematopus ostralegus (L.) with Mytilus edulis (L.); curlew Numenius arquata (L.) with Nereis diversicolor (L.); redshank Tringa totanus (L.) with Nereis diversicolor ; knot Calidris canutus (L.) with Cerastoderma edule (L.) and dunlin Calidris alpina (L.) with Nereis diversicolor . For four species site characteristics also apparently affected feeding patterns [bar-tailed godwit Limosa lapponica (L.), area; redshank, exposure sequence; knot, area, configuration and exposure sequence; dunlin, area]. Overall, significant variables explained 43–96% of the variation in wader feeding-hours km −2 on the estuary and 41–87% of numbers km −2 . It is emphasised however, that the observed variation in feeding patterns on the Forth Estuary represents only a part of the total variation in usage of estuaries by feeding waders and further studies are necessary to explain the extent and causes of this additional variability.

Energetics of free existence in swallows and martins (hirundinidae) during breeding: a comparative study using doubly labeled water

Energy metabolism of three sympatric swallows (Hirundinidae) was investigated during the breeding season using doubly labeled water (2H218O). Interspecific and intraspecific differences in energy metabolism were examined in relation to the habits, size and environment of the birds. To facilitate comparisons we expressed energy metabolism (M) as the ratio of average daily metabolic rate (ADMR, cm3CO2g-1h-1) to basal metabolic rate (BMR). We observed adults during incubation and when feeding nestlings. Then, both sexes of Sand Martins Riparia riparia and House Martins Delinchon urbica were either at the nest or on the wing. Incubation reduced activity levels during the day resulting in M (incubation) being 17–26% lower than during rearing. Differences in energy costs for rearing chicks depended mainly on flight behaviour, the smaller Sand Martin doing nearly twice as much flapping during flight as the House Martin, giving higher values for M. In Swallows Hirundo rustica the female incubates alone, alternating between short feeding trips and incubating in daytime. This pattern was linked with a relatively high value for M in the only individual behaving like our controls. Both sexes of Swallows feed the chicks, and they showed similar values of M. They also closely resembled House Martins, despite contrasts in the time spent flying and their behaviour during flight. Feeding conditions affected activity, and thereby M, in a species specific way. The House Martin did more gliding in poor weather, taking less mobile prey, reducing M. Swallows reduced foraging costs further by using body reserves, as in the House Martin. The smaller Sand Martin, in contrast, showed a high expenditure in poor weather. Over two breeding seasons ADMR reached values around 5 BMR for all three species.

Hatching asynchrony, sibling competition and siblicide in nestling birds: Studies of swiftlets and bee-eaters

Abstract The consequence of hatching asynchrony for fledging success and sibling competition in white-bellied swiftlets, Collocalia esculenta, and blue-throated bee-eaters, Merops viridis, was studied by observation and manipulation of nest contents at breeding colonies in Malaysia. A greater synchrony of hatching tended to increase fledging success amongst swiftlets, a result consistent with most other similar manipulative studies to date. Sibling competition in blue-throated bee-eaters invariably led to nest fatalities, which were induced by sibling attacks with an apparently unique, but developmentally temporary, hook at the tip of the upper mandible. Measurements of energy expenditure using the doubly labelled water technique showed that synchrony also reduced the energy cost of nestling competition in bee-eaters, thereby providing a possible mechanism for improved growth and survival of synchronous broods. No universal adaptive function for hatching asynchrony was identified, so it is proposed that a primary function is to encourage distribution of food amongst the brood in a way that permits resource tracking, but can also allow brood reduction under extreme adversity (i.e. a sustained food ‘crash’, parental death or desertion).

Extra-pair fertilizations and paternity defence in house martins,Delichon urbica

Abstract DNA fingerprinting showed that 15% of 62 house martin nestlings at study colonies in central Scotland were not related to their putative fathers, and 32% of 19 broods contained at least one extra-pair chick. There was no evidence of intraspecific brood parasitism. Birds were never seen to mate outside nests in the vicinity of nesting colonies, and the single copulation that was observed during this study was a pair mating that took place inside a nestbox. Extra-pair birds were often seen to enter nests, with seven of eight identified intruders being males from other nests where laying had already taken place. Pair males mate guarded by ensuring that females spent very little time alone at the nest from about 7 days before the first egg of the clutch was laid, and by accompanying females on up to 70% of flights away from the nest during the 4-5 days before laying commenced. Mate guarding seemed to slacken after egg laying began, with a gradual transition to incubation behaviour taking place. Associated with this was a higher likelihood that the youngest nestling would be fathered by an extra-pair male. Male removal experiments indicated that extra-pair birds were more likely to enter the nest of a fertile female when the pair male was absent, but in three cases where DNA fingerprints were obtained male removal during the fertile period of the pair female had no apparent influence on paternity.