Backyard bird feeders help a variety of bird species survive periods of food scarcity. The easy nutrition can also help lower stress levels and boost reproductive success during mating season.

Now, a new study -- published Tuesday in the Journal of Applied Ecology -- suggests feeding bluebirds can help the songbirds fend off parasitic flies.

Parasitic flies lay eggs in the nests of bluebirds. Once the eggs hatch, the fly larvae drill holes through the thin skin of newborn birds and feed on their blood.

The parasites are mostly tolerated by nestlings, but the larvae can siphon off significant amounts of blood from the baby birds. For some birds, the blood loss can have long-term negative healthy effects.

"Bluebirds do not have a detectable immune response to the parasitic flies," researcher Sarah Knutie, professor of ecology and evolutionary biology at the University of Connecticut, said in a news release. "Since backyard bird feeding by humans is so popular, I was interested in how giving these birds food could influence their immune response against the parasite, and whether there is a particular time during the breeding season when supplemental feeding is most effective."

For the study, scientists positioned nest boxes at more than 200 test sites throughout northern Minnesota. Researchers monitored the boxes for eggs, and later, for chicks. Scientists returned to feed some of the baby birds mealworms. For all of the birds, researchers measured growth and survival rates, as well as the presence of parasites in the nests.

The birds fed mealworms were more likely to survive and lost less blood to parasites on average.

"When the nestlings were not fed, every nest had parasites, with up to 125 flies in a single nest," Knutie said. "When the nestlings had been fed, I found very few or no parasites. These results suggest that food supplementation could be increasing the birds' ability to kill the parasites."

To better understand the discrepancy, Knutie and her colleagues measure the presence of antibodies in the birds. Birds that weren't fed mealworms had low-to-no detectable antibodies. Birds fed mealworms shortly after birth benefited from a significant antibody response.

Researchers also analyzed the gut microbiota in supplemented and unsupplemented bluebirds. The community of microbes were mostly the same in each set of birds, but birds fed mealworms had higher numbers of one specific type of bacteria.

"The relative abundance of Clostridium species was much higher in supplemented birds, and there were correlations with antibody levels and parasites," Knutie said. "More Clostridium meant more antibodies and fewer parasites."

For birders who feed their backyard visitors, the latest research suggest bluebirds will benefit most from mealworm feedings early in the breeding season. The extra nutrition can help the chicks get a head start developing antibodies.

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