David M. Larson
Common Loon on nest. Photograph by Brian Henderson. (CC BY-NC 2.0).
One of the scourges of the Great North Woods is the black fly. Arriving in insatiable hordes, biting through flesh to feed on blood, female black flies torment vertebrates during their brief life span. Humans have tried repellants from smoke to chemicals, barriers from clothing to creosote and lard and, of course, avoidance. For nonhuman mammals and birds, life during the two- to three-week life span of black flies is torturous. Nesting birds are particularly vulnerable since they are unable to escape without endangering eggs or nestlings.
Piper et al., 2018, report on a long-term project about how Common Loons, Gavia immer, deal with black flies, in this case Simulium annulus, during breeding season. The authors studied loons and black flies from 1993–2017 in a 2000 km2 study area in northern Wisconsin, containing 200 glacial lakes surrounded by mixed hardwood and conifer forests. Loons were captured using spotlighting at night from boats and banded with unique color bands for individual identification.
The goal of this project was to test the hypothesis that nest abandonment by loons was associated with harassment by black flies. Four predictions were that (1) nest abandonment should increase in years with high black fly abundance; (2) loons on nests should spend less time incubating due to biting flies; (3) in years of high abandonment, single chicks should predominate, due to hatching of only one of the two eggs; and (4) abnormally cool weather should exacerbate nest abandonment by extending the lives of the black flies. A further goal of this project was to look for influences of black flies on population dynamics of loons, along with other biotic and abiotic variables. Finally, the authors tested the loons' behavior after nest abandonment—did they renest in the same or a different location?
From late April through early August, breeding pairs were monitored at least weekly in each territory. Observers noted loon breeding behavior and nesting success. They also estimated numbers days with large numbers of black flies, defined as more than 10 around a loon's head or more than 50 around a nest. Nesting success (47.8% of over 2000 nests) was indicated by the presence of chicks, by many small eggshell fragments, or by largely intact shell membranes in or adjacent to the nest. Empty nests or large shell fragments with shell membranes near the nest or within five meters of the nest indicated predation (33.9%), usually by raccoons. Abandoned nests (15.2%) were indicated by one or two intact eggs that were not being incubated or defended by the territorial pair. Infertile eggs or prehatching embryo deaths (2%) were indicated by incubation for at least a week beyond normal incubation period. Flooded nests (1.1%) had eggs that were at least half submerged and not incubated or defended.
Adults that incubated eggs during black fly activity periods showed persistent head shakes and ruffled nape feathers, possibly because of inflammation due to bites. All four of the predictions noted above were supported by observational data. High counts of black flies around loons' heads and nests were strongly correlated with nest abandonment, and high numbers of flies were correlated with reduced incubation. High abandonment years also showed high single-egg clutches, and cool springs were correlated with high abandonment.
The proportion of nest abandonment varied from year to year in this study, ranging from less than 10% to 50%. In 44% of nest abandonments, the pairs attempted renesting, though overall black fly nest abandonment was negatively correlated with breeding success—an estimated 6.1% reduction of fledglings per year. In some years the effect was negligible (1.7% reduction in chicks fledged); in others it was substantial (23% fewer fledglings).
The response of loon pairs to nest failures depended on the cause of failure. Loons relocated nests greater distances after predation than after abandonment. Renests after abandonment were moved no more than after successful hatching. Pairs that nested in the same site following abandonment produced chicks in 55% of attempts. Abandonment due to flies is not likely to reoccur in a season, because of the short life span of the flies, whereas a predator is much more likely to still be active on a subsequent nesting attempt.
Though it may seem that a 6.1 % reduction in fledglings per year is a small impact on Common Loon population dynamics, it is important to note that nesting adults can suffer hundreds of black fly bites. Disease transmission from bites is a problem. S. annulus is a vector for the Leucocytozoon protozoa, and infections could reduce loon survival, producing another long-term effect on populations.
The best predictors of nest abandonment were breeding on small lakes, older females, and a long open-water fetch in relation to the prevailing winds. The authors suggest that increased abandonment on small lakes may be due to poorer food availability, leading to reduced adult health and less robust immune systems to fend off the effects of blood loss and parasitism. The increased abandonment in older females may be caused by lower tolerance of stressors due to energetic cost, disease, and discomfort. Although males and females senesce at similar rates, with adult survival decreasing steeply in their mid-20s, abandonment failure was not correlated with age of males. The effect of nest orientation regarding prevailing winds seems counterintuitive, since one might expect that more wind would be protective for the nesting birds by keeping fly numbers down. However, wind is fickle, and the flies are persistent. Ultimately, it is more likely that the long fetch means larger waves that may drown nests, causing abandonment.
Clearly, pestering black flies can have negative consequences for nesting Common Loons, not limited to painful bites and including infection, parasitism, and reduced nesting success. Fortunately, Common Loons apparently deal with only one species of short-lived black fly. Other mammals and birds in this habitat may suffer deleterious effects of multiple black fly species over longer periods of time. How other bird species adapt to dealing with these insects is a subject of ongoing investigation. Nestling mortality due to black flies has been shown in various avian species, including Peregrine Falcons, Great Horned Owls, and Red-tailed Hawks, and nest abandonment has been reported in Snowy Owls and Whooping Cranes.
- Piper, W.H., K.B. Tischler, and A. Reinke. 2018. Common Loons respond adaptively to a black fly that reduces nesting success. The Auk 135: 788-97.
David M. Larson, PhD, is the Science and Education Coordinator at Mass Audubon's Joppa Flats Education Center in Newburyport, the Director of Mass Audubon's Birder's Certificate Program and the Certificate Program in Bird Ecology (a course for naturalist guides in Belize), a domestic and international tour leader, President of the Nuttall Ornithological Club, and a member of the editorial staff of Bird Observer.