Parasite-host interactions between Protocalliphora Spp. (Diptera: Calliphoridae) and tree swallows Tachycineta bicolor

Many host-parasite relationships exist, and while some have been studied extensively, for others, our understanding of the influences the parasite and host impose on each other is limited. Ectoparasites live in close proximity to their host, often living on the host or within the nest of their host, and so are influenced not only by the individual host they exploit, but by the habitat choices of their host as well. Bird blow flies in the genus Protocalliphora (Diptera: Calliphoridae) are ectoparasites of the altricial young of many species of birds; the nests of tree swallows Tachycineta bicolor in particular are frequently infested by a number of species of Protocalliphora. Through the use of baited traps, experimental manipulation of the nests and nest sites of hosts, and observing the behaviours of adult tree swallows, I studied: (1) host location and host selection strategies of blow flies by testing the importance of nest feathers to blow flies; (2) the influence of the density and distribution of hosts and a reduction in available hosts on the size and density of parasites; (3) individual host selection; (4) the effect of sanitation behaviour by hosts on the size and density of blow flies. Traps baited with potential cues used by blow flies in host location were unsuccessful in attracting blow flies; however, traps installed in nests containing nestlings successfully captured adult blow flies, suggesting that a specific concentration or combination of cues may be necessary to attract blow flies. Additionally, the number of blow flies per nestling in nests where preen oil was removed from feathers used in nest construction was significantly higher than in control nests, which suggests that preen oil may play a role in masking olfactory cues used by blow flies to locate hosts. No clear influence of the number of occupied neighbouring nest boxes on the density of blow flies in nests was observed, although size (i.e., length) of pupae decreased as the number of occupied neighbouring nest boxes increased. A reduction in available hosts resulted in an increase in the number of blow flies per nestling, and unexpectedly, an increase in the weight of larvae and weight and body length of adults, perhaps because of higher quality hosts acquiring the remaining nest sites over lower quality hosts. The ability of blow fly larvae to feed and acquire resources from nestling tree swallows did not appear to be influenced by previous exposure of hosts to parasites, although lighter larvae were more likely to feed than heavier larvae, and acquired a larger blood meal. There also was evidence that the size of the blood meal varied among species of blow fly. The presence of blow flies in the nest did not appear to influence the rate or time spent performing sanitation behaviour by adult tree swallows, and there was similarly no significant influence of sanitation behaviour of hosts on density or size of blow flies, although there was some suggestion of a negative influence on the length of adult blow flies. Overall, more research is needed to understand better the host location strategies of blow flies. While the influence of hosts on the size of blow flies may be subtle, a positive influence was much more apparent after a reduction in available hosts, which may have unknown effects on future generations of hosts and implications for conservation efforts of hosts.