Nestling Mass Research Articles

Fitness benefits of alternated chick provisioning in cooperatively breeding carrion crows.

In most bird species, parents raise offspring cooperatively. In some cases, this cooperation extends to helpers-at-the-nest who assist the breeders with a range of tasks. While cooperative food provisioning might merely arise incidentally, as a result of the efforts of carers that act independently from each other, recent studies suggest that birds may coordinate by taking turns in visiting the nest. However, evidence that such coordination emerges because individuals actively respond to each other's behaviour is controversial, and the potential benefits of carers' alternation remain unknown. We addressed this knowledge gap by analysing a multiyear dataset for cooperatively breeding carrion crows Corvus corone, comprising 8693 nest visits across 50 groups. Our results reveal that turn-taking does occur in this species and that all group members, regardless of their sex and social role (breeder/helper), tend to alternate at the nest with other carers rather than to make repeat visits. Importantly, we found that the body mass of nestlings increased significantly with the degree of carers' alternation, possibly because well-coordinated groups provided food at more regular intervals. Using earlier monitoring data, the observed increase in body mass is predicted to substantially boost postfledging survival rates. Our analyses demonstrate that alternation in nestling provisioning has measurable fitness benefits in this study system. This raises the possibility that cooperatively breeding carrion crows, as well as other bird species with similarly coordinated brood provisioning, exhibit specialized behavioural strategies that enable effective alternation.

Variable choice affects estimations of vulnerability to climate change

For practical reasons, assessments of species' vulnerability to rising temperatures are often limited to measuring responses to a single ecological response variable, but this could result in an underestimation of vulnerability. Using the Cape Rockjumper Chaetops frenatus (‘Rockjumper’) we examined the thermal risk to nestling Rockjumpers for sublethal (i.e. reduced nestling mass gain) and lethal (i.e. increased nest predation) consequences of sustained hot weather under both current and predicted future climatic conditions (RCP 8.5). We used a direct approach to examine these risks, first as independent ecological responses and then as combined risk driven by both response variables (mass gain and predation risk). This study revealed that the inclusion of multiple climate‐related responses affected the predicted vulnerability to climate change. Further, our analyses showed that increased vulnerability to climate change will vary within the Rockjumper's habitat. Our results demonstrate that the variability in predicted thermal risk depends on which response variable was used, with implications for how and where conservation practitioners direct their already limited resources.

High nest failure but better nestling quality for early breeders in an alpine population of Northern Wheatear (Oenanthe oenanthe)

Climate change is leading to the advancement of spring conditions, resulting in an earlier snowmelt and green‐up, with highest rates of change in highly seasonal environments, including alpine habitats. Migratory birds breeding at high elevations need to time their arrival and lay dates accurately with this advancement, but also with the annually variable spring conditions at their breeding sites, to maximize nest survival probabilities and reproductive output. Nest survival probability and mean nestling mass were analysed in relation to lay date and habitat conditions in an alpine population of the migratory Northern Wheatear Oenanthe oenanthe collected over six consecutive breeding seasons in the Western Italian Alps. This open grassland species showed the lowest nest survival probability in years with an early onset of spring conditions. Within‐season, nest survival was highest when breeding late, at lower elevations, and when grass cover and grass height were higher. Both across‐ and within‐season, severe weather conditions may indirectly lead to higher early season nest failure rates by increasing predation risk. By contrast, mean nestling mass, and thus the quality of the fledglings, was lower when breeding late. This might be driven by a mismatch with the peak in food abundance. Breeding early is thus generally advantageous in terms of chick quality in our high‐elevation population, but reproductive success is limited by the risk of nest failure that is higher in early springs and early in the season. This trade‐off between breeding early and late may thus allow Northern Wheatears to maximize fitness under highly variable spring conditions. However, climate change may cause disruption to this trade‐off, and shifts in phenology could become a threat for migratory alpine birds that might not be able to keep track of advancing spring conditions.

