Beak of the pinch: anti-parasite traits are similar among Darwin’s finch species

INFLUENCE OF GROOMING ON PERMANENT ARTHROPOD ASSOCIATES OF BIRDS: CATTLE EGRETS, LICE, AND MITES

Birds have a diverse community of "permanent" arthropods that complete their entire life cycle on the body of the host. Because some of these arthropods are parasites that reduce host fitness, birds control them by grooming, which consists of preening with the beak and scratching with the feet. Although preening is the primary component of grooming, scratching is essential for controlling arthropods on the head and neck, which cannot be preened. Several unrelated groups of birds have evolved comb-like pectinate claws on the middle toenail of each foot. We tested the role of these claws in the control of arthropods by experimentally removing teeth from the claws of captive western cattle egrets (Bubulcus ibis) infested with chewing lice (Insecta: Phthiraptera), feather mites (Acari: Sarcoptiformes), and nasal mites (Acari: Mesostigmata). After a period of 4 mo, we compared the abundance of arthropods on experimental birds to that of control birds with intact teeth. We used video to quantify the grooming rates of the captive birds, which groomed twice as much as wild birds. Experimental and control birds did not differ significantly in grooming time. Both groups virtually eradicated the chewing lice, but not feather mites or nasal mites. We found no support for the hypothesis that pectinate claws increase the efficiency of arthropod control by grooming. Experiments with wild birds are needed to test the hypothesis further under conditions in which birds devote less time to grooming.

Change in beak overhangs of cliff swallows over 40 years: Partly a response to parasites?

Some birds exhibit a maxillary overhang, in which the tip of the upper beak projects beyond the lower mandible and may curve downward. The overhang is thought to help control ectoparasites on the feathers. Little is known about the extent to which the maxillary overhang varies spatially or temporally within populations of the same species. The colonial cliff swallow (Petrochelidon pyrrhonota) has relatively recently shifted to almost exclusive use of artificial structures such as bridges and highway culverts for nesting and consequently has been exposed to higher levels of parasitism than on its ancestral cliff nesting sites. We examined whether increased ectoparasitism may have favored recent changes in the extent of the maxillary overhang. Using a specimen collection of cliff swallows from western Nebraska, USA, spanning 40 years and field data on live birds, we found that the extent of the maxillary overhang increased across years in a nonlinear way, peaking in the late 2000's, and varied inversely with cliff swallow colony size for unknown reasons. The number of fleas on nestling cliff swallows declined in general over this period. Those birds with perceptible overhangs had fewer swallow bugs on the outside of their nest, but they did not have higher nesting success than birds with no overhangs. The intraspecific variation in the maxillary overhang in cliff swallows was partly consistent with it having a functional role in combatting ectoparasites. The temporal increase in the extent of the overhang may be a response by cliff swallows to their relatively recent increased exposure to parasitism. Our results demonstrate that this avian morphological trait can change rapidly over time.

Nestling behaviour predicts naris deformation in Darwin’s finches parasitized by the avian vampire fly

Although in-nest parasitism can reduce the fitness of avian hosts, the severity of these effects may vary with host physiology and behaviour. If certain nestling behaviours are beneficial for resisting parasitism, then selection may favour some behavioural phenotypes over others. Here, we tested whether differences in nestling behaviour mediate the negative effects of parasitism, using small ground finches (Geospiza fuliginosa), on Floreana Island, that had been parasitized by the invasive avian vampire fly (Philornis downsi). We first established, using 4 years of breeding data (2005, 2006, 2010 and 2020), that nestlings exposed to more parasites had larger nares and, among older nestlings only, lower body mass. We then examined, using data from the 2020 season, whether each nestling’s behaviour (specifically, its response to human handling) predicted the severity of its naris deformation. When faced with high-intensity parasitism, more responsive nestlings (i.e. those that struggled more during handling) had larger nares compared to more docile nestlings. This suggests that more responsive nestlings suffer greater fitness costs due to parasitism, although we also discuss alternative explanations. Future work should consider the stability and heritability of these nestling behavioural differences and whether parasite-induced selection shapes behavioural variation at the population level.