A simple tool for calculating egg shape,volume and surface area from digital images

The geometrical properties of eggs – such as volume and surface area – have uses ranging from ecological, physiological and morphological studies in birds, to predictions of chick condition in the poultry industry. Although measurements of an egg's length and breadth can be used to approximate its geometry, the coefficients used in these models are specific to the original test population, and intraspecific variation in egg shape means these methods cannot be used reliably beyond that population. Here I present a novel mathematical formula to describe the curvature of a bird's egg that can be used to calculate the shape, volume and surface area of an egg precisely from digital images. Using data from several species I demonstrate that the model has a greater level of accuracy than length‐ and breadth‐based methods, and release the user‐friendly tool for others to use for measuring eggs from digital images.     

Great tits growing old: selective disappearance and the partitioning of senescence to stages within the breeding cycle

Deterioration of reproductive traits with age is observed in an increasing number of species. Although such deterioration is often attributed to reproductive senescence, a within-individual decline in reproductive success with age, few studies on wild animals have focused on direct fitness measures while accounting for selective disappearance and terminal effects, and to our knowledge none have determined how senescence effects arise from underlying reproductive traits. We show for female great tits that such an approach helps understanding of the onset, impact and architecture of senescence. Cross-sectional analysis of 49 years of breeding data shows annual recruit production to decline from 3.5 years of age, this decline affecting 9 per cent of females each year. Longitudinal analyses, however, show that selective disappearance of poor-quality breeders partly masks senescence, which in fact starts at 2.8 years and affects 21 per cent of females each year. There is no evidence for abrupt terminal effects. Analyses of underlying traits show no deterioration in clutch size, but significant declines in brood size and fledgling number. Furthermore, these traits contribute −9, 12 and 39 per cent to the senescent decline in recruit production, respectively. Besides providing detailed knowledge of the patterns and architecture of senescence in a natural population, these results illustrate the importance of modelling individual variation, and facilitate study of the underlying mechanisms of senescence.     

DO THE SIBERIAN TITS PARUS CINCTUS IN SCANDINAVIA AND SIBERIA SPEAK THE SAME LANGUAGE?

ABSTRACT

The vocabulary of Siberian tits Parus cinctus in South Norway and East Siberia is compared by means of tape recordings and spectrograms. The species has many different calls that are greatly confused in the literature. In this paper ten main call types are examined. On the whole, the equivalent utterings are geographically so similar that a distinction is usually impossible or questionable. This similarity also applies to the contextual use of the calls. Consistent structural differences in certain calls were nevertheless found. The sit foraging call embraced on average a broader frequency range and was of a slightly longer duration in Siberia than in Norway. The complex ‘gargle’ system could at both places be divided into three main groups equally represented in the populations: (1) trilled, (2) tonal and (3) simple gargles. The tonal gargles are characterized by a terminal tonal element (T) that varies in shape. In Norway, tonal gargles terminating with an even or upslurred T dominated, in Siberia those with a chevron-formed T. Historically, the west-east distribution throughout Eurasia was apparently continuous until quite recently, allowing an effective gene flow. Considering the huge distance from Norway to East Siberia and the markedly resident behaviour of the Siberian tit, it is nevertheless somewhat unexpected that so few vocal differences have evolved. Several of the calls of the Siberian tit are structurally very similar to equivalent (most likely homologous) calls in other species belonging to the subgenus Poecile, e.g. the willow tit Parus montanus and the black-capped chickadee P. atricapillus, suggesting that the multicall repertoire of their common ancestor remained largely intact despite Poecile's differentiation into different species.     

Dispersal,kinship and inbreeding in an island population of the Great Tit

We investigated the kinship structure of an island population of the Great Tit (Parus major). Kinship of birds could be inferred by comparing their family trees. Dispersal was also studied to explain the observed pattern of kinship. On the island of Vlieland the tits breed in several wooded areas. Both males and females preferred to breed in their natal area; males did so more strongly than females. Hence gene flow between the areas is restricted. However, within the largest wooded area females showed random dispersal, while males showed a slight tendency to breed near their natal site. The degree of kinship of neighbouring birds is a suitable control group for the relatedness of partners that takes into account the effects of dispersal. In the largest wooded area, birds were on average equally related to their partner and to their neighbours. Moreover, the mean coefficient of kinship between male and female neighbours was equal to the average kinship in this part of the population. We conclude that mating is random with respect to kinship. There is no evidence for avoidance of inbreeding. It is unlikely that kin recognition plays an important role in the process of mate choice in this population of Great Tits. We suggest that ecological factors are the main causes for the observed patterns of dispersal and mating. On the island more female than male immigrants enter the population each year. Incidental data indicate an exchange of birds between the population studied and surrounding populations. Ancestries of immigrants are not known, and indeed a first analysis of all birds, including immigrants, showed that males were more closely related than females. However, differential immigration could not fully explain the observed difference in kinship. The presence of local adaptation in males is suggested as a possible additional cause.     