The interactive effect of ambient temperature and brood size manipulation on nestling body mass in blue tits: an exploratory analysis of a long-term study

BackgroundRelatively few studies have examined the interactive effects of ecological factors on physiological responses in wild animals. Nearly all of them have been short-term investigations that did not include experimental manipulations, limiting our ability to understand how climate change will affect natural populations. Using a 10-year brood size manipulation experiment in wild blue tits (Cyanistes caeruleus), we quantified the impact of weather conditions and brood competition on the body mass and structural size (tarsus length) of nestlings just prior to leaving the nest.ResultsWe found that variation in nestling body mass on day 14 after hatching was explained by an interactive effect between average ambient temperature experienced during nestling period and brood size treatment. Specifically, in control broods nestling body mass was correlated with temperature in a non-linear manner (concave) with the vertex point (maximum body mass) at ca. 13 °C. In contrast, in enlarged broods nestling body mass permanently increased (also non-linearly) as temperature advanced.ConclusionsOur results highlight the importance of considering the effects of brood rearing conditions alongside other environmental factors experienced during growth while investigating early-life environmental effects on body condition.

Higher temperatures are associated with reduced nestling body condition in a range‐restricted mountain bird

Demonstrated negative effects of increased temperatures on avian reproductive success suggest a mechanism by which climate change may impact species persistence. High temperatures can result in reduced parental care and reduced nestling condition in passerines with dependent young, resulting in lowered fledging success and population recruitment. We examined provisioning rate and nestling condition in a South African mountain endemic, the Cape rockjumper Chaetops frenatus, whose population declines correlate with warming habitat. Our aim was to determine whether rockjumper reproductive success could be affected by high air temperatures. We set up video cameras on nests at three nestling age classes (≤ 7 days old; 8–12 days old; ≥ 13 days old) for 8 hours on 37 separate days. We successfully collected full‐day footage on 25 of the 37 days (four days with predation, eight with equipment failure). Nestlings were weighed at the beginning and end of each film day, barring the four days with mid‐day predation (n = 65 nestling measures from 33 of the 37 days). Average mass gain across all nestlings per nest was positively correlated with provisioning rate (0.78 g provisions−1 hr−1, CI: 0.26–1.30), and provisioning rate decreased at increasing temperatures (−0.08 provisions hr−1 °C−1, CI: −0.15 to −0.01). Daily change in mass of individual nestlings was negatively correlated with air temperatures above a significant temperature threshold (22.4°C; −0.30 g °C−1, CI: −0.40 to −0.19). This suggests nestling energy requirements were not being met on higher temperature days – perhaps because nestling energy and water demands for thermoregulation are elevated and provisioning rate is not correspondingly maintained or increased. These results suggest that higher temperatures negatively affect nestling mass gain. While in our study this did not directly affect fledging rates, it may affect post‐fledging survival.

The time-activity budgets of breeding Blue Swallows Hirundo atrocaerulea and the effects of weather on nestling growth

The Blue Swallow is an intra-African migrant that breeds in its fragmented range from South Africa to southern Tanzania, from October to March annually. The birds migrate north and are on their non-breeding range in NE DRC, Uganda, NW Tanzania and W Kenya, from April to September annually. The global Blue Swallow population, estimated at <1 500 pairs, is classified as ‘Vulnerable’, because of its small and declining population. Only the female Blue Swallow incubates the eggs and broods the nestlings for the first nine days after they have hatched from the eggs. The growth rate of Blue Swallow nestlings is reduced by adverse weather conditions characterised by low temperatures (<14 °C), 3–4 consecutive days of fog and intermittent rainfall. In the Blue Swallow Natural Heritage Site (25°36′ S, 30°45′ E), food shortages, because of three or more consecutive days of adverse weather conditions resulted in Blue Swallows shifting entirely to self-maintenance by abandoning eggs and nestlings and attempting to breed again when weather conditions improved. The post-fledging survival of the nestlings affected by adverse weather conditions is reduced. The reason is that at the time of fledging the mass of these nestlings was below the mean mass of the adults and because the mass of nestlings developing under good weather conditions is equal to or greater than the mean mass of the adult birds. Circumstantial evidence suggests that Common Fiscal Lanius collaris may be the predator of Blue Swallow eggs and nestlings. Adult longevity and the breeding success of Blue Swallows may have been reduced by adverse weather affecting their entire meta-populations in the small and isolated patches of suitable habitat remaining for the birds. These characteristics of the Blue Swallow meta-populations may be contributing to their continued decline.

Pre-fledging quality and recruitment in an aerial insectivore reflect dynamics of insects, wetlands and climate.

