What is behind the mysterious decline of the Ridgway’s Hawk?

Dominican Republic, Los Haitises National Park
Female Ridgway’s Hawk keeping watch over her nest, Los Haitises National Park, Dominican Republic. Photo © David L. Anderson

 

Male Ridgway's Hawk looks angrily at a climber entering his nest in Los Haitises National Park, Dominican Republic. Photo © David L. Anderson
Male Ridgway’s Hawk looks angrily at a climber entering his nest in Los Haitises National Park, Dominican Republic. Photo © David L. Anderson
Dominican Republic
This valley is actually a farm field in Los Haitises National Park. Ridgway’s seem to like to nest in open areas like this. Note the ball of sticks at the top of the palm – a colonial nest of the Palmchat. Photo © David L. Anderson
Los Haitises National Park, Dominican Republic
Dominican field technician ascending a royal palm (Roystonea borinquena) so he can treat the nestlings of a Ridgway’s Hawk. Photo © David L. Anderson

 

¡Viene! ¡Viene! (“Here she comes!”) As the ground crew shouts a warning the climber waves his arm rapidly over his head. Sometimes it wards off the attacking Ridgway’s Hawk, and sometimes a rusty streak lands a two-fisted strike on a climbing helmet, reminding everyone who is in charge here. The climber works his way slowly up the side of a 35-foot royal palm, approaching a nest containing one to two downy nestlings. At three days old each is a mere ball of puff that fits into the palm of your hand. It’s hard to fathom that such a fragile piece of life could in two years comet out of the sky and clock a climber on the head. It’s also essential to understand how such a life could be critically endangered on a tropical island like Hispaniola, so full of emerald vitality.

Ridgway's Hawk, Dominican Republic
Look to the right of the helmet and you see a rusty blur with talons outstretched for the strike. Photo © David L. Anderson

Critically endangered is only one label we can pin on the Ridgway’s Hawk. Their population numbers in the mere 100’s. Once distributed across the island nations of Haiti and the Dominican Republic, their last stronghold is in Los Haitises National Park, far northeastern sector of the “DR,” where an estimated 250 to 300 pairs hangs on in a mosaic of lowland rain forest and farm fields.

Another label for the Ridgway’s: generalist. Pairs can breed in the interior of forest or in a farm plot of manioc, squash, and root vegetables. They aren’t picky eaters either. Peer into a nest and feast your eyes on a melee of snakes, lizards, rats, mice, small birds, giant centipedes, and the occasional chicken. They nest in forest trees and also in the tops of royal palms.

Dominican Republic, Los Haitises National Park
This male Ridgway’s Hawk spent several minutes obsessing over the remains of a snake he had cached in a tree. Photo © David L. Anderson

The big mystery is this: how can a generalist species that lives everywhere and eats everything go from being common to almost gone? We can’t blame deforestation directly, that would be too easy. Many rain forest specialists like Harpy Eagles suffer from habitat loss; as the forest shrinks so do their populations. For generalists like the Ridgway’s, deforestation creates habitat in the form of forest edges, complete with palms for nesting and abundant prey to feed their nestlings. Instead, biologists working for The Peregrine Fund have discovered that a parasitic fly is killing hawk nestlings. Before the nestlings can grow and leave the nest, fly larvae burrow under their skin and eat them from the inside out. Why this happens is part of the mystery. Both the fly (Philornis pici, a type of botfly) and the Ridgway’s Hawk are native to Hispaniola. They have coexisted and coevolved for as long as either has been on Earth. What has happened to this island system that a parasite is driving a hawk over the cliff of extinction?

Ridgway's Hawk, Dominican Republic, Los Haitises National Park
A Dominican technician measures a hawk nestling so that researchers can study the effects of parasitism on nestling growth. Photo © David L. Anderson

In a long complex loop of interrelated events, deforestation may ultimately be to blame after all. It goes like this. Ridgway’s Hawks like to build their nests on top of the stick nests of Palmcats. Palmchats build their nests most often in royal palms. Like the fly and the hawk, the royal palm is a native to Hispaniola, but it is a colonizing species: when forests are cut down, royal palms invade the new clearings. Palmchats are also a host to the Philornis pici botfly, but from what we know so far Palmchats don’t die from the parasite. Instead, they may serve as a reservoir, a vast and spreading warehouse of Philornis botflies that then parasitize hawks. In the ecological equivalent of a conspiracy novel, deforestation spreads royal palms, palms spread Palmchats, Palmchats spread Philornis botflies, and Ridgway’s Hawks takes it in the tank. What we have is a forest out of balance.

The Peregrine Fund is working to break this curse. First, biologists and Dominican technicians climb Ridgway’s Hawk nests every week to treat the chicks with fipronil, an insecticide that causes no harm to chicks but which makes the fly disappear. This solution is expensive and labor intensive, and that makes it unsustainable. Meanwhile, the search is on to find ways for flies and hawks to coexist. Can repellants be placed in nests to keep flies at bay? Are there places on the island yet suitable for hawks but not so much for the fly? This is a tough spot to be in. Without the efforts from The Peregrine Fund over the last 10 years the Ridgway’s may already have been lost. Humans most likely created this problem, and humans will have to fix it.

To read more about The Peregrine Fund’s excellent work to save the Ridway’s Hawk from extinction – click here.

Dominican Republic
Time will tell if the sun is setting or rising over the enigmatic Ridgway’s Hawk. Photo © David L. Anderson

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