Thermoregulation and protection against solar radiation determine black plumage patterns in gulls.
When you pay close attention to the distribution of animals across the globe, some interesting patterns emerge. For example, some animals tend to be darker in warm and humid areas. This pattern – known as Gloger’s Rule – has been described for several animal groups, but the underlying mechanisms are still a matter of debate. Proposed explanations include camouflage, protection against parasites and dealing with solar radiation (recently reviewed by Delhey 2019). A related pattern is Bogert’s Rule which states that darker animals occur in colder regions because dark coloration absorbs more solar radiation and thus ensures proper thermoregulation. In both cases, we have clear predictions that can be tested with climatic data on solar radiation, temperature and precipitation. All we need is a group of animals that shows variation in dark plumage patterns…
A recent study in the journal Global Ecology and Biogeography found the ideal study system for this challenge: gulls. In these black-and-white birds, the researchers decided to focus on the mantle color and the proportion of black on the wing tips. These traits are sexually monochromatic (i.e. males and females look alike), suggesting that there is little sexual selection for certain plumage patterns that could complicate the analyses. They quantified these plumage colors for 80 subspecies (representing 52 species) and correlated them with several climatic variables. Did the results follow Gloger’s Rule of Bogert’s Rule?
Thermoregulation and Solar Radiation
Statistical analyses revealed that climatic conditions experienced during the non-breeding season had a stronger effect than those of the breeding season. Let’s start with the mantle color. It turned out that darker mantle coloration was negatively correlated with air temperature, and positively with solar radiation (see figures below). These findings can be explained by the fact that darker-mantle species winter in colder regions and experience more solar radiation compared to lighter species. Their black plumage allows the gulls in these regions to retain heat in freezing conditions (i.e. black objects trap heat better). And the dark plumage protects against solar radiation because melanin pigments increase the resistance of feathers against harmful UV radiation.
Similarly, the proportion of black color on the wingtips was mostly influenced by the positive interaction between solar radiation and migration distance. In other words, gulls migrating over long distances and overwintering in areas with high level of solar radiation have more black on wing tips, whereas sedentary species in areas of low solar radiation have less black feathers on their wingtips. The researchers explain these results are follows: “Species with a high proportion of black are thus more likely to tolerate long migration distances, which may cause mechanical damage, and more likely to spend the winter in highly insolated conditions where UV radiation can also damage their plumage.”
In summary, the black coloration of gull plumage plays a crucial role in both thermoregulation and protection against solar radiation. The reported patterns are in line with Bogert’s Rule which predicted darker animals in colder regions. In contrast, there was little support for Gloger’s Rule (darker animals in warm and humid areas) because there was no clear relationship with precipitation. The present study focused on plumage patterns of adult birds. Hence, these results remain to be confirmed with immature plumage patterns. But as any birdwatcher knows, immature gull plumage is more complex (just open any field guide to experience the mindboggling variation) and will be more daunting to analyze. I wish the researchers that will pick up this challenge the best of luck.
Dufour, P., Guerra Carande, J., Renaud, J., Renoult, J. P., Lavergne, S., & Crochet, P. A. (2020). Plumage colouration in gulls responds to their non‐breeding climatic niche. Global Ecology and Biogeography, 29(10), 1704-1715.
Featured image: Great Black-backed Gull (Larus marinus) © Ken Billington | Wikimedia Commons