Wetland systems, including shallow palustrine ponds, are hotspots for emergent aquatic insects but are globally threatened by land-use practices and climate change. Loss of insects is hypothesized as a key driver of population declines in aerial insectivores, but studies of climate-driven fluctuations in pond abundance during wet-dry periods and aerial insects on nestling quality and apparent recruitment are lacking. Using tree swallow (Tachycineta bicolor) data spanning 14-28years we evaluated: (1) whether nestling quality based on pre-fledging (~ 12days old) body mass changed over the time series; (2) how annual estimates of aerial insect biomass and variability, temperature, and pond abundance influenced nestling mass; and (3) whether the annual number of recruits produced was related to the annual mean mass of nestlings, aerial insects, and pond abundance in their year of hatching. Average nestling body mass varied annually but no long-term temporal trends were detected. Nestlings were heavier when raised during periods of stable insect biomass, warmer temperatures, and higher pond abundance. Pond abundance consistently had strong effects on nestling mass and inter-annual apparent recruitment, suggesting that this metric provides a complementary index of either higher prey abundance or higher-quality aquatic prey. Overall, pre-fledging quality and annual recruitment of nestling tree swallows reflects dynamic interannualchanges in climate, pond availability, and aerial insect food supply. Our findings further suggest the abundance of ponds in this semi-arid prairie landscape is likely a strong predictor of regional population stability in tree swallows and possibly other ecologically similar species.

Weather effects on nestling survival of great tits vary according to the developmental stage

Organisms change breeding investments as a function of the environment, thereby maximizing reproductive success. Climate change studies of avian life‐history have long focused on plasticity of laying dates and clutch sizes in response to weather conditions prior to clutch initiation. By contrast, effects of unpredictable weather events occurring after initial reproductive decisions are made have largely been ignored, despite becoming increasingly important with ongoing climate change. We studied the detrimental effects of fluctuations in temperature and precipitation during various nesting phases of great titsParus major, identifying the developmental age windows where weather fluctuations affected hatching, nestling mass and fledging success. We used a longitudinal (8‐year) dataset of great tits breeding in nest boxes in southern Germany and applied a recently introduced explorative approach that does not require a priori assumptions on the time windows (range of ages) over which weather may affect reproductive parameters. Lower temperatures and higher precipitation during the nestling phase negatively affected nestling mass and survival: nestlings between the ages 6 and 9 days were most susceptible to this form of variation in weather. Effects of weather did not differ between years, i.e. there was no evidence for ‘good' versus ‘bad' years. Future research should focus on the mechanisms underpinning effects of weather on nestling growth and survival in other populations and consider if age‐dependent weather consequences occur in a wide variety of taxa, as specific developmental stages may serve as weak spots in a scenario of increasing recurrence of weather extremes.

Testing the utility of condition indices in nestling swallows: a quantitative magnetic resonance approach

Nutritional condition is a key factor influencing the survival of migratory birds, and condition indices derived from morphometric measurements are commonly used to assess nutrient and energy stores. The performance and utility of condition indices used to examine energetics of growing juvenile birds remain uncertain. Recently, quantitative magnetic resonance (QMR) has been used to provide a non-invasive and non-lethal way to obtain measures of body composition. We calculated three widely used body condition indices using both wing length and tarsus length for nestling Barn Swallows Hirundo rustica erythrogaster at two ages (Day 8 and 15) and compared them to relative fat and lean mass calculated using QMR measurements. Additionally, we used QMR to develop multiple regression formulas to predict nestling fat mass, lean mass, and fat-free lean body mass (LBM) from easily obtainable body metrics. In Day 8 birds, variation in both relative fat mass and relative lean mass were adequately explained by total body mass alone, but this relationship was strongest for lean mass. Almost every condition index evaluated failed to explain variation in relative fat or lean mass in Day 15 birds. While our best multiple regression formulas for predicting fat mass and LBM had low predictive power, our formulas for predicting lean mass were precise (R2 > 90%) in both ages, providing a method to estimate the lean mass of nestlings from more readily obtainable metrics. Estimates of fat mass calculated from LBM and total body mass showed poor precision (R2 ≤ 30%).