Cost of reproduction: parental survival and production of recruits in the Willow Tit Parus montanus

Summary Brood sizes of the Willow Tit were altered experimentally by subtracting or adding two nestlings in 1986 and 1987 in the vicinity of Oulu, northern Finland. The manipulated broods were within the normal range observed in natural conditions. Unaltered broods were used as controls. Data from natural broods from 1978–1985 were available for comparison. When the nestlings were 13 days old they were ringed and weighed and their tarsus, wing, and tail lengths were measured. On the same day the parents were caught, weighed, and measured. In 1986 there were no differences in nestling mortality between the reduced, control, or enlarged broods; i.e. parents were able to fledge the two extra young. In 1987 starvation was most pronounced in the enlarged broods. This resulted in the number of fledglings being practically the same in each manipulation category. Especially the body weight, but also the other indices of body size, decreased as a function of the brood size category, suggesting that there may be quality differences between the young reared in different experimental groups. In 1986 there was a non-significant trend towards lower body weight of the parents attending reduced, control, or enlarged broods, in that order. In 1987 the differences were much smaller. These results were not due to size differences between the groups, so possibly the increased reproductive effort of raising extra young was responsible for the trend observed in 1986. There were no significant differences in parental survival associated with the manipulation category, although the trend in the females was consistent with the hypothesis of reproductive cost. It is possible that environmental conditions in 1986 were so favourable that the tits were not unduly stressed even when attending two extra young. Correlative data from 1978–1985 did not support the cost hypothesis either. A non-significant trend towards reduced post-fledging survival and recruitment of the young was observed with increased brood size. The average fitness value of parents, incorporating parental survival and number of recruits, showed that the success of the adults raising enlarged broods may be lower than that of others. It seems that the reproductive cost, if it exists, decreases individual fitness value by reducing the chances of recruiting descendants into the next generation. The reproductive stress may be insufficient to reduce the subsequent survival of parents. More data are however needed to confirm these results.     

Linking individual foraging behavior and population spatial distribution in patchy environments: a field example with Mediterranean blue tits

 In spite of recent theoretical interest, few field studies have addressed the links between individual behavioral decisions and population distribution. This work analyzes the foraging behavior of individuals and the spatial distribution of a population of blue tits (Parus caeruleus) just before the main breeding season, when blue tit foraging was not affected by central-place or flocking behaviors. The study was carried out in open holm oak Quercus ilex woodlands (dehesas) that are patchy for canopy-foraging birds because of the scattered arrangement of trees. Residence time on each tree was not correlated either with previous flight time or with prey abundance in trees. Flight distances between trees were larger than average distances estimated in random samples of holm oaks taken close to foraging birds. Trees were not selected by birds on the basis of their expected energy costs and rewards. Bird abundance was not related to food availability in trees or to tree size across dehesas. However, bird abundance was strongly correlated with tree density and with the availability of tree holes for nesting, to the extent that the proportion of tits matched the proportions of both tree abundance and hole abundance across study plots. Overall, neither the behavior of individuals nor the distribution of the population of blue tits corresponded with food resources, which appeared superabundant; instead, tits appeared to behave and be distributed according to the distribution of structural resources such as trees and tree holes for nesting. Received: 18 August 1996 / Accepted: 14 March 1997     

NATURAL SELECTION ON AVIAN BREEDING TIME: CAUSALITY,FECUNDITY-DEPENDENT,AND FECUNDITY-INDEPENDENT SELECTION

Natural selection on breeding time through postfledgling survival was investigated in a population of blue tits, Parus caeruleus, between 1993 and 1996. In particular, selection acting through large brood sizes among early breeders (fecundity-dependent selection) was separated from selection acting through fitness benefits unrelated to the numerical benefits of large brood sizes (fecundity-independent selection). There was evidence that natural selection favored early breeding in 1993 and 1995 (significantly so in 1993), whereas in 1994 there was a tendency for selection to be stabilizing. Analyses of selection gradients further showed that there was direct selection for both early breeding and large brood sizes in 1993. Experimentally delayed pairs produced a lower proportion of surviving young among their fledglings compared to early pairs, indicating that the seasonal fitness trend is a causal one and not caused by differences between early and late breeders in territory or individual quality. There was no evidence for significant heritability for four reproductive traits (clutch sizes, laying dates, hatching dates, and brood size at fledging) although repeatability values ranged from 0.26 to 0.53. The importance of year-specific selective regimes in the evolution of avian breedings times are discussed and a role for population density in this context is proposed.     

Reproductive investment and parasite susceptibility in the Great Tit

1. Reproduction and parasite defence are assumed to be costly activities for hosts, and therefore trade-offs might exist between reproduction and parasite defence.
2. Brood sizes were manipulated in a population of Great Tits ( Parus major L.) to assess trade-offs between reproduction and parasite defence. Blood samples were taken from males and females during the late nestling phase, and parasite prevalence was compared among parents raising enlarged, reduced and control broods.
3. A higher prevalence of protozoan blood parasites dominated by Haemoproteus majoris was found among parents with enlarged broods, as compared with parents with control and reduced broods, respectively. Brood size manipulation did not affect parasite prevalence between the sexes or between age classes differently.
4. Mean parasite intensity (number of parasites per microscope field) among adult birds only infected with H. majoris was higher for parents with experimentally enlarged broods than controls and reduced broods, respectively.
5. These results support the hypothesis of a trade-off between parasite defence and reproduction.