Aim The laying of eggs and the building of a nest structure to accommodate them are two of the defining characteristics of members of the class Aves. Nest structures vary considerably across avian taxa and for many species the design of the completed nest can have important consequences for both parents and their offspring. While nest characteristics are expected to vary adaptively in response to environmental conditions, large-scale spatial variation in nest characteristics has been largely overlooked. Here, we examined the effects of latitudinal variation in spring temperatures on nest characteristics, including insulatory properties, and reproductive success of blue tits, Cyanistes caeruleus, and great tits, Parus major. \ud \ud Location Great Britain. \ud \ud Methods Nests and reproductive data were collected from seven study sites, spread over five degrees of latitude. Then, the nest insulatory properties were determined before the nests were separated into either nest base material or cup lining material. \ud \ud Results As spring temperatures increased with decreasing latitude, the mass of the nest base material did not vary in either species, whilst the mass of the cup lining material and nest insulatory properties decreased in both species. This suggests that in response to increasing temperatures the breeding female reduces the mass of the cup lining material thereby maintaining an appropriate microclimate for incubating and brooding. The mean laying dates of both species advanced with decreasing latitude and increasing spring temperatures, although reproductive success did not vary. \ud \ud Main conclusions This is the first study to demonstrate that birds are systematically adjusting their nest structure in response to variation in ambient temperatures across large spatial scales. Therefore, nest composition reliably indicates environmental conditions and we suggest that studies of nest structure may be sentinels for the early signs of rapid climate change

One of the most striking aspects of animal groups is their remarkable variation in size, both within and between species. While a number of mechanistic models have been proposed to explain this variation, there are few comprehensive datasets against which these models have been tested. In particular, we only vaguely understand how environmental factors and behavioral activities affect group-size distributions. Here we use observations of House sparrows (Passer domesticus) to investigate the factors determining group-size distribution. Over a wide range of conditions, we observed that animal group sizes followed a single parameter distribution known as the logarithmic distribution. This single parameter is the mean group size experienced by a randomly chosen individual (including the individual itself). For sparrows, the experienced mean group size, and hence the distribution, was affected by four factors: morning temperature, place, behavior and the degree of food spillage. Our results further indicate that the sparrows regulate the mean group size they experience, either by groups splitting more or merging less when local densities are high. We suggest that the mean experienced group size provides a simple but general tool for assessing the ecology and evolution of grouping.

Eggshell maculation of most passerines is due to the deposition of the pigment protoporphyrin which is produced during biosynthesis of blood haem. Its functional signifi cance has only received empirical attention in recent years. Th is interest has generated a number of hypotheses of which some remain untested partly because the quantifi cation of protoporphyrin is analytically challenging and can be prohibitively expensive. Many studies have therefore used the extent of eggshell spotting as a proxy for total eggshell protoporphyrin concentration, although this has not been formally tested. Pigment scoring involves recording visible eggshell pigment attributes, such as spot intensity, distribution and size. Since even immaculate eggs can contain some protoporphyrin, there remains doubt over the degree to which visible pigment correlates with total pigment content of the shell. In this study, we test whether visible pigment scoring can be used as a proxy for protoporphyrin concentration of an eggshell. We use pigmented eggshells of two common British passerine species to compare eggshell spot intensity, distribution and spot size (as used by the visual pigment scoring method) with direct measures of eggshell protoporphyrin concentration. In addition, we compared an alternative method of pigment scoring, the pixel pigment scoring method, using a computer programme to quantify the number of pixels exceeding a specifi ed colour threshold. We demonstrate that although results from both scoring methods were positively correlated with eggshell protoporphyrin concentrations, the correlations were not suffi ciently strong to be used as surrogates in studies where actual pigment concentrations are required.

Summary 1.Climate change has caused a phenological mismatch between the timing of reproduction and the local food peak in many bird species. Late breeding birds therefore experience reduced food availability during chick rearing and are thus predicted to have an increased energy expenditure. Observational studies, however, show mixed results, perhaps because they compare energy expenditure across rather than within individuals at different levels of food availability. 2. In a cross foster experiment, we measured daily energy expenditure (DEE) twice within individuals during chick feeding (when chicks were 6 and 14 days old) for 28 free-living female great tits (Parus major). To avoid confounding effects of chick age, these females reared on both occasions a standardized foster brood of eight 10-day-old chicks during the 24-h measuring period. For all birds, food availability declined between the two measurements. 3. We show that DEE during chick feeding increased within females when food availability decreased. Variation in DEE within females is partly explained by brood visit rates, food availability and temperature. 4. DEE during chick feeding could be affected by the investment in previous stages of the reproductive attempt. However, energy expenditure during chick feeding was not correlated to energy expenditure during egg laying, measured in these same females. 5. Understanding of energetic costs during all phases of the reproductive cycle is important to forecast the consequences of climate warming on timing of reproduction.

In many bird populations, variation in the timing of reproduction exists but it is not obvious how this variation is maintained as timing has substantial fitness consequences. Daily energy expenditure (DEE) during the egg laying period increases with decreasing temperatures and thus perhaps only females that can produce eggs at low energetic cost will lay early in the season, at low temperatures. We tested whether late laying females have a higher daily energy expenditure during egg laying than early laying females in 43 great tits (Parus major), by comparing on the same day the DEE of early females late in their laying sequence with DEE of late females early in their egg laying sequence. We also validated the assumption that there are no within female differences in DEE within the egg laying sequence. We found a negative effect of temperature and a positive effect of female body mass on DEE but no evidence for differences in DEE between early and late laying females. However, costs incurred during egg laying may have carry-over effects later in the breeding cycle and if such carry-over effects differ for early and late laying females this could contribute to the maintenance of phenotypic variation in laying dates.