Little Fly, Big Impact: First Record of Philornis Parasitism of Snail Kite

Meet the flies from the genus Philornis… or preferably, don’t. Our Puerto Rico and Dominican Republic teams are far too familiar with these insects, as both Puerto Rican Sharp-shinned Hawks and Ridgway’s Hawks have experienced population declines due in part to Philornis parasitism. The larvae of some species of these flies burrow under the skin of nestlings and feed off their tissues and blood, causing developmental delays, anemia, disease transmission, and even death for infected nestlings. Over the past decade, our teams have developed an effective form of nest treatment to significantly reduce Philornis parasitism. 

While the prevalence and impact of Philornis species on Ridgway’s Hawks and Puerto Rican Sharp-shinned Hawks are well-documented due to their profoundly negative effects, the occurrence of these flies in raptor species in other regions of Central and South America is less known. Our Neotropical Program provided a grant to Maria Lorena Rojas for her MSc project wherein she studied the effects of Philornis parasites on birds in western Ecuador. She documented the first known parasitism of Philornis parasitism in Snail Kites in Ecuador, but with a potentially more optimistic outcome.

The team monitored Snail Kite nests in western Ecuador in 2020 and 2021 and found that 12 out of 72 nests contained evidence of Philornis infestation. This prevalence of Philornis is lower than that of the Ridgway’s Hawk, but similar to other bird species in Ecuador. In contrast to the species of Philornis that parasitize the Ridgway’s Hawk and Puerto Rican Sharp-shinned Hawk, the species most present in Snail Kite nests do not burrow under the skin of nestings. As such, the nestling survival rate appears to be higher and the impacts of Philornis seem to be lower. However, further research is needed to evaluate the specific impacts of Philornis on nestling growth and survival.