• Energy Research

In bird species where offspring growth and survival rely on parents’ food provisioning, parents can maximise their fitness by increasing the quantity and/or the quality of preys delivered to their offspring. Many studies have focused on inter-individual variation in feeding rate, yet this measure may not accurately reflect the total amount of food (i.e. energy) provided by parents if there is large variation in the quantity and quality of preys at each feeding. Here, we explored the relative role of individual (sex, age, body condition), breeding (hatching date, brood size) and environmental (temperature) factors on feeding rate, prey number, size and quality, and their contribution to total prey biomass delivered to the nestlings of 164 Collared Flycatcher (Ficedula albicollis) parents in 98 nests. Preys delivered to the nest were mainly larvae (53.6%) and flying insects (45.6%). Feeding rate increased with brood size and age, and was higher in males than females. Mean prey number decreased, but mean prey size increased, as the season progressed and parents feeding their brood with primary larvae brought more preys per visit. Relationships between feeding rate, mean prey number and size remained when taking into account the provisioning quality: parents brought either a large number of small prey or a small number of larger items, and the force of the trade-offs between feeding rate and mean prey number and size depended on the quality of the provisioning of the parents. Whatever the percentage of larvae among preys in the provisioning, the variance in total prey biomass was foremost explained by feeding rate (65.1% to 76.6%) compared to mean prey number (16.4% to 26%) and prey size (2.7% to 4%). Our study shows that variation in feeding rate, prey number, size, but not quality (i.e. percentage of larvae), were influenced by individual factors (sex and age) and breeding decisions (brood size and timing of breeding) and that, whatever the provisioning strategy adopted, feeding rate was the best proxy of the total biomass delivered to the nestlings.

AbstractThe size and growth patterns of nestling birds are key determinants of their survival up to fledging and long‐term fitness. However, because traits such as feathers, skeleton and body mass can follow different developmental trajectories, our understanding of the impact of adverse weather on development requires insights into trait‐specific sensitive developmental windows. We analysed data from nestling Alpine swifts in Switzerland measured throughout growth up to the age of 50 days (i.e. fledging between 50 and 70 days), for wing length and body mass (2693 nestlings in 25 years) and sternum length (2447 nestlings in 22 years). We show that the sensitive developmental windows for wing and sternum length corresponded to the periods of trait‐specific peak growth, which span almost the whole developmental period for wings and the first half for the sternum. Adverse weather conditions during these periods slowed down growth and reduced size. Although nestling body mass at 50 days showed the greatest inter‐individual variation, this was explained by weather in the two days before measurement rather than during peak growth. Interestingly, the relationship between temperature and body mass was not linear, and the initial sharp increase in body mass associated with the increase in temperature was followed by a moderate drop on hot days, likely linked to heat stress. Nestlings experiencing adverse weather conditions during wing growth had lower survival rates up to fledging and fledged at later ages, presumably to compensate for slower wing growth. Overall, our results suggest that measures of feather growth and, to some extent, skeletal growth best capture the consequences of adverse weather conditions throughout the whole development of offspring, while body mass better reflects the short, instantaneous effects of weather conditions on their body reserves (i.e. energy depletion vs. storage in unfavourable vs. favourable conditions).

Although climate change is considered to be partly responsible for the size change observed in numerous species, the relevance of this hypothesis for ungulates remains debated. We used body mass measurements of 5635 yearlings (i.e. 1.5 years old) of Alpine chamois ( Rupicapra rupicapra ) harvested in September in the Swiss Alps (Ticino canton) from 1992 to 2018. In our study area, during this period, yearlings shrank by ca 3 kg while temperatures between May and July rose by 1.7°C. We identified that warmer temperatures during birth and the early suckling period (9 May to 2 July in the year of birth) had the strongest impact on yearling mass. Further analyses of year-detrended mass and temperature data indicate that this result was not simply due to changes in both variables over years, but that increases in temperature during this particularly sensitive time window for development and growth are responsible for the decrease in body mass of yearling chamois. Altogether, our results suggest that rising temperatures in the Alpine regions could significantly affect the ecology and evolution of this wild ungulate.

Climate change affects timing of reproduction in many bird species, but few studies have investigated its influence on annual reproductive output. Here, we assess changes in the annual production of young by female breeders in 201 populations of 104 bird species (N = 745,962 clutches) covering all continents between 1970 and 2019. Overall, average offspring production has declined in recent decades, but considerable differences were found among species and populations. A total of 56.7% of populations showed a declining trend in offspring production (significant in 17.4%), whereas 43.3% exhibited an increase (significant in 10.4%). The results show that climatic changes affect offspring production through compounded effects on ecological and life history traits of species. Migratory and larger-bodied species experienced reduced offspring production with increasing temperatures during the chick-rearing period, whereas smaller-bodied, sedentary species tended to produce more offspring. Likewise, multi-brooded species showed increased breeding success with increasing temperatures, whereas rising temperatures were unrelated to reproductive success in single-brooded species. Our study suggests that rapid declines in size of bird populations reported by many studies from different parts of the world are driven only to a small degree by changes in the production of young.