Analyses of photographs support the “solar glare hypothesis”.
Many falcon species have a malar stripe, a distinctive patch of dark feathers below the eye. The exact function of this plumage trait is a matter of debate among ornithologists. It could, for instance, play a role in vision (absorbing excess light in bright conditions) or thermoregulation (helping animals heat up faster in colder environments). A recent study in the journal Biology Letters provided evidence for the “solar glare hypothesis”. According to this explanation, the malar stripe reduces the amount of light that is reflected into the eyes, potentially increasing the hunting efficiency of falcons under bright conditions.
Michelle Vrettos and her colleagues inspected pictures of more than 2100 Peregrine Falcons (Falco peregrinus) from across the globe. They measured several characteristics of the malar stripes, which they correlated with local environmental conditions. The analyses revealed a positive relationship between annual solar radiation and four malar stripe measurements (i.e. width, contiguity, prominence and length). In general, Peregrine Falcons with “wider and more prominent malar stripes or overall darker heads were associated with areas of higher solar radiation.” These patterns are in line with the solar glare hypothesis.
However, the authors do indicate that their findings are only correlational (we all know that correlation does not imply causation) and that the statistical effect sizes were relatively small. Hence, these results remain to be confirmed with additional analyses, and perhaps with an experimental approach. Who doesn’t want to apply eye-liner to a Peregrine Falcon?
Although this study supports the solar glare hypothesis, it is always worthwhile to investigate alternative hypotheses. That is why the authors focused on two biogeographical patterns: Gloger’s Rule and Bogert’s Rule.
Gloger’s Rule predicts darker individuals in wetter areas. Proposed explanations for this pattern include camouflage, protection against parasites and dealing with solar radiation (recently reviewed by Delhey 2019). Bogert’s Rule states that darker animals occur in colder regions because dark coloration absorbs more solar radiation and thus ensures proper thermoregulation (as found in gulls, see this blog post).
If the malar stripe in Peregrine Falcons followed these rules, we would expect significant correlations with rainfall (for Gloger’s Rule) or temperature patterns (for Bogert’s Rule). This was, however, not the case. Rejection of these alternative hypotheses does not automatically support the solar glare hypothesis – that reasoning would be a black-and-white fallacy – but it does narrow down the search for the potential function of the malar stripe.
You might be wondering why I decided to cover this study on the Avian Hybrids website. What is the connection with hybridization? While reading this paper, I remembered a study in Science on hybridization between wolves and domestic dogs. I summarized the findings of this study in an article for the journal Frontiers for Young Minds.
If you watch nature documentaries, you might have noticed that these wolves mostly have gray fur. But perceptive scientists observed that there were some wolves with darker fur. Where did that dark fur come from? The scientists studied the DNA of these wolves and discovered that the dark fur was caused by a particular variant of a gene. Surprisingly, wolves normally do not have this variant of the gene. But dogs do! Further analyses revealed that, in the past, dogs and wolves had pups together. The mixing of these two species led to the exchange of DNA, including the variant gene that gave wolves darker fur. Because of this darker shade, these wolves were better camouflaged in the forest, making them better hunters. The exchange of DNA—or introgression—helped the wolves adapt to their environment.
Perhaps a similar process could occur in falcons? Several falcon species are known to hybridize, both in captivity and in nature (see this page for an overview). Some of these hybrids might develop darker malar stripes, providing them with an advantage when hunting in bright conditions. Time to inspect more photographs!
Vrettos, M., Reynolds, C., & Amar, A. (2021). Malar stripe size and prominence in peregrine falcons vary positively with solar radiation: support for the solar glare hypothesis. Biology Letters, 17(6), 20210116.
Featured image: Peregrine Falcon (Falco peregrinus) © Mosharaf hossain ce | Wikimedia Commons