Contrasting sensitivity of nestling and fledgling Barn Swallow Hirundo rustica body mass to local weather conditions

Local weather can influence the growth and development of young birds either indirectly, by modifying prey availability, or directly, by affecting energetic trade‐offs. Such effects can have lasting implications for life history traits, but the nature of these effets may vary with the developmental stage of the birds, and over timescales from days to weeks. We examined the interactive effects of temperature, rainfall and wind speed on the mass of nestling and fledgling Barn Swallows Hirundo rustica both on the day of capture and averaging weather across the time since hatching. At the daily timescale, nestling mass was negatively correlated with temperature, but the strength of this association depended on the level of rainfall and wind speed; nestlings were typically heavier on dry or windy days, and the negative effect of temperature was strongest under calm or wet conditions. At the early lifetime timescale (i.e. from hatching to pre‐fledging), nestling mass was negatively correlated with temperature at low wind speed. Fledgling body mass was less sensitive to weather; the only weather effect evident was a negative correlation with temperature at the daily scale under high rainfall that became slightly positive under low rainfall. These changes are consistent with weather effects on the availability and distribution of insects within the landscape (e.g. causing high concentrations of flying insects) and with the effects of weather variation on nest microclimate. These results together demonstrate the impacts of weather on chick growth, over immediate (daily) and longer term (nestling/fledgling lifetime) timescales. This shows that sensitivity to local weather conditions varies across the early lifetime of young birds (nestling–fledgling stages) and illustrates the mechanisms by which larger scale (climate) variations influence the body condition of individuals.

Effects of Food Availability and Parental Risk Taking on Nestling Period Duration: A Field ExperimenT on the Pied Flycatcher Ficedula hypoleuca

The timing of fledging is critical in altricial birds as it affects offspring survival probability and thereby both parental and offspring fitness. The high vulnerability of offspring to predation soon after fledging may induce a parent-offspring conflict with respect to the age of fledging, as parents would benefit more than offspring from reducing the period of whole-brood vulnerability before fledging. Parents also benefit from inducing early fledging through reduced commuting costs and a decreased exposure to predators. A reliable food resource near the nest could thus favour a relaxation of parental drives to induce fledging and enable an approach to the optimal fledging age from the offspring point of view. A population of a cavity nester, the European Pied Flycatcher Ficedula hypoleuca, was offered food supplements (live mealworms) at the nest-box to randomly selected pairs throughout the nestling period and fledging ages were compared with those at nests without supplements. Provisioning parents were also trapped at the nest-box to estimate their willingness to incur predation risks in terms of easiness of capture (trappability). Fledging ages ranged from 15 to 20 days. Fledging was delayed in food-supplemented nests (n = 21) compared with control nests (n = 20) by one day on average, a significant difference. This was not due to the size and mass of nestlings at 13 days, which were similar under both treatments. Moreover, fledging age was positively related to parental trappability. Parents with easy access to food near the nest and those more willing to risk predation were those whose nestlings fledged at older ages. Parents apparently adjust the timing of offspring fledging to their foraging costs.—Moreno, J. (2020). Effects of food availability and parental risk taking on nestling period duration: a field experiment on the Pied Flycatcher Ficedula hypoleuca. Ardeola, 67: 29-38.

Anthropogenic noise is associated with telomere length and carotenoid-based coloration in free-living nestling songbirds

Growing evidence suggests that anthropogenic noise has deleterious effects on the behavior and physiology of free-living animals. These effects may be particularly pronounced early in life, when developmental trajectories are sensitive to stressors, yet studies investigating developmental effects of noise exposure in free-living populations remain scarce. To elucidate the effects of noise exposure during development, we examined whether noise exposure is associated with shorter telomeres, duller carotenoid-based coloration and reduced body mass in nestlings of a common urban bird, the great tit (Parus major). We also assessed how the noise environment is related to reproductive success. We obtained long-term measurements of the noise environment, over a ∼24-h period, and characterized both the amplitude (measured by LAeq, LA90, LA10, LAmax) and variance in noise levels, since more stochastic, as well as louder, noise regimes might be more likely to induce stress. In our urban population, noise levels varied substantially, with louder, but less variable, noise characteristic of areas adjacent to a highway. Noise levels were also highly repeatable, suggesting that individuals experience consistent differences in noise exposure. The amplitude of noise near nest boxes was associated with shorter telomeres among smaller, but not larger, brood members. In addition, carotenoid chroma and hue were positively associated with variance in average and maximum noise levels, and average reflectance was negatively associated with variance in background noise. Independent of noise, hue was positively related to telomere length. Nestling mass and reproductive success were unaffected by noise exposure. Results indicate that multiple dimensions of the noise environment, or factors associated with the noise environment, could affect the phenotype of developing organisms, that noise exposure, or correlated variables, might have the strongest effects on sensitive groups of individuals, and that carotenoid hue could serve as a signal of early-life telomere length.

