Songbirds do not create long-term stores of calcium in their legs prior to laying: results from high-resolution radiography

We prepared high-resolution radiographs of the leg bones of three passerine species using X-rays at the Cornell High Energy Synchrotron Source (CHESS), which allows imaging with exceptionally high contrast and resolution. In studies of living Tree Swallows before and after laying and in studies of Brown-headed Cowbirds and Great Tits collected before, during or after laying, there is no evidence of calcium storage in the leg bones. By contrast, Great Tit nestlings raised in environments with extremely low calcium availability exhibited calcium deprivation in the development of their leg bones.     

Latitudinal variation in blue tit and great tit nest characteristics indicates environmental adjustment

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 structure of the completed nest can have important consequences both for 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 examine 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. Location Great Britain. Methods Nests and reproductive data were collected from seven study sites, spread over 5° of latitude. The nest insulatory properties were then determined before the nests were separated into nest base material and cup lining material. Results As spring temperatures increased with decreasing latitude, the mass of the nest base material did not vary in either species, while 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 first egg date of both species advanced with decreasing latitude and increasing spring temperatures, although clutch size and brood size at hatching and fledging did not vary. Main conclusions This is the first study to demonstrate that the nest‐construction behaviour of birds varies in response to large‐scale spatial variation in ambient temperatures. 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.     

Stressful colours: corticosterone concentrations in a free-living songbird vary with the spectral composition of experimental illumination

Organisms have evolved under natural daily light/dark cycles for millions of years. These cycles have been disturbed as night-time darkness is increasingly replaced by artificial illumination. Investigating the physiological consequences of free-living organisms in artificially lit environments is crucial to determine whether nocturnal lighting disrupts circadian rhythms, changes behaviour, reduces fitness and ultimately affects population numbers. We make use of a unique, large-scale network of replicated field sites which were experimentally illuminated at night using lampposts emanating either red, green, white or no light to test effect on stress hormone concentrations (corticosterone) in a songbird, the great tit (Parus major). Adults nesting in white-light transects had higher corticosterone concentrations than in the other treatments. We also found a significant interaction between distance to the closest lamppost and treatment type: individuals in red light had higher corticosterone levels when they nested closer to the lamppost than individuals nesting farther away, a decline not observed in the green or dark treatment. Individuals with high corticosterone levels had fewer fledglings, irrespective of treatment. These results show that artificial light can induce changes in individual hormonal phenotype. As these effects vary considerably with light spectrum, it opens the possibility to mitigate these effects by selecting street lighting of specific spectra.     

Does coping style predict optimization? An experimental test in a wild passerine bird

A number of studies have suggested that avian brood size is individually optimized. Yet, optimal reproductive decisions likely vary owing to among-individual differences in environmental sensitivity. Specifically, ‘proactive’ individuals who do not track environmental changes may be less able to produce optimal brood sizes than ‘reactive’ individuals who have more precise local environmental knowledge. To test this, we quantified exploratory behaviour (a proxy for proactivity) in a great tit (Parus major) population, manipulated brood sizes (reduced, control, enlarged) and evaluated whether individuals of dissimilar coping style differed in their level of optimization. If reactive females behaved optimally, any deviation from their original brood size should lower fitness, whereas this should not be the case for proactive females. Reactive females indeed performed best at their natural brood size, whereas proactive females performed best when raising an enlarged brood. These findings imply that proactive females produced sub-optimal brood sizes. We speculate that proactive females might (i) take decisions based on biased perception of their environment, (ii) face energetic constraints in offspring production and/or (iii) be more willing to invest into current reproduction when given the option. Our findings provide experimental evidence for coping style-related differences in optimal reproductive decisions and life-history strategies.     

Contrasting the seasonal and elevational prevalence of generalist avian haemosporidia in co‐occurring host species

Understanding the ecology and evolution of parasites is contingent on identifying the selection pressures they face across their infection landscape. Such a task is made challenging by the fact that these pressures will likely vary across time and space, as a result of seasonal and geographical differences in host susceptibility or transmission opportunities. Avian haemosporidian blood parasites are capable of infecting multiple co‐occurring hosts within their ranges, yet whether their distribution across time and space varies similarly in their different host species remains unclear. Here, we applied a new PCR method to detect avian haemosporidia (genera Haemoproteus, Leucocytozoon, and Plasmodium) and to determine parasite prevalence in two closely related and co‐occurring host species, blue tits (Cyanistes caeruleus, N = 529) and great tits (Parus major, N = 443). Our samples were collected between autumn and spring, along an elevational gradient in the French Pyrenees and over a three‐year period. Most parasites were found to infect both host species, and while these generalist parasites displayed similar elevational patterns of prevalence in the two host species, this was not always the case for seasonal prevalence patterns. For example, Leucocytozoon group A parasites showed inverse seasonal prevalence when comparing between the two host species, being highest in winter and spring in blue tits but higher in autumn in great tits. While Plasmodium relictum prevalence was overall lower in spring relative to winter or autumn in both species, spring prevalence was also lower in blue tits than in great tits. Together, these results reveal how generalist parasites can exhibit host‐specific epidemiology, which is likely to complicate predictions of host–parasite co‐evolution.     

Manipulation of male attractiveness induces rapid changes in avian maternal yolk androgen deposition

Avian eggs contain maternal androgens that may adjust offspringdevelopment to environmental conditions. We review evidenceand functional explanations for the relationship between androgenconcentrations in avian eggs and male attractiveness. Experimentalstudies in captive birds show generally positive relationships,but results from correlational and experimental field studiesare less consistent, perhaps because they lack a within-femaledesign to control for confounding between-female variation.We analyzed the effect of male attractiveness on yolk levelsof maternal androgens in a wild bird, using a correlationaland experimental approach with a within-female design. We manipulatedthe sexually selected UV coloration of the crown feathers ofmale blue tits (Cyanistes caeruleus) after their female hadlaid the second egg and measured the subsequent effect on androgenconcentrations (testosterone and androstenedione) in the fifth,seventh, and ninth eggs relative to that in the second egg.Levels of testosterone, but not androstenedione, in eggs 5 and7 were higher for control (attractive) than for UV-reduced (unattractive)males. This effect disappeared in the ninth egg, coincidingwith the recovery of UV coloration after manipulation. Thissuggests that females are capable of rapid adjustments of testosteronedeposition in response to changes in their mate's ornamentalplumage. However, androgen concentrations in the second eggand pretreatment male crown coloration were not correlated.Possibly, the combination of relatively small variation in UVcoloration before treatment and the influence of unknown confoundingvariables in the correlative approach resulted in insufficientstatistical power to detect such a correlation.