Bluebirds mainly eat insects, so they often show little interest in backyard bird feeders — unless you're serving mealworms. There are potential pros and cons with feeding wild birds, but if it's done right, it can provide a valuable boost for many songbirds. And according to a new study, feeding bluebirds may also offer another benefit: protection from parasites.
As with many bird species, bluebird nests are commonly plagued by parasitic fly larvae. Adult flies lay their eggs in a bird's nest, and once the larvae hatch, they feed on blood from nestlings by burrowing through the young birds' skin. For some birds, parasitic flies can have significant effects on nestling survival.
Baby bluebirds seem pretty resilient to this threat, according to Sarah Knutie, a professor of ecology and evolutionary biology at the University of Connecticut and author of the new study, which was published in the Journal of Applied Ecology. They can handle lots of fly larvae without big drops in growth or survival, but they do lose a lot of blood, which could have longer-term effects.
"Bluebirds do not have a detectable immune response to the parasitic flies," Knutie says in a statement. "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."
Knutie conducted her study in northern Minnesota, where she and her father set up 200 nest boxes for eastern bluebirds. (There are three bluebird species across North America: eastern bluebirds, which live east of the Rocky Mountains, and western and mountain bluebirds, which range from the Rockies to the Pacific coast.) Knutie monitored all the nest boxes for bird eggs, and as those eggs hatched, she fed live mealworms to some of the nestlings. She tracked the growth and survival of all the nestlings until they fledged, and once they left the nest, she also recorded the number of parasites in each box.
All the nestlings were fed by their parents, but only some received supplementary mealworms from Knutie. Those birds seemed to reap significant benefits from the extra food, with a higher overall survival rate and less blood loss than the control group.
"When the nestlings were not fed, every nest had parasites, with up to 125 flies in a single nest," Knutie says. "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."
But why would extra food have that effect? Knutie also measured the chicks' antibody responses, which help them fend off the parasites. "With unsupplemented nestlings, there is a low-to-no detectable antibody response. With supplemented nestlings, there was a significantly higher antibody response," she says. "Higher antibody levels mean fewer parasites."
That might be because the nestlings who got extra food had more nutrient resources available, letting them mount an earlier immune response before things got out of hand. The timing of supplemental feeding seems important, with feeding earlier in the breeding season apparently helping the young birds more than later in the season. "If food availability is driving the nestlings' immune response to parasites, feeding early could really help the birds," Knutie says.
(Bluebirds begin nesting as early as February or March, depending on the species and location, with chicks hatching a few weeks after the nest is built. Eastern bluebirds typically have more than one successful brood per year, according to the Cornell Lab or Ornithology, and in warmer climates they may have as many as four in a year. Western bluebirds produce one to three broods per year, and mountain bluebirds tend to have just one or two.)
Birds' gut bacteria might also play a role in the immune response, Knutie adds. While gut bacteria were similar in supplemented and unsupplemented nestlings, Knutie did find some intriguing differences. The relative abundance of Clostridium bacteria was "much higher" in supplemented birds, she says, and birds with more of these bacteria also had more antibodies and fewer parasites. More research is needed to reveal whether the gut bacteria actually cause that effect, but for now, this study at least hints at substantial benefits for bluebirds who receive extra food from their human neighbors.
"The interesting piece of this work suggests that if you feed your birds, it can really reduce the parasite load for the young birds, and that timing of feeding matters," Knutie says.