Stress and age may influence common crossbill coloration, research shows

Some of the recognized functions of bird plumage are thermoregulation, waterproofing, courtship and flight. Birds tend to molt their plumage partially or completely to keep it in good condition due to the wear and tear caused by the passage of time. This is the basis on which the pamplonesa Blanca Fernández Eslava has completed his doctoral thesis at the Faculty of Science of the University of Navarra under the direction of Drs. David Galicia (University of Navarra) and Juan Arizaga (Aranzadi Science Society).

The study model used was the common crossbill (Loxia curvirostra), and research has been conducted on the molting process and plumage coloration, as well as the environmental factors that influence them. "To know when the molt begins in the species, its duration and extent, and to determine if it has a significant effect on the molt process and the coloration of the plumage.elation to plumage color, breeding or environmental factors. I have also delved into plumage coloration and whether there is a link with body size, the age of the birds or their survival over time," reports Dr. Fernandez Eslava.

The capture areas for the common crossbill have been the Navarre municipalities of Uztarroz (Pyrenees) and Bigüézal (Sierra de Leyre), and the Soria municipality of El Royo. The methodology used by the biologist has consisted, on the one hand, in the capture of species with a net and the subsequent identification of the species, sex, and age. In addition, biometric measurements were taken from each specimen (beak, skull, tarsus, wings and tail) in order to study the morphological differences between birds. Finally, he has worked with a database -belonging to Daniel AlonsoThe Aranzadi Science Society ornithologist, who has been keeping records of the common crossbill for more than two decades.   

Although this is basic research, Dr. Fernandez's work yields some interesting results. For example, the number of feathers replaced during the molt period depends on sex, age and plumage coloration. "We have observed that adult females and adult yellow males do not replace all the feathers during their molt and that this is probably related to the high reproductive cost in females and to stress or poor quality of yellow individuals".

Another conclusion of this study is that yellow males molt later and in less time and acquire a poorer quality plumage compared to red males. "We have calculated that red individuals have a longer survival, almost twice as long as yellow individuals. And also that yellow coloration is an index of lower individual quality."

For Blanca, the interrupted molts of birds (when they keep some old feathers) can be an indicator of the quality or stress level of a population (e.g. food shortage or the presence of parasites).   

Blanca Fernández Eslava has a degree in Biology (2006) and a PhD from the Faculty of Sciences of the University of Navarra (2022). Currently, she continues with the same line of research in several projects, one of them with the Consejo Superior de Investigaciones Científicas (CSIC), on the plumage coloration of these birds and the relationship between dietary resources, stress and gene expression.

Bibliographical references

→ Fernández-Eslava, B., Alonso, D., Galicia, D., Arizaga, J., Alonso-Alvarez, C. (2022). Bigger or long-winged male common crossbills exhibit redder carotenoid-based plumage coloration, Current Zoology, zoac038, https://doi.org/10.1093/cz/zoac038.

→ Fernández-Eslava, B., Alonso, D., Galicia, D., Arizaga, J. (2021) 'Strong evidence supporting a relationship between colour pattern and apparent survival in common crossbills', Journal of Ornithology. Springer Berlin Heidelberg, 163(1), pp. 243-249. doi: 10.1007/s10336-021-01927-4.