Linked paper: Sex-specific differences in preen gland size of Zebra Finches during the course of breeding by S. Golüke and B.A. Caspers, The Auk: Ornithological Advances 119:4, October 2017.
Almost all birds possess a preen gland that produces a waxy secretion used by birds for feather maintenance. Several studies have found that the preen gland is enlarged during breeding, but it is currently not clear why.
We investigated the temporal pattern of gland size changes during breeding in a captive population of domesticated zebra finches. Zebra finches are small passerines, only weighing 13 grams on average, and the gland is therefore quite small. Additionally, the gland itself is really flexible, and measuring the gland manually with a caliper is therefore quite challenging and error-prone. So, how to measure precisely this flexible, heart-shaped gland in such small birds?
We took pictures of preen glands and calculated the gland surface area using digital picture analysis. This method worked out well, and we started to investigate gland size changes over the breeding period. We measured preen gland sizes of breeding pairs at key points that are relevant during the reproductive period, such as pre-mating, egg laying, hatching, rearing and independence of the chicks.
We found that gland sized increased in both parents—that is, they produced more secretion—during breeding. The maximum size of the gland was reached around the time the chicks hatch. We assume that the need for secretions is highest at this point. After breeding, the gland size was similar to what it was before the reproductive season, indicating that the size increase is due to breeding. Similarly, at the same time, non-reproducing birds showed no size increase.
More specifically, our results show that the temporal pattern of gland size increase differs for males and females, with males already enlarging the gland around the time of egg-laying, while females start increasing the gland size later.
Knowing the temporal pattern of preen gland size enlargement of males and females allows us to think about different factors that might explain the enlargement pattern we observed. First, an investment in gland secretions might reduce the odor of the birds inside the nest, which is advantageous against olfactory-hunting predators. Second, in a breeding-related context, the ingredients of preen gland secretions might be necessary for self-protection and/or to protect offspring against harmful microbes. The nest is an ideal environment for microbial growth, which could reduce plumage condition and health and could further impact egg viability and hatching success. There is evidence that transferring gland secretions to the plumage and onto eggs or offspring might reduce harmful microbes. Third, there might be a different need for chemical communication during breeding. As the gland secretions are spread on the plumage during preening, they might be an essential contribution to a bird’s body odor. In our group, we are especially interested in the role of odors for social communication.