Differential fitness effects of moonlight on plumage colour morphs in barn owls

The Moon cycle exposes nocturnal life to variation in environmental light. However, whether moonlight shapes the fitness of nocturnal species with distinct colour variants remains unknown. Combining long-term monitoring, high-resolution GPS tracking, and experiments on prey, we show that barn owls (Tyto alba) with distinct plumage colourations are differently affected by moonlight. The reddest owls are less successful hunting and providing food to their offspring during moonlit nights, which associates with lower body mass and survival of the youngest nestlings and with female mates starting to lay eggs at low moonlight levels. Although moonlight should make white owls more conspicuous to prey, hunting and fitness of the whitest owls are positively or un-affected by moonlight. We experimentally show that, under full-moon conditions, white plumages trigger longer freezing times in the prey, which should facilitate prey catchability. We propose that the barn owl’s white plumage, a rare trait among nocturnal predators, exploits the known aversion of rodents to bright light, explaining why, counterintuitively, moonlight impacts less the whitest owls. Our study provides evidence for the long-suspected influence of the Moon on the evolution of colouration in nocturnal species, highlighting the importance of colour in nocturnal ecosystems.

Differential larval phenology affects nestling condition of Green-backed Tit (Parus monticolus) in broadleaf and coniferous habitats, subtropical Taiwan

Trophic interactions between birds and their prey often vary among habitat types, but they are poorly studied in low-latitude regions. We assessed the seasonal effect of larval abundance on the breeding performance of the Green-backed Tit (Parus monticolus), a caterpillar specialist, in broadleaf (mixed-oak forest) and coniferous (conifer plantation) habitats in subtropical Taiwan. We measured the biomass of Lepidoptera and Hymenoptera larvae in five tree species: two evergreen oaks and one deciduous alder in broadleaf habitat and two conifers in coniferous habitat. Alder supported a high larval biomass peak in early spring, dominated by noctuid and sawfly larvae, while the two oaks had later, lower peaks. The diversity of trees in the broadleaf habitat supported a broad food peak, spiking in early spring and gradually declining. In contrast, the coniferous habitat had a comparatively lower larval biomass that increased slightly over the season and included a high percentage of hairy lithosiines. Habitat-specific seasonality in larval abundance affected nestling conditions. Early-brood nestlings in the broadleaf habitat were heavier than those in the coniferous habitat. However, the between-habitat difference disappeared in the late broods because the mass of nestlings in the coniferous habitat increased significantly. We found a linear relationship between nestling condition and the total larval biomass available at demand peaks, but there were no differences in annual breeding density, late-brood frequency, laying date, clutch size, or fledging success between the two habitats. Food availability constrained nestling growth but not survival. Habitat-related trophic interactions are present in these subtropical montane forests.

Mixed effects of urbanization on density, nest survival, and nestling corticosterone of a generalist passerine

AbstractGeneralist species occupy a wide range of habitats and typically respond well to environmental disturbance. Highly disturbed urban areas may contain habitat that generalists can use and exploit; however, habitat use and quality of areas within urban landscapes are poorly understood. Here, we investigated habitat use of a generalist passerine, the red‐winged blackbird (Agelaius phoeniceus), in urban and rural grassland patches in greater metropolitan Chicago, USA. From 2012 to 2013, we conducted point counts on 30 grassland patches and searched for nests at a subset of these sites. We measured and weighed nestlings and collected fecal samples for corticosterone metabolite analysis. We found that population densities were greater in grassland patches in more rural areas, despite higher rates of nest predation and brood parasitism. We found that clutch size and number of young fledged did not differ between urban and rural areas. Average body mass and tarsus length of nestlings did not vary with urbanization, but nestlings in rural environments had significantly greater fecal corticosterone metabolites than urban nestlings. These results suggest that, even for widely successful generalist species, novel habitat can create a disconnection between habitat use and quality. The reason for this disconnection is unclear but may be related to perceived predation risk or other biotic or abiotic cues. Addressing the mechanisms behind undervaluation of habitat within urban landscapes will aid in wildlife conservation and management in novel environments.

