Hungry or angry? Experimental evidence for the effects of food availability on two measures of stress in developing wild raptor nestlings

Abstract

Food shortage challenges the development of nestlings; yet, to cope with this stressor, nestlings can induce stress responses to adjust metabolism or behaviour. Food shortage also enhances the antagonism between siblings, but it remains unclear whether the stress response induced by food shortage operates via the individual nutritional state or via the social environment experienced. In addition, the understanding of these processes is hindered by the fact that effects of food availability often co-vary with other environmental factors. We used a food supplementation experiment to test the effect of food availability on two complementary stress measures, feather corticosterone (CORTf) and Heterophil/Lymphocyte-ratio (H/L) in developing red kite (Milvus milvus) nestlings, a species with competitive brood hierarchy. By statistically controlling for the effect of food supplementation on the nestlings’ body condition, we disentangled the effects of food and ambient temperature on nestlings during development. Experimental food supplementation increased body condition, and both CORTf and H/L were reduced in nestlings of high body condition. Additionally, CORTf decreased with age in non-supplemented nestlings. H/L decreased with age in all nestlings and was lower in supplemented last-hatched nestlings compared to non-supplemented ones. Ambient temperature showed a negative effect on H/L. Our results indicate that food shortage increases the nestlings’ stress levels through both, a reduced food intake affecting nutritional state and the nestlings’ social environment. Thus, food availability in conjunction with ambient temperature shape between- and within-nest differences in stress load, which may have carry-over effects on behaviour and performance in further life-history stages.

Determinants of departure to natal dispersal across an elevational gradient in a long‐lived raptor species

“…In red kites, food supplementation during the nestling period mainly affected survival of nestlings and growth rate during the exponential part of the growth curve, rather than the final fledging body mass (Nägeli et al, 2022 ). Nonetheless, we recently showed that the feeding treatment strongly influenced the stress physiology of red kite fledglings (Catitti et al, 2022 ). Thus, our results suggest that condition‐dependent departure decisions in red kites are due to physiological components of body condition rather than due to the body mass component.…”

“…Experimental food supplementation represents a valid approach to investigate condition‐dependent dispersal behavior (Fattebert et al, 2019 ; Kapota et al, 2016 ); the additional food is well known to increase body condition (e.g., Bańbura et al, 2011 ; Catitti et al, 2022 ; Ruffino et al, 2014 ). However, it also allows for disentangling the effect of spatial food gradients at natal sites from other environmental gradients on dispersal behavior.…”

Productivity drives the dynamics of a red kite source population that depends on immigration

“…Productivity varied strongly between years and nestlings were in better condition in years with enhanced productivity. Possible reasons causing temporal variability are variation in food supply (Gottschalk et al 2015, Catitti et al 2022, Nägeli et al 2022), notably the availability of voles (Gottschalk et al 2015), rainy weather (Nägeli et al 2022) and nest predation by goshawks Accipiter gentilis or raccoons Procyon lotor , with the latter varying more in space than in time (Gottschalk et al 2019).…”

“…Productivity and population density in raptors often have strong links to food abundance (Houston and Schmutz 1995, Salamolard et al 2000, Rooney et al 2015), and the red kite is no exception. Empirical studies have shown that food supply positively affects red kite productivity (Gottschalk et al 2015), and nestling food supplementation experiments revealed that growth and condition of nestlings are reduced and their stress levels increased when food is short (Catitti et al 2022, Nägeli et al 2022). We found a positive correlation between productivity and nestling condition suggesting that under high food availability, more nestlings of a nest survive, and they are in good condition.…”

“…Red kites raise one brood per year comprising 1–4 nestlings and broods that failed are usually not replaced. The condition of the nestlings as well as their number per brood depends on the amount of food that they receive (Catitti et al 2022, Nägeli et al 2022). Despite red kites are opportunistic scavengers, their productivity shows a link with vole density (Gottschalk et al 2015).…”