By Ilsa Griebel
Linked paper: Benefits of an anti-parasite treatment are influenced by within-brood size variation in Tree Swallows (Tachycineta bicolor) by I.A. Griebel and R.D. Dawson, The Auk: Ornithological Advances.
Many bird species, particularly those that are altricial, hatch asynchronously. This means that the eggs in a clutch don’t necessarily hatch at the same time, creating a hierarchy of size within broods. In Tree Swallows, a clutch typically hatches over the span of one or two days. While working on Tree Swallows as an undergrad, I noticed this variation in size within broods, and it made me curious about how size variation could influence a nestling’s chances of success.
Fast forward to my master’s degree. I’m thinking about a topic for my thesis, and I’m fascinated by parasites and their interaction with hosts. Young nest-bound birds face a variety of ectoparasites, such as fleas, mites and larval blow flies, that live amongst the nest material. Many of these ectoparasites feed on the blood of nestlings to obtain the nutrients and energy they need to complete their life cycle. As I dug through the literature to learn more about the interaction between blood feeding, nest-dwelling ectoparasites, and nestlings, I began to recognize that nestlings could differ in their susceptibility to ectoparasites. Within broods, the size variation created by hatching asynchrony increases differences in physiological factors such as immune function among nest mates, which can influence how vulnerable individual nestlings are to ectoparasites. The parasites themselves may also selectively feed on particular nestlings within a brood.
To explore how susceptibility to ectoparasites could influence the success of both entire broods and individual nestlings, we manipulated the susceptibility of nestlings to parasites by treating nestling Tree Swallows with a broad-spectrum anti-parasite drug called ivermectin (IVM). At our field site located near Prince George, British Columbia, we assigned half of the broods involved in our experiment as “IVM broods” and the other half as “control broods.” In IVM broods, we first ordered the nestlings by mass and then injected every other nestling along the size hierarchy with IVM diluted in sesame oil. The nestlings that did not receive IVM were injected with pure sesame oil as a control. In control broods, all nestlings received pure oil injections.
We injected nestlings at six days post-hatch — picture a pink, naked chick with its eyes still closed and a few wispy feathers. Six-day-old nestlings are very wiggly, and so it was a two-person job to do the injections. A field assistant carefully held the squirming nestling, securing its flapping, rubbery wings, kicking legs, and wobbling head (no easy feat). I then slid the needle of a loaded syringe under the skin on the back (the upper area between the wings), going far enough that there would be space for the oil, depressed the plunger to expel the oil, pulled the needle back out, and applied light pressure to the injection point for a couple seconds to let it seal back up. The nestling’s skin is very transparent at this age, so you could see the needle the entire time and the subsequent yellow bubble of oil!
Among broods, we found fewer larval blow flies, the most prevalent ectoparasite in our population that also takes the largest blood meal size, in IVM broods than in control broods. Nestlings from IVM broods, regardless of whether they actually received IVM injections or just control injections, had higher hemoglobin levels than nestlings from control broods. Hemoglobin levels of nestlings are often reduced by blood-feeding ectoparasites. Decreasing the parasite susceptibility of broods using IVM also improved survival, with a greater proportion of young fledging (successfully leaving the nest) from IVM broods than control broods. The way that partial treatment of broods with IVM benefited all nestlings in a brood is an example of the herd effect, a concept where disease transmission is reduced at the population level when a sufficient proportion of the population is vaccinated or immune. In our case, we have shown a herd effect at the brood, rather than population, level.
Within broods, we found that the chance of an individual nestling surviving increased with relative size in IVM broods but not control broods. In other words, larger nestlings were more likely to survive than their smaller nest mates when parasite abundance was reduced (IVM broods), but when parasite abundance was not reduced (control broods), the size hierarchy within broods had no effect on nestling survival. Larger nestlings within broods thus benefited more from the IVM treatment, suggesting they may be more susceptible to parasites than smaller nestlings. Why would this be the case? Are ectoparasites selectively feeding on the largest nestlings within Tree Swallow broods, as has been found in European Bee-eaters? Are larger-sized nestlings using a more costly anti-parasite strategy than smaller-sized nestlings? Further research will be required to answer these questions.