Birth hierarchy, not egg effects, determines gull chick behavior

February 3, 2017

Researcher Javier Díaz Real, from the Behavioral Ecology Laboratory, has just dismissed a theory widely accepted by the scientific community. And that is that the different behavioral strategies between yellow-legged gull chicks are determined by environmental factors after hatching and not by the position of the egg in the original nest. This finding rejects the general applicability of the assumption that the very different behavior of the first and third gull chicks is programmed by the substances that the mothers deposit in the eggs. That is, in general these results indicate that the chicks’ behavioral strategies are plastic and are influenced by their early social experience.

The results of their study were recently published in the journal Behavioral Ecology and show that chicks that were hatched in third position emitted more calls to their parents, were less likely to react to a parental alarm call and had a lower respiratory rate than chicks that were hatched in first position. “The original egg order did not affect the chicks’ behavior,” explains the researcher. In addition, third eggs were smaller and chicks hatched from these eggs grew slower than those hatched from first eggs. That is, regardless of the egg order in the original nest, chicks hatched first in the experimental nest grew faster than their siblings.

The aim of the study

In many species of birds with parental care, siblings often compete for resources and care from their parents, although the competitive strategy varies between siblings according to their birth order in the nest. For example, third-laying gull chicks receive more androgens maternally in the egg, and these influence their early behavioral patterns. Therefore, the objective of this study was to find out what maternal influence factors – substances deposited by mothers in the eggs, or the birth hierarchy – affect early behavior in gull chicks. To do this, doctoral student Díaz Real carried out an experiment of crossing first and third-laying eggs in yellow-legged gull (Larus michahellis) nests. The aim was to separate the natural covariation between the order of laying eggs and the order of birth of chicks. This study thus contributes, according to the researcher, to a greater understanding of the behavioral ecology of such an emblematic species as the yellow-legged gull.

The Behavioral Ecology Laboratory

The Behavioral Ecology Laboratory, a group led by professors Alberto Velando and Sin-Yeon Kim, has been developing a line of research for years on the effect of natural selection on the life cycle and behavior of animal populations in the wild, paying special attention to the conservation of populations and the consequences that human activities have on biodiversity. Its lines of research are very varied, from the evaluation of complex phenotypes, gene expression, coloration, the role of oxidative stress and maternal effects, the study of senescence or the biology of seabird conservation.

Although this group focuses mainly on seabirds such as cormorants (Phalacrocorax), gannets (Morus bassanus), gulls, shearwaters (Calonectris diomedea) or gannets (Alca torda), it is also carrying out research work with fish and other vertebrate species.