Interpopulation Variation in Contour Feather Structure Is Environmentally Determined in Great Tits

Background: The plumage of birds is important for flying, insulation and social communication. Contour feathers cover most of the avian body and among other functions they provide a critical insulation layer against heat loss. Feather structure and composition are known to vary among individuals, which in turn determines variation in the insulation properties of the feather. However, the extent and the proximate mechanisms underlying this variation remain unexplored. Methodology/Principal Findings: We analyzed contour feather structure from two different great tit populations adapted to different winter regimes, one northern population in Oulu (Finland) and one southern population in Lund (Sweden). Great tits from the two populations differed significantly in feather structure. Birds from the northern population had a denser plumage but consisting of shorter feathers with a smaller proportion containing plumulaceous barbs, compared with conspecifics from the southern population. However, differences disappeared when birds originating from the two populations were raised and moulted in identical conditions in a common-garden experiment located in Oulu, under ad libitum nutritional conditions. All birds raised in the aviaries, including adult foster parents moulting in the same captive conditions, developed a similar feather structure. These feathers were different from that of wild birds in Oulu but similar to wild birds in Lund, the latter moulting in more benign conditions than those of Oulu. Conclusions/Significance: Wild populations exposed to different conditions develop contour feather differences either due to plastic responses or constraints. Environmental conditions, such as nutrient availability during feather growth play a crucial role in determining such differences in plumage structure among populations

Winter feeding influences the cost of living in boreal passerines

Abstract The plastic regulation of internal energy reserves is acknowledged as the main adaptive response to winter conditions of resident small birds in northern latitudes, a strategy that may be altered whenever human‐supplemented food is available. We investigated the effects of supplementary feeding on the energy management strategy of two wild passerine species, the Willow Tit Poecile montanus and Blue Tit Cyanistes caeruleus, wintering in boreal conditions by measuring body mass and the energy cost of living, i.e. basal metabolic rate. Individuals of both species were heavier, larger and exhibited a higher energy cost of living when captured at the feeders than were individuals captured away from feeders. Fed Willow Tits expended more energy in maintenance, although this difference disappeared once mass was accounted for. Conversely, Blue Tits at feeders had higher mass‐adjusted energy cost of living, but only at low ambient temperatures. The results indicate that winter feeding has species‐specific effects on overall energy management strategy and modifies the response to environmental conditions of wintering passerines

Northward expanding resident species benefit from warming winters through increased foraging rates and predator vigilance

Abstract Species distributions shift northwards due to climate change, but the ecological mechanisms allowing range expansions are not fully understood. Most studies have concentrated on breeding seasons, but winter warming may also be important. Wintering distributions are restricted by food availability and temperature, which may also interact. Foraging in cold conditions requires adaptations as individuals have to be efficient in foraging, while staying warm and vigilant for predators. When the ambient temperature declines, foraging rates should be reduced due to increased time spent on warming behaviours. In addition, predator vigilance should decline, because more time has to be invested in foraging. Cold weather should limit northward expanding southern species in particular, while northern species should perform better in cold conditions. We tested this by studying temperature responses (between 0 and − 35 °C) among wintering birds at feeders. We compared foraging behaviours of two northward expanding southern species, the great tit (Parus major) and the blue tit (Cyanistes caeruleus) to a northern species, the willow tit (Poecile montanus). Foraging rate and vigilance decreased, and warming behaviour increased when temperatures declined. Importantly, the performance in these traits was poorer in the southern species compared to the willow tit. Furthermore, the response to decreasing temperatures in foraging rates and warming behaviour was stronger in the great tits than willow tits. As the winters become warmer, these mechanisms should increase wintering success of southern species wintering at high latitudes, and lead to higher survival, increased population growth, and consequent range expansion

Prehatching temperatures drive inter-annual cohort differences in great tit metabolism

Abstract Basal metabolic rate (BMR) constitutes the lowest metabolic rate in a resting animal and is, therefore, considered to reflect the energetic cost of maintenance in endotherms. BMR is a reversible plastic trait that changes with environmental and ecological circumstances, albeit being heritable and susceptible to selection. Inter-individual variation within populations of small birds is substantial, and while many of the drivers of such variation have been identified, many remain unexplained. We studied winter BMR variation of juveniles over a 15-year period in a wild population of great tits Parus major at the northern border of their distribution. BMR during winter consistently changed between years, even after controlling for environmental factors, suggestive of a non-reversible developmental plasticity shaping the adult metabolic phenotype. BMR in cohorts of wintering great tits varied among winters as a response to minimum ambient temperatures experienced early in life, during the prehatching period. This developmental plasticity might be adaptive if temperatures experienced by growing embryos would metabolically prime them to an environment that they will likely encounter in future life. However, in line with a more unpredictable future climate, the risk of phenotype-environment mismatch is likely to lead to certain cohorts being poorly adapted to prevailing winter conditions, resulting in wider annual fluctuations in population size

