A Theoretical Investigation of the Effect of Latitude on Avian Life Histories

Temporal Components of Reproductive Variability in Eastern Kingbirds (Tyrannus Tyrannus)

Geographical trends in demographic traits were studied in Swallow Hirundo rustica and House Martin Delichon urbica from populations in the western Palaearctic. Clutch size increased with latitude in Swallow second clutches and with longitude in House Martin second clutches. The number of broods per season decreased latitudinally and for the House Martin longitudinally as well. The number of eggs per pair per breeding season thus decreased with latitude in both species and with longitude in the House Martin. It is suggested that the difference in the longitudinal response between the two species is due to the higher and thus more weatherinfluenced foraging niche of the House Martin. Hatchability of House Martin eggs increased with latitude. Between year population size variations increased with latitude in the Swallow. Ashmole's (1961) hypothesis did not account for geographical trends in clutch size. Geographical variation in nest predation could not explain variance in clutch size. Royama's (1969) model on clutch size trends taking food availability, time available for feeding and the temperature-dependent energy requirements of young into consideration explained a large fraction of the variance in clutch size among both species. The absolute metabolic level of the nestlings decreased from north to south.

SEASONAL AND LATITUDINAL TRENDS IN CLUTCH SIZE: THERMAL CONSTRAINTS DURING LAYING AND INCUBATION

The breeding success and chronology of Wood Storks Mycteria americana were studied at eight colonies in northern and central Florida during 1981–1985. Mean ± s.d. clutch size for all colony‐years was 3.07 ± 0.56 (n = 2694 nests), with three‐egg clutches (72%) most frequent. Mean clutch size among all colonies and years ranged from 2.73 ± 0.55 to 3.41 ± 0.61. Many colonies exhibited significant negative trends in clutch size with, hatching date because of a proportional decrease in four‐egg clutches later in the season. Mean colony clutch size was not correlated with nest numbers, nesting density or mean hatching date within most years. Mean ± s.d. number of fledglings for all colonies and years was 1.29 ± 1.16 fledglings per nest (n = 2812 nests). Mean annual fledging rates in colonies ranged from 0 (colony failed) to 2.66 fledglings per nest. Most breeding failure occurred prior to egg hatching, and the second highest mortality occurred between hatching and 2 weeks of age. Four‐egg clutches fledged more storks than three‐egg clutches, which in turn were more successful than two‐egg clutches. However, all clutch sizes showed similar fledgling per egg rates. The seasonal decline in productivity was associated proportionally with smaller clutch sizes later in the breeding season. An increase in mean hatching date was correlated with an increase in latitude. There was greater within‐year breeding synchrony among colonies than interyear breeding synchrony within each colony. Breeding synchrony was not correlated with mean hatching date, latitude, longitude, nest numbers or nesting density.

Seasonal Variation in Reproductive Output of House Sparrows: The Determination of Clutch Size

We studied the breeding biology of the south temperate Cape Penduline Tit (Anthoscopus minutus) in order to compare its life history traits with those of related north temperate members of the family Remizidae, namely the Eurasian Penduline Tit (Remiz pendulinus) and the Verdin (Auriparus flaviceps). We used this comparison to test key predictions of three hypotheses thought to explain latitudinal variation in life histories among bird species—the seasonality and food limitation hypothesis, nest predation hypothesis and adult mortality hypothesis. Contrary to the general pattern of smaller clutch size and lower adult mortality among south-temperate birds living in less seasonal environments, the Cape Penduline Tit has a clutch size larger than that of the Verdin and similar to that of the Eurasian Penduline Tit, and higher adult mortality than both of the other two species. The most notable difference between the Cape Penduline Tit and the two other species is in parental behavioural strategy, with the former exhibiting bi-parental care at all stages of nesting together with facultative cooperative breeding, whereas the Eurasian Penduline Tit has uni-parental care and the Verdin has a combination of female-only incubation but bi-parental nestling care. Consequently, in comparison to the other two species, the Cape Penduline Tit exhibits greater nest attentiveness during incubation, a similar per-nestling feeding rate and greater post-fledging survival. Its relatively large clutch size, high parental investment and associated high adult mortality in a less seasonal environment are consistent with key predictions of the adult mortality hypothesis but not with key predictions of the seasonality and food limitation hypothesis in explaining life history variation among Remizidae species. These results add to a growing body of evidence of the importance of age-specific mortality in shaping life history evolution.

