What Bird Is This?

What bird is this? It’s a Merlin! This surprise visitor delighted watchers at Decorah North yesterday by perching, eating, preening, and showing off its yoga skills before flying away.

According to most sources, North America is home to five falcon species. From largest to smallest, they stack up like this:

Like Peregrine falcons, Merlins feed primarily on other birds that they catch in the air, although they will also eat mammals, insects, and small reptiles. With their slate backs, barred tails with white tips, rufous wash, small size, and quick wingbeats, I think they look like someone mashed up a falcon and a Coopers Hawk. They have the long pointed wings and proportionally shorter tail of a falcon and lack the ‘flap-flap-glide’ pattern of accipiters, as banding station attendant Sophia Landis pointed out in this blog.

Merlins breed in the northern boreal forest, the northeastern United States, and the northern Rocky Mountains, migrating south as far as the Caribbean, Columbia, and Venezuela. The northernmost populations tend to ‘leapfrog’, migrating farther south than their counterparts that nest at lower latitudes. While we don’t know where this Merlin came from, it joins the long list of species we have seen and heard migrating through the Valley of the Norths. We often talk about preserving habitat for nesting birds, but high-quality migratory habitat is equally important to their survival.

Amy’s note: This was an adult male Taiga Merlin. Was it a local or migratory bird? Given the time of year, I suspect it was migratory, but Pauline Harrison reminded us that Merlins were found nesting in Iowa in 2016 after an absence of a century: https://www.thegazette.com/subject/news/nesting-merlins-are-first-seen-in-iowa-in-a-century-20160426.

Falcons and Other Birds of Prey

Back to the mashup! Falcons are diurnal birds of prey, but they are genetically more closely related to parrots than other raptors. Similar morphological traits have obscured the taxonomy of raptors for centuries, since two species that aren’t closely related can still develop similar traits if they live in similar ways. This phenomenon is called convergent evolution.

Phylogeny of core landbirds modified from Mindell et al. (2018)

Phylogeny of core landbirds modified from Mindell et al. (2018). Image credit: J. of Raptor Research, 53(4):419-430 (2019). https://doi.org/10.3356/0892-1016-53.4.419

The image above shows the phylogenetic relationships between the birds we call ‘raptors’, plus a few close cousins. Like all falcons, Merlins are more closely related to parrots than to other raptors. So why do falcons look more like accipiters than parrots? While they aren’t closely related, species that live in similar ways can develop (or conserve) similar traits and body plans, while relatives that live in different ways can develop (or conserve) different traits and body plans.

Convergent or Divergent?

Did falcons converge on or conserve an accipiter-like body plan? We think that early birds were (mostly) predatory and that birds diversified rapidly after the Cretaceous–Paleogene mass extinction about 65 million years ago. Falcons emerged about 60 million years ago: the next-to-last step on a lineage that radiated into parrots and passerine birds. Could falcons and some accipiters have retained the body plan of their last common ancestor across 60+ million years?

Ancient keystone predators like sharks retained their basic body plans for  much longer than 60 million years, feeding on fish as fish evolved and changed. Perhaps the common ancestor of falconiformes and accipitriformes looked like a falcon. Some accipitriformes could have evolved new body plans and lifestyles in response to new kinds of prey, while others retained an older, but still successful, body plan. Convergence or divergence, it all comes down to who survived and how they lived!

Similarities and Differences

North America’s falcons and accipiters breed in arctic, boreal, and temperate habitat and hunt birds in flight. Hunting birds in flight requires a body plan that maximizes speed and maneuverability, something we see in falcons, Cooper’s Hawks, and Sharp-shinned Hawks. While not identical, their body plans are more similar to one another than to more closely-related species that don’t specialize in hunting birds.

What about plumage color? North America’s falcons are strongly countershaded, with dark backs and light undersides: more like accipitriformes than parrots. This reflects their habitat and hunting style: starkly patterned northern forests of conifer and birch, sun-dappled oak savannah, flat tundra and prairie, and wide open skies. In these directly-illuminated habitats, a predatory bird’s limited color palette and countershading provides excellent camouflage. A parrot’s brightly colored plumage would be conspicuous in the North Valley, but helps hide it in the shady green tropical forests it calls home. Did falcons converge or did parrots diverge? Either way, each species is spectacularly suited to the place it calls home.

In Vesper Flights, Helen McDonald writes “We so often think of science as somehow subtracting beauty and mystery from the world. But it’s the things I’ve learned from scientific books and papers that are making what I’m watching almost unbearably moving.” I’m almost unbearably moved by this little Merlin’s beauty and mystery. Where did it come from? Where is it going? What can we learn from it? It fills me with joy. Beautiful and fierce, long may it fly!

Things to read…