Peregrine Falcons with ‘eye make-up’ might be better hunters

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?

Different environmental conditions correlated with several malar stripe measurements. Notice that “average solar radiation” is associated with the first four measurements, even though the effect sizes are small. From: Vettos et al. (2021).

Biogeographic Patterns

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.

Hybrid Falcons

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 Letters17(6), 20210116.

Featured image: Peregrine Falcon (Falco peregrinus) © Mosharaf hossain ce | Wikimedia Commons

The genetic basis of long-distance migration in Peregrine Falcons

Extensive analyses point to selection on ADCY8, a gene involved in long-term memory.

Many bird species undertake impressive migrations. Think of the Bar-tailed Godwit (Limosa lapponica), a small wader that can cover more than 29,000 kilometers in one year – travelling from Alaska to New Zealand and back. These achievements become even more mindboggling when you consider that several bird species are able to return to their exact breeding grounds. These birds must have a solid long-term memory. And indeed, a recent study in the journal Nature provided convincing evidence for selection on memory-related genes in long-distance migrants of the Peregrine Falcon (Falco peregrinus). Let’s have a look at the details of this exciting finding.

Migration Routes

First, the researchers used GPS-transmitters to track the migration of 41 Peregrine Falcons from several Russian locations. The resulting patterns pointed to five distinct migration routes which could be divided into short-distance (Kola and Kolguev) and long-distance (Yamal, Popigai, Lena and Kolyma) migration strategies. Attentive readers might have counted six locations even though I mention only five migration routes. That is because the short-distance migrants from Kola and Kolguev followed the same migration route and were thus clustered together.

Tracking Peregrine Falcons from six populations revealed five distinct migration routes which could be divided into short-distance (blue) and long-distance (red) strategies. From: Gu et al. (2021).

Genetic Variants

Time for some genomic analyses. The researchers sequenced the genomes of 35 Peregrine Falcons and compared the genetic make-up of short-distance and long-distance migrants. This comparison uncovered signatures of 149 selective sweeps – targeting 37 genes – between the two groups. The most significant outlier in this analysis was the gene ADCY8. A closer look at this candidate gene revealed an interesting genetic locus with two alleles: C or T. All long-distance migrants possessed the T-variant, suggesting that there has been strong selection for this particular variant.

When the researchers inspected the DNA-letters surrounding this genetic variant, they recognized the sequence CGTCA, which is a binding motif for the transcription factor CREB. Transcription factors are proteins that control the expression of particular genes by binding with specific DNA sequences. The presence of this motif suggests that the expression of ADCY8 might be tightly regulated. And indeed, when the researchers quantified the expression levels of the different ADCY8-variants in brain tissue, they found that the T-variant was expressed at higher levels than the C-variant. Previous research has shown that this gene is involved in long-term memory by regulating the activity of other memory-related genes. The precise molecular details remain to be unraveled, but the researchers can already conclude:

The higher activity of ADCY8 that we identified in long-distance peregrine migrants may increase their long-term memory. Our analysis reveals a unique mutation that facilitates the binding of the transcription factor CREB1 to ADCY8, and fixation of this variation happened after the divergence of long-distance and short-distance populations. Our work thus not only reveals a causative gene that may explain migratory differences, but also provides a mechanistic basis for these differences.


Gu, Z., Pan, S., Lin, Z., Hu, L., Dai, X., Chang, J., … & Zhan, X. (2021). Climate-driven flyway changes and memory-based long-distance migration. Nature591(7849), 259-264.

Featured image: Peregrine Falcon (Falco peregrinus) © Mosharaf hossain ce | Wikimedia Commons

How did the population of Sussex Peregrine Falcons recover?

Searching for the source population of these UK birds.

The history of the Peregrine Falcon (Falco peregrinus) is a string of ups and downs. This enigmatic falcon species has been prosecuted numerous times and suffered from pollution, only to bounce back afterwards. During the Second World War, Peregrine Falcons were shot because they predated on homing pigeons, which carried important military messages. After the World War, they were targeted by grouse-moor gamekeepers (i.e. the hunting of Red Grouse, a field sport in the UK) and pigeon fanciers. In addition, the use of pesticides – specifically DDT – posed another threat. These chemicals flow through the food chain, eventually accumulating to high doses in predators where they lead to the thinning of eggshells and a reduction in breeding success.

Luckily, the use of these chemicals has been banned and the prosecution of Peregrine Falcons has diminished. In several regions, such as Scandinavia and North America, re-introduction programs have restored the breeding populations of Peregrine Falcons. In the southern part of the UK, however, the number of Peregrine Falcons seems to have recovered without any human assistance. In Sussex, the number of breeding pairs increased from ca. 10 in 1954 to more than 40 by 2016. This remarkable population growth raises an important question: where did these birds come from?

Source Populations

In a recent Conservation Genetics study, Angela Weaving and her colleagues attempted to determine the origin of the Sussex Peregrine Falcons. Using a combination of microsatellites and mitochondrial markers, they tested several hypotheses about the identity of the founding birds:

  • Migrants from the UK
  • Migrants from mainland Europe
  • Escaped captive birds (possibly including hybrids)

To discriminate between these possible source populations, the researchers compared the genetic material of the Sussex birds with a historical sample (before the 1950s) of Peregrine Falcons from the UK, domestic-bred birds from the UK, and wild birds from Germany, the Republic of Ireland and the Mediterranean.


Analyses of the mitochondrial DNA were not very helpful to solve this mystery. One main haplotype was shared by all the sampled populations. This low genetic diversity is probably a remnant from a rapid population expansion from a small population (with limited genetic diversity) at the end of the Pleistocene. The microsatellites, on the other hand, did provide insights into the origin of the Sussex population. The researchers found that “the contemporary wild Peregrine population in Sussex is genetically similar, but not identical, to the pre-pesticide UK population, and it is genetically different from other European populations and from the domestic stock.” It thus seems that the Sussex population increased through the influx of other Peregrine Falcons from the UK. More sampling is needed to pinpoint the exact locations where these birds came from.

The contemporary population (dark-blue) clusters with the UK population from the pre-pesticide period (light-blue). There is a clear difference with the domestic birds (orange). From: Weaving et al. (2021) Conservation Genetics.

Hybrid Falcons

Hybrid falcons, such as Peregrine Falcon x Gyrfalcon (Falco rusticolus), are quite common in captivity and regularly escape into the wild. Vincent Fleming and his colleagues estimated that since 1980 at least 30 Peregrine Falcon hybrids per year have been reported as lost. This is probably an underestimate because it is now no longer required to report escapees. The findings from the Sussex study suggest that escaped hybrids did not markedly influence the genetic make-up of the wild population. It could be that these hybrids might have lower survival rates or difficulties in finding a partner. However, these birds could still affect the wild population by occupying breeding territories or engage in failed nesting attempts with “pure” Peregrine Falcons. More research is thus needed to understand the fate of escaped hybrids, and their impact on wild populations of Peregrine Falcons.

The number of falcon individuals that were reported as lost in the UK. From: Fleming et al. (2011).


Weaving, A., Jackson, H. A., Nicholls, M. K., Franklin, J., & Vega, R. (2021). Conservation genetics of regionally extinct peregrine falcons (Falco peregrinus) and unassisted recovery without genetic bottleneck in southern England. Conservation Genetics22(1), 133-150.

Featured image: Peregrine Falcon (Falco peregrinus) © Mosharaf Hossain Ce | Wikimedia Commons