Nestling sex predicts susceptibility to parasitism and influences parasite population size within avian broods

Abstract

Vertebrate hosts differ in their level of parasite susceptibility and infestation. In avian broods, variation in susceptibility of nestlings to ectoparasites may be associated with non‐uniform distributions of parasites among brood mates, with parasites concentrating feeding on the most vulnerable hosts. The presence of a highly susceptible nestling in a brood can benefit the remaining young by reducing the parasite pressure they experience; however, from a parasite’s perspective, broods with fewer susceptible hosts may provide effectively fewer resources than broods of the same size containing a greater abundance of susceptible hosts, and this could limit the number of parasites that a host brood can sustain. To test whether variation in number of susceptible hosts affects the number of parasites in bird nests, we first examined the role of host sex and induced immunity (via methionine supplementation) on susceptibility of mountain bluebirds Sialia currucoides to parasitism by blow flies Protocalliphora spp. We then assessed the effect of variation in number of susceptible hosts on the number of parasites inhabiting the nest. Only females showed a benefit of methionine supplementation, gaining mass more rapidly following supplementation compared to males. This suggests that females are more susceptible to parasites in this system; this was further supported by parasite feeding trials, in which parasites extracted larger blood meals from female than male hosts. Finally, the abundance of parasites in nests was predicted by brood sex ratio: broods containing more female young harboured more parasites. Hence, within‐brood variation in host susceptibility to parasites can not only influence the costs of parasitism for individual nestlings, but may also have consequences for the size of parasite populations within nests. If patterns of maternal investment affect the abundance of nest‐dwelling parasites, these interactions may be important for understanding fitness consequences of maternal resource allocation in many vertebrate hosts.

Experimental effects of within-brood genetic variation on parasite resistance in a wild bird host

“…Nestlings were weighed (to the nearest 0.1 g) using an Ohaus CS200-100 portable compact scale and measured (bill length [0.1 mm], right tarsus length [0.1mm], and left first primary feather [0.1mm]) using analog calipers from Avinet. Since nestling sex can affect feeding preferences of the parasite (O’Brien & Dawson 2013), nestlings were sexed on day 13 using visual differences in feather pigmentation (O’Brien & Dawson 2009). Starting on day 15, nests were checked every other day from a distance to determine the age of fledging.…”

Parasites suppress immune-enhancing effect of methionine in nestling great tits

“…Our study thus suggests that the trade-off between investment in life history traits and immune function is only partly dependent on available resources, but shows that parasites may influence this trade-off in a more complex way, by also inhibiting important physiological functions. compensated mass gain after methionine supplementation (O'Brien and Dawson 2013). In a great tit study, methionine led to reduced growth during supplementation and augmented growth after the treatment in the presence of haematophagous ectoparasites only, suggesting a parasitemediated allocation trade-off between growth and immune function (Tschirren and Richner 2006).…”

Trade-offs in the early development of tree swallows (Tachycineta bicolor): offspring characteristics, offspring quality, and parental care

“…A study similar to mine but carried out over many more years could possibly show this. I also may have found parasite effects if I had been able to quantify sex of nestlings, as larval blow flies have been found to extract larger blood meals from female than male nestling mountain bluebirds (Sialia currucoides), and more parasites have been found in broods containing more female young (O'Brien & Dawson 2013). Unfortunately determining the sex of nestlings was outside the scope of my study, but would be a useful inclusion in future studies on parasite burden.…”