Among individuals of female three‐spined sticklebacks Gasterosteus aculeatus from a population in the Camargue, southern France, studied in 12 successive years, adult LT ranged from 31–64 mm, clutch size ranged from 33–660 eggs, and mean egg diameter per clutch ranged from 1.15–1.67 mm. Because the population was strictly annual, inter‐annual variation corresponded to variation among generations having experienced different environmental conditions. Body mass varied significantly among years, suggesting an effect of varying environmental conditions. Gonad mass and clutch size increased with body mass, but mean egg diameter was not correlated to body mass. Body mass‐adjusted gonad mass, interpreted as reproductive effort per clutch, did not vary significantly among years, suggesting that this trait was not influenced by environmental conditions. Body mass‐adjusted clutch size and egg size varied significantly among years. Inter‐annual variation in body length at breeding, clutch size and egg size was of the same order of magnitude as inter‐population variation reported by other authors for this species. During the breeding season, reproductive effort and clutch size tended to increase. Egg size tended to decrease during the breeding season but this seasonal pattern varied among years. Observed life‐history variation is discussed both in terms of its evolutionary significance and methodological implications in the study of life‐history variation among populations.

In seasonal environments, a temporal decline in breeding performance (e.g. clutch size, nestling condition, and fledging success) of altricial bird species is a well-known phenomenon. In this study, we present the effect of laying phenology on the physiological condition of nestling great tits (Parus major) in 14 consecutive breeding seasons. We used blood haemoglobin and baseline glucose concentrations as indicators of nestling physiological condition. Nestling blood haemoglobin reflects food base quality and availability during the breeding period. Blood glucose concentration can be used as a supplementary reverse index of condition, since it is negatively related to environmental quality. It might be indicative of the stress caused by unfavourable extrinsic factors, though, due to potential confounding factors such as adverse weather conditions, low food quality, or feeding interruptions, glucose levels should be used in this ecological context with caution. Great tit nestlings from earlier broods were characterised by higher mean haemoglobin concentrations, indicating a seasonal decline in food quality and availability. The blood glucose concentration displayed an opposite pattern, with nestlings from earlier broods being characterised by lower mean concentrations than those from later broods. However, very little of the variation in blood glucose concentration can be explained by the variation in laying date, which suggests that blood glucose concentration is of little importance in the context of breeding phenology. Our results show that the physiological condition of nestlings of this species decreases as the breeding season progresses, most probably due to environmental factors.

We argue that a relationship between life history variation and population processes may form the foundation for developing a theory for variation in population growth rate. An examination of the distribution of 104 European bird species in relation to their clutch size and adult survival rate showed three different clusters. First, there is a large group of species which lay a large number of eggs and have low adult survival rate. The second cluster consists of species with very high survival rates and a clutch size of only one egg. The third group is characterized by species with high survival rates but still a relatively large clutch size. From these clusters of life history characteristics we argue that the species can be classified according to the quality of their survival and breeding habitats, respectively. The high-reproductive species live in favourable breeding habitats, but poor survival habitat. In contrast, the survival habitat of the survivorship species are very good, but the breeding habitats are poor. The bet-hedging species live in favourable breeding and survival habitats, but the annual variation in the quality of the breeding habitats is very large, favouring the evolution of a larger clutch size than in the survivorship species. In order to examine the effects of these patterns of covariation between life history traits on population dynamics we calculated the sensitivity and elasticity of population growth rate to a change in age-specific fecundity or mortality rates for one species from each of the three life history types. These analyses showed that population growth rates of high-reproductive species were more sensitive and elastic to changes in the fecundity among the younger age-classes, compared to the species from the two other groups. Furthermore, elasticity to variation in mortality rates was higher than to variation in fecundity rates in all three species. To provide a further link between life history variation and population dynamics the results from key-factor analyses of population fluctuations in birds and mammals were reviewed. In most altricial birds, the key-factor appears during the non-breeding season. In contrast, in precocial birds key-factors from the breeding season explained a higher proportion of the variance in the total losses than the losses during the non-breeding season. In the majority of the cases density-dependence was found in the losses during the non-breeding season. According to the Allee-effect, we would expect that the population growth rate should decrease with density at low population sizes. No evidence was found for the presence of an Allee-effect in the studies of 11 bird species which were reduced to very low population levels during the study period. We suggest however that such an Allee-effect still may be important due to a reduction in the defence efficiency among predators or parasites, reduction in mating efficiency, or reduction in the foraging efficiency at low population densities. These results may have some important implications for overall priorities in the development of strategies for conserving species diversity. In particular, we focus on the securing of survival habitats for especially longlived species outside the breeding season.

CapsuleFood independently affects both laying date and clutch size, suggesting that seasonal decline in clutch size is related to a decrease in food availability. Aim To test the effect of food abundance on laying date and clutch size of the White Stork and identify the cause of seasonal decline in the number of eggs laid. Methods During 1991 and 1996 we recorded clutch size and laying date of pairs breeding next to rubbish dumps (food abundant and constant throughout the breeding season) and birds breeding far from rubbish dumps (using natural food sources). Results In 1991 there was no difference in mean laying date between pairs nesting at rubbish dumps and control pairs. Clutch size was significantly larger at rubbish dump nests. In contrast, mean laying date was earlier in control pairs in 1996 and there was no significant differences in clutch sizes, even when controlling for laying date effect. Conclusion The results support the hypothesis that food availability independently affects both laying date and clutch size. The seasonal decline in clutch size close to rubbish dumps was negligible (1991) or much smaller than in the control zone (1996) suggesting that a progressive deterioration of natural food sources is the most probable reason for a decline in clutch size as the season advances.

