How do hosts defend themselves against parasites?

A pair of two day old mockingbird chicks

A pair of two day old mockingbird chicks

Animals are attacked by parasites and pathogens every day. How they fight off attack or compensate for its costs is a key focus of my research. During my Ph.D. I studied tolerance, which refers to the ability of the host to compensate for damage done by parasites without reducing parasite burden. I found that tolerance can vary in a single host population in response to environmental conditions.

Philornis downsi and Galápagos mockingbirds

A mockingbird chick a few days before fledging. The blood around its nares (nostrils) is a sign of parasitism by  P. downsi .

A mockingbird chick a few days before fledging. The blood around its nares (nostrils) is a sign of parasitism by P. downsi.

I study tolerance of Galápagos mockingbirds (Mimus parvulus) to an introduced, parasitic nest fly called Philornis downsi. The larvae of P. downsi live in the nests of birds and feed on the nestlings, often killing them. This parasite is considered a significant threat to the survival of several species of Darwin’s finches. Unlike most finches in the Galápagos, mockingbirds seem able to tolerate P. downsi. In work lead by Dr. Sarah Knutie, we found that P. downsi had no effect on mockingbird fledging success (Knutie et al. 2016).

The importance of the environment

Our field site during the rainy season

Our field site during the rainy season

Our field site during the drought

Our field site during the drought

While mockingbirds can survive P. downsi they may not be able to do so in all conditions. The Galápagos Islands have a famously variable environment and mockingbird rely on seasonal rains to have enough food to feed their offspring. In drought years, food is limited and reproductive success decreases. Although mockingbirds can tolerate P. downsi in “normal” rainy years, tolerance decreases in dry years. These results help us understand how the environment mediates host-parasite interactions.