Parental body condition and high energy value of fish determine nestling success in Magellanic penguin (Spheniscus magellanicus)

Nestling growth and survival of marine birds are influenced by multiple factors. Oceanic conditions influence food availability and energy value of the diet and affect nestling condition. Climate variables, like heavy rains, could drown or cause hypothermia of nestlings. Besides oceanic conditions, parental conditions and hatching asynchrony can also affect nestling growth and survival. Magellanic penguins are long-lived seabirds with multiple breeding opportunities that lay a two-egg clutch, but they may raise only one of the two chicks depending on the breeding season. We studied nestling growth and survival for four breeding seasons (2010–2013) and related these to parents’ body condition and hatching asynchrony. We also analysed the effect of nestling diet, rainfall, chlorophyll a concentrations (c-a), and sea surface temperature (SST) of known foraging sites to evaluate nestling performance. We found that parents’ body condition, especially that of males, were positively related to nestling growth. Hatching asynchrony appears not to have an important role in nestling growth/survival. Nestling growth/survival, rain, diet, c-a and SST differed drastically among breeding seasons. During the breeding season with the highest mortality and lowest asymptotic mass of nestlings (2012), we registered heavy daily rains within the first stage of the raising period. We also found that high/low concentrations of c-a were related to high/low mortality of nestlings, respectively, and that SST was inversely related to asymptotic mass. Nonetheless, neither of these oceanic variables was related clearly to nestling growth/survival during breeding seasons with intermediate breeding success. On the contrary, the estimated proportion of low quality compared to high quality items in nestling diets coincided in every breeding season with the survival/growth of nestlings. We concluded that c-a and SST were not precise enough to determine nestling success or survival of nestlings during breeding seasons with intermediate nestling success, but that the proportion of high quality compared to low quality items in the diet were key for nestling survival and growth. Although the breeding seasons did not seem to be so “unfavourable” in terms of c-a and SST, when low quality items predominated in the diet, this had a strong impact on the condition of the chicks.

Reproductive and developmental changes in tree swallows (Tachycineta bicolor) are influenced by multiple stressors, including polycyclic aromatic compounds, in the Athabasca Oil Sands

Mining in the Athabasca Oil Sands Region (AOSR) has contributed extensively to increased exposure of wildlife to naturally occurring polycyclic aromatic compounds (PACs), yet little is known about the toxicity of PACs to wildlife in this region. We identified reproductive and developmental changes in tree swallows (Tachycineta bicolor) breeding in close proximity to mining-related activities in the AOSR, and determined these changes in relation to the birds' exposure and accumulation of 41 PACs (parent-, alkylated-PAHs), dibenzothiophenes (DBTs; previously published), diet (carbon (δ13C), nitrogen (δ15N)), volatile organic compounds, and weather variables. Tree swallow pairs (N = 43) were compared among mining-related (OS1, OS2) and reference (REF1, REF2) sites. At OS2, clutch initiation was slightly advanced (2012) but reproductive success (65%) was much lower than at the other sites (≥ 79%). Fledgling production by each pair was influenced by the timing of clutch initiation (years combined); in a highly inclement brood rearing period (2013), additional influences included the nestlings' exposure to ΣDBTs, accumulation of C1-naphthalene, the trophic position of the prey in their diet (δ15N), and record-breaking rainfall. Nestlings at OS2 were significantly lighter at day (d) 9 and d14, and in poorer body condition (d9). Nestling body mass was influenced by multiple stressors that varied by site: mass of younger nestlings (d9) was related to dietary source (δ13C; e.g., wetlands, terrestrial fields), exposure and/or accumulation of C1-phenanthrenes, C2-fluorenes, Σalkyl-PAHs and ΣDBTs, while for older nestlings (d14), body mass was related to sex, hatch date and/or rainfall during brood rearing. The swallows' exposure and accumulation of parent-PACs, alkyl-PACs and DBTs, the timing of hatching, their diet and exposure to highly inclement rains, contributed to their reproductive and developmental changes.

