Kind to kin: weak interference competition among white stork Ciconia ciconia broodmates

Abstract

Altricial nestlings in structured families show a diverse array of behavioural mechanisms to compete for food, ranging from signalling scrambles to aggressive interference. Rates of filial infanticide are moderately high in white storks. It has been hypothesized that this unusual behaviour is an adaptive parental response to the absence of efficient mechanisms of brood reduction (aggression or direct physical interference) by nestlings. To test this latter assumption, we analyzed video recordings of 41 complete feeding episodes at 32 broods during the first half of the nestling period, when nestlings complete 90% of growth and chick mortality and size asymmetries are highest. Parents delivered food to all nestlings simultaneously by regurgitating on the nest floor. No direct (bill to bill) feeding was recorded. Senior nestlings were never observed to limit their junior nestlings from eating food, either by aggression or physical interference. Experimental feeding tests revealed that heavier nestlings handled prey items more efficiently and ate food at a higher speed. The high degree of tolerance shown by senior nestlings is unusual among birds with similar ecological and phylogenetic affinities, such as herons. Tolerance by seniors cannot be easily explained by absence of parental favouritism or proximate factors known to affect the occurrence of sibling aggression in other species (rate of food transfer, brood size, hatching asynchrony or length of nestling period).

Broodmate aggression and life history variation in accipitrid birds of prey

“…Moreover, the hypothesis that chicks behave more aggressively when accompanied by a smaller number of broodmates has received little experimental support (Drummond & Rodríguez, 2009). This finding seems at odds with a general trend for aggression to be weaker in species with larger broods (Drummond, 2001;; and this study), although brood (or clutch) size alone may also be of little predictive value for explaining broodmate aggression in other bird taxa, such as herons and storks (Ciconiiformes, Romero & Redondo, 2017). By contrast, a direct causal effect of provisioning rate on aggression was strongly supported by path analysis.…”

Antioxidant supplementation slows telomere shortening in free-living white stork chicks

“…White storks begin incubation with the first or second egg, and laying occurs at intervals of 2 days. Furthermore, egg mass also tends to decrease with laying order, and this effect, combined with hatching asynchrony, results in a marked size hierarchy among nest-mates [24]. Birds were randomly assigned to two treatment groups, while ensuring that both groups were represented in all nests.…”