Abstract. Clutch size is an important life history trait, and factors such as nest predation and food availability can both be of crucial importance for its variation in nature. The aim of the present study was to evaluate the effects of extra food on clutch size, laying date and hatching success in the White Stork. Three different colonies of White Storks were studied in northern Algeria over a three-year period (2002–2004) that was characterised by considerable variation in both food availability and precipitation. This study demonstrated that an extra food supply during the pre-laying period had a positive effect on clutch size — nests with extra food had larger clutches. There was also an advance in laying date and a greater hatching success in nests with access to extra food. In addition to food supply, clutch size was independently affected by the year, which could have been due to differences in rainfall. Furthermore, the results of this study suggest that extra food during the incubation period co...

Though many authors have demonstrated an adaptive relationship between food supply or territory quality and clutch size, the proximate mechanism by which clutch size is determined remains largely unknown. A series of supplementary feeding experiments on lesser black-backed gulls (Larus fuscus) is used in conjunction with a new technique to measure pectoral muscle protein levels in live birds to investigate the relationships between food supply, body condition, and clutch size. Comparisons across treatments revealed a positive relationship between the level of flight muscle protein at the start of laying and clutch size, but not egg size. In a year when the clutch sizes of unfed birds were reduced, suggesting low food availability, gulls provided with a protein supplement had higher levels of flight muscle protein at the start of laying and laid larger clutches than birds that received no food supplement. In contrast, gulls provided with an additional energy source (fat) did not attain higher protein levels or lay larger clutches than unfed gulls. In a second year, when unfed gulls laid larger clutches, which were typical for the species, supplements of two different protein types (fish and egg) had no effect on clutch size, though egg-fed females laid considerably (10%) larger eggs. However, there were no detectable differences in condition between treatments at the start of laying, suggesting that though flight muscle protein levels may be of importance in determining clutch size, they may be of lesser significance for egg size, which may be largely influenced by dietary intake.

We manipulated hatching pattern and brood size in 102 European Starling (Sturnus vulgaris) broods to test the "brood reduction" hypothesis for the adaptive significance of asynchronous hatching. According to this hypothesis, asynchronous hatching in food-limited broods reduces mortality or improves the condition of nestlings in comparison with synchronous hatching.

This paper describes the density fluctuations of the adder Vipera berus and its prey during seven years. The reproduction during high adder density and low food availability was compared with that seven years later in the same population during low adder density and high food availability. Female length was similar but weight status (mass/length 2.56) was significantly higher during high prey density. Female mass loss when giving birth and clutch mass were significantly higher during high availability of food, but relative clutch mass and relative mass loss were similar in the two situations. However, the variance in relative clutch mass was significantly lower with high prey density. Clutch size and mass per young were significantly higher during high prey density. During high food availability there were significant positive correlations between female length (~ age) and birth mass loss, female length and clutch mass, female length and clutch size, clutch mass and mass per young and between relative clutch mass and mass per young. During low food availability there were significant positive correlations between clutch mass and clutch size and relative clutch mass and clutch size. A significant negative correlation was found between female length and relative mass loss. This study do not conform entirely to general theories on reproductive adaptations to varying food availability (r-K strategies). During high prey density both more and heavier young were produced than during scarcity of food. A high correlation of reproductive parameters during high food availability indicates a maximal utilization of reproductive capacity in most females, while the lack of such correlations during low food availability indicates a varying access to food and fat storage possibility among females.

Summary The rates of clutch size, brood size and brood‐size fledged recorded during the period 1956≈63 (2 years interrupted) in the rural and the urban colony of the grey starling were analysed comparatively. The clutch size was significantly larger in the urban than in the rural one and the brood size was also larger significantly in the urban population. But, the difference in brood‐size fledged was insignificant. This reflected that the hatching rate was similar in both colonies (the rates in total differed but possible unusual rate was included in the clutch size of 5 eggs) and this may be determined physiologically but not depending upon food supply since smaller clutches showed higher hatching rate. However, the fledging rate was higher in the rural and lower and more variable in the urban colony. This is apparently due to food which was nutritive animal food in the rural but largely mixed with fruits in the urban. The fledging rate was rather irregularly variable with brood size suggesting that this is dependent upon parents' adaptability in feeding. However, the rate of 100% fledging becomes higher from brood sizes of 6 and less chicks. Thus, in general larger clutch and brood sizes produced larger absolute numbers of chicks and chicks fledged respectively. But, 5 was the most frequent size in the clutch and brood sizes and 4 in the brood‐size fledged. From the above, the ecological evolution of urban population was discussed.