Incubation temperature impacts nestling growth and survival in an open-cup nesting passerine.

For oviparous species such as birds, conditions experienced while in the egg can have long‐lasting effects on the individual. The impact of subtle changes in incubation temperature on nestling development, however, remains poorly understood, especially for open‐cup nesting species with altricial young. To investigate how incubation temperature affects nestling development and survival in such species, we artificially incubated American robin (Turdus migratorius) eggs at 36.1°C (“Low” treatment) and 37.8°C (“High” treatment). Chicks were fostered to same‐age nests upon hatching, and we measured mass, tarsus, and wing length of experimental nestlings and one randomly selected, naturally incubated (“Natural”), foster nest‐mate on days 7 and 10 posthatch. We found significant effects of incubation temperature on incubation duration, growth, and survival, in which experimentally incubated nestlings had shorter incubation periods (10.22, 11.50, and 11.95 days for High, Low, and Natural eggs, respectively), and nestlings from the Low treatment were smaller and had reduced survival compared to High and Natural nestlings. These results highlight the importance of incubation conditions during embryonic development for incubation duration, somatic development, and survival. Moreover, these findings indicate that differences in incubation temperature within the natural range of variation can have important carryover effects on growth and survival in species with altricial young.

Parasites favour intermediate nestling mass and brood size in cliff swallows.

A challenge of life-history theory is to explain why animal body size does not continue to increase, given various advantages of larger size. In birds, body size of nestlings and the number of nestlings produced (brood size) have occasionally been shown to be constrained by higher predation on larger nestlings and those from larger broods. Parasites also are known to have strong effects on life-history traits in birds, but whether parasitism can be a driver for stabilizing selection on nestling body size or brood size is unknown. We studied patterns of first-year survival in cliff swallows (Petrochelidon pyrrhonota) in western Nebraska in relation to brood size and nestling body mass in nests under natural conditions and in those in which hematophagous ectoparasites had been removed by fumigation. Birds from parasitized nests showed highest first-year survival at the most common, intermediate brood-size and nestling-mass categories, but cliff swallows from nonparasitized nests had highest survival at the heaviest nestling masses and no relationship with brood size. A survival analysis suggested stabilizing selection on brood size and nestling mass in the presence (but not in the absence) of parasites. Parasites apparently favour intermediate offspring size and number in cliff swallows and produce the observed distributions of these traits, although the mechanisms are unclear. Our results emphasize the importance of parasites in life-history evolution.

Do glucocorticoids predict fitness? Linking environmental conditions, corticosterone and reproductive success in the blue tit, Cyanistes caeruleus.

Glucocorticoids, including corticosterone (CORT), have been suggested to provide a physiological link between ecological conditions and fitness. Specifically, CORT, which is elevated in response to harsh conditions, is predicted to be correlated with reduced fitness. Yet, empirical studies show that CORT can be non-significantly, positively and negatively linked with fitness. Divergent environmental conditions between years or study systems may influence whether CORT is linked to fitness. To test this, we monitored free-living blue tits (Cyanistes caeruleus) during breeding over 3 years. We quantified foraging conditions during brood rearing, and examined whether they were correlated with parental baseline CORT and reproductive success. We then tested whether CORT predicted fitness. Elevated parental CORT was associated with lower temperatures, greater rainfall and lower territory-scale oak density. Whereas asynchrony with the caterpillar food peak was correlated with reduced nestling mass and fledging success, but not parental CORT. Only low temperatures were associated with both reduced nestling mass and elevated parental CORT. Despite this, parents with elevated CORT had lighter offspring in all years. Contrarily, in 2009 parental CORT was positively correlated with the number fledged. The absence of a direct link between the foraging conditions that reduce nestling quality and elevate parental CORT suggests that parental CORT may provide a holistic measure of conditions where parents are working harder to meet the demands of developing young. As the positive correlation between parental CORT and fledging success differed between years, this suggests that contrasting conditions between years can influence correlations between parental CORT and fitness. Ultimately, as CORT concentrations are intrinsically variable and linked to the prevalent conditions, studies that incorporate environmental harshness will improve our understanding of evolutionary endocrinology.