Mass or pace? Seasonal energy management in wintering boreal passerines

Abstract Research on winter energy management in small vertebrates has focused on the regulation of body mass (BM) within a framework of starvation-predation trade-off. Winter-acclimatized birds exhibit a seasonal increase in both BM and basal metabolic rate (BMR), although the patterns of co-variation between the two traits remain unknown. We studied this co-variation in three different species of wild titmice, great, blue and willow tits, originating from two boreal regions at different latitudes. Seasonal change in BM and BMR was inter-dependent, particularly in the great tit; however, by contrast, no seasonal change was observed in the willow tit. BMR changed non-linearly in concert with BM with a peak in midwinter for both blue and great tits, whereas such non-linear pattern in willow tit was opposite and independent of BM. Surprisingly, BMR appears to be more sensitive to ambient temperatures than BM in all three species studied. Energy management is a multifaceted strategy that cannot be fully understood without considering reserve levels and energy expenditure simultaneously. Thus, our study indicates that the prevailing conceptual framework based on variation in BM alone is insufficient to understand seasonal energy management in small wintering passerines

Elucidating mechanisms for insect body size:partial support for the oxygen-dependent induction of moulting hypothesis

Abstract Body size is a key life history trait, and knowledge of its mechanistic basis is crucial in life history biology. Such knowledge is accumulating for holometabolous insects, whose growth is characterised and body size affected by moulting. According to the oxygen-dependent induction of moulting (ODIM) hypothesis, moult is induced at a critical mass at which oxygen demand of growing tissues overrides the supply from the tracheal respiratory system, which principally grows only at moults. Support for the ODIM hypothesis is controversial, partly because of a lack of proper data to explicitly test the hypothesis. The ODIM hypothesis predicts that the critical mass is positively correlated with oxygen partial pressure (PO2) and negatively with temperature. To resolve the controversy that surrounds the ODIM hypothesis, we rigorously test these predictions by exposing penultimate-instar Orthosia gothica (Lepidoptera: Noctuidae) larvae to temperature and moderate PO2 manipulations in a factorial experiment. The relative mass increment in the focal instar increased along with increasing PO2, as predicted, but there was only weak suggestive evidence of the temperature effect. Probably owing to a high measurement error in the trait, the effect of PO2 on the critical mass was sex specific; high PO2 had a positive effect only in females, whereas low PO2 had a negative effect only in males. Critical mass was independent of temperature. Support for the ODIM hypothesis is partial because of only suggestive evidence of a temperature effect on moulting, but the role of oxygen in moult induction seems unambiguous. The ODIM mechanism thus seems worth considering in body size analyses

Mitochondrial 2,4-dienoyl-CoA reductase (Decr) deficiency and impairment of thermogenesis in mouse brown adipose tissue

Abstract A large number of studies have demonstrated significance of polyunsaturated fatty acids (PUFAs) for human health. However, many aspects on signals translating PUFA-sensing into body homeostasis have remained enigmatic. To shed light on PUFA physiology, we have generated a mouse line defective in mitochondrial dienoyl-CoA reductase (Decr), which is a key enzyme required for β-oxidation of PUFAs. Previously, we have shown that these mice, whose oxidation of saturated fatty acid is intact but break-down of unsaturated fatty acids is blunted, develop severe hypoglycemia during metabolic stresses and fatal hypothermia upon acute cold challenge. In the current work, indirect calorimetry and thermography suggested that cold intolerance of Decr−/− mice is due to failure in maintaining appropriate heat production at least partly due to failure of brown adipose tissue (BAT) thermogenesis. Magnetic resonance imaging, electron microscopy, mass spectrometry and biochemical analysis showed attenuation in activation of lipolysis despite of functional NE-signaling and inappropriate expression of genes contributing to thermogenesis in iBAT when the Decr−/− mice were exposed to cold. We hypothesize that the failure in turning on BAT thermogenesis occurs due to accumulation of unsaturated long-chain fatty acids or their metabolites in Decr−/− mice BAT suppressing down-stream propagation of NE-signaling