Chicks in the same nest compete for food because the parents do not have total control over feeding

The way the food is distributed amongst chicks in the same brood is one of the most important challenges for evolutionary ecology. According to a study recently published in the journal called 'Oikos' there are conflicts between the parents and brood of the lesser kestrel (Falco naumanni) because of food supply. The fact that in some species the chicks are born at different times and may be of different sizes (and competitive capacity) increases the complexity of the conflict.

The authors of the study have put forward a simulation model using various food distribution strategies, considering the daily growth of each clutch over the 37-day nesting period. To do this, they have set up different hatching asynchrony scenarios.

According to Carlos Rodríguez, a researcher from the University of Jena, Germany and the principal author of the study, the hatching asynchrony 'is a frequent phenomenon in nature which generates asymmetry of size between the members of a brood'. The researcher also tells SINC about 'the possibility of parents not having total control over which chick to feed, because of difficult situations (such as tubular nests and very aggressive chicks) thus forcing them to accept the outcome of competition between siblings'.

The study shows the importance in the quality of the habitat in raising different species of birds. The results suggest that in a clutch where chicks bigger than others exist because they were hatched at different times, this can be an advantage in poor surroundings, but this advantage lessens at the same time as feeding conditions improve.

In order to reach these conclusions, the scientists have considered three classic patterns of feeding controlled by the parents: preferentially feed the bigger ones, feed the ones who are hungriest and feed at random. Furthermore the experts have considered a further scenario in which it is the differences in weight between the siblings, and not the decision of the parents, that determines how food is distributed (more despotic when the differences in size between the chicks are greater).

The study considers the total number chicks that fly, their weight (this may be an indicator of their survival capacity outside the nest), and the number of times the parents feed (which can be considered a measurement of parental effort) as measurements of reproductive success. Depending on the ecological context, one of these measurements may take precedence over the others, for which reason the results of this study can be applied beyond this species.

Background to the study

The idea of this article comes from a previous study published in the Journal of Applied Ecology in 2006 and which received the Southwood prize from the British Ecological Society. In this study the researchers constructed a model that simulates the growth of the chicks from a small colonial falcon, the lesser kestrel.

'We simulated different environmental conditions by modifying two of the model parameters: the prey biomass and the time necessary to capture it. During the elaboration of this study we realised that we were unaware of the strategy the parents used to feed the chicks (whether they gave the food to the biggest, the smallest, the hungriest, randomly, etc.)' Rodríguez points out. The differences between the food distribution strategies that were found enabled the development of the present study to be possible.

Baby lesser kestrels in a nest from a grain silo in La Palma del Condado, Huelva, where the differences in size between chicks from the same nest can be seen: on the right the biggest (with all its feathers), and on the left the smallest (still with feathers).