Sibling rivalry increases stress for hungry red kite chicks

Abstract

Feeding a hungry brood can be tough. Faced with a mass of demanding beaks when returning from a foraging flight, red kite (Milvus milvus) parents are under pressure. And it only gets worse when food is scarce. Stress caused by starvation has life-long implications for chicks and tensions rise as hungry clutches begin to squabble, especially for the youngest at the bottom of the pecking order. ‘Food shortage enhances the antagonism between siblings’, explains Benedetta Catitti, from the Swiss Ornithological Institute. But it wasn't clear whether the stress experienced by famished youngsters was a direct consequence of food deprivation or bullying by their siblings. Together with Martin Grüebler, Urs Kormann and Susanne Jenni-Eiermann, also from the Swiss Ornithological Institute, Catitti decided to monitor red kite hatchlings over three consecutive breeding seasons to tease apart the impact of starvation and sibling rivalry on the youngsters’ stress levels.‘It was challenging to find and monitor closely so many nests in a short time period’, says Catitti, who set up a total of 78 feeding platforms over a 3 year period in the region around the Swiss cities of Bern and Fribourg to provide nesting red kite parents with a dependable supply of chicken chicks to supplement their ravenous offspring's diets. During that time, Catitti with Valentijn van Bergen, Patrick Scherler and Stepanie Witczak from the Swiss Ornithological Institute and a dedicated team of field assistants kept track of 82 red kite youngsters, climbing 20–30 m to the nests to weigh and measure the chicks, as well as collecting neck feathers and a minute blood sample on the first visit (∼22 days after hatching) and larger body feathers when the chicks were older (∼40 days after hatching) – both of which carry evidence of the amount of stress each individual was experiencing. ‘Climbing [up to] red kite nests is a physical challenge rewarded by the privilege of entering their world, even if just for a few minutes’, says experienced ornithologist and tree climber van Bergen.After analysing the signs of stress in the blood samples and feathers, it was clear to Catitti and colleagues that the chicks whose parents could access the feeding platforms did better and were less stressed than the chicks of parents that could not supplement their diets. ‘It strongly supports the idea that nestlings’ stress levels are lowered in optimal food conditions’, says Catitti. In addition, as all of the chicks grew – food supplemented or not – their stress levels dropped. However, the youngest chicks in the nests, which are usually the most stressed because of having to compete with their older siblings for food, were more relaxed during the scientists’ first visit when the parents had access to extra food. Not only were the youngest getting more to eat but there was also potentially less rivalry with their siblings, creating more harmonious conditions and reducing their overall stress levels. ‘Our results clearly support the social challenge hypothesis and, therefore, are in line with the idea that food shortage can drive changes in stress levels through an alteration of the nestlings’ social environment’, says Catitti.So food shortages – often caused by poor weather, which also threatens nestlings – are stressful for red kite chicks, leaving them hungry and at risk from bickering. And the youngest are particularly vulnerable, as older red kite nestlings are not averse to tucking into their younger and weaker siblings when feeling particularly ‘hangry’.