Trait-specific sensitive developmental windows: Wing growth best integrates weather conditions encountered throughout the development of nestling Alpine swifts

Abstract

The 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).

Keywords: Apodiformes; Tachymarptis(Apus)melba; climate change; early‐life conditions; feathers; heat stress; meteorological conditions; multi‐trait; offspring development.

FIGURE 1

Two Alpine swift nestlings at ca. 50 days of age. Photo by G. Masoero.

FIGURE 2

Developmental trajectories, from hatching to fledging, of nestling Alpine swifts. Dots represent individual measurements of wing and sternum length and body mass of nestlings measured between 1999 and 2023, and smooth lines represent the average growth pattern of nestlings. The sensitive developmental windows during which weather conditions most affected the phenotype at 50 days (see Section 3) are highlighted with a purple background. N = 15,866 measures from 3194 nestlings for wing length, 12,662 measures from 2887 nestlings for sternum length and 15,968 from 3188 nestlings for body mass.

FIGURE 3

Variation in wing length, sternum length and body mass of 50‐day‐old nestling Alpine swifts in relation to weather conditions encountered earlier in their development. Weather conditions resulted from a principal component analysis (PC1) between daily average temperature and daily rain, and traits were affected by weather conditions over different time windows, that is, PC1_1‐48d between 1 and 48 days for wing length, PC1_12‐34d between 12 and 34 days for sternum length and mean ambient temperature between 48 and 50 days for body mass (Ta_48‐50). Low PC1 values indicate cold and rainy days and high values indicate sunny and warm days. Solid lines (and 95% confidence intervals) are predictions from the models presented in Table 2. Climatic windows are reported using the nestling's age as a reference.

FIGURE 4

Variation in body mass of 50‐day‐old nestling Alpine swifts in relation to daily mean ambient temperature in the 2 days before the measurement (Ta_48‐50d). The analysis using the segmented package identified a threshold at 19.2°C. Solid lines (and 95% confidence intervals) are predictions from the models presented in Data S1. Climatic windows are reported using the nestling's age as a reference.

FIGURE 5

Variation in wing and sternum growth rates of nestling Alpine swifts in relation to weather conditions encountered during development. Weather conditions resulted from a principal component analysis (PC1) between daily average temperature and daily rain, and traits were affected by weather conditions over different time windows. PC1 has low values with cold and rainy days and high values with sunny and warm days. Dashed lines indicate non‐significant relationships. Lines (and 95% confidence intervals) are predictions from the models presented in Table 3. Climatic windows are reported using the nestling's age as a reference.

FIGURE 6

Variation in (a) the nestling survival (calculated as the weighted proportion of nestlings that fledged from a nest with at least one hatchling) and (b) age at fledging (in days) in relation to weather conditions encountered during development in nestling Alpine swifts. Weather conditions are the result of a principal component analysis between daily average temperature and daily rain in the window that best explains the variation in wing length at 50 days. Predicted lines are obtained using average values of the other continuous variables. Climatic windows are reported using the nestling's age as a reference.