Irritating insects affect diver nesting success


New research has highlighted how biting insects can directly affect the nesting success of Great Northern Divers.

When sitting on a nest to incubate eggs, a bird is physically stuck and at its most vulnerable to attacks of any kind, so coping with stress and other significant costs is important. For Great Northern Divers, black flies are a common blood-feeding pest and can cause nest abandonment and decreased fledging rates. This has impacts on not only individual pair success, but on population dynamics as well.

The new study has presented some of the best data to date to support hypotheses about the effects that black flies have on diver nesting behaviour and success. Chapman University’s Walter Piper and his colleagues monitored Great Northern Diver nests for 25 years in northern Wisconsin, USA. They marked individuals to track each bird’s behaviour, nesting success and interaction with black flies. More than 2,050 nests were included in the study to assess the impacts of black flies on divers at population level.

If the black fly concentration around an individual bird was high or it was a particularly intense fly outbreak year, diver incubation time decreased and nest abandonment increased. It was discovered that nest abandonment could be predicted using lake size, female age and the amount of wind. The team found that the smaller the lake, the older the female and the greater the distance across water that wind has to travel to reach the nest, the more likely it is that the nest will be abandoned.

Black flies can irritate Great Northern Divers to the point that they may abandon their nests (Tony Davison).

The cost associated with severe black fly outbreaks appears to be high enough that a nest can be abandoned and a second attempt made following the peak of the outbreak. The second nest is likely to be in the original location unless a predator destroyed the nest. In that case, the pair is more likely to choose a new and, hopefully, safer nest site.

Lead author Walter Piper commented: “Black flies, which we think of as a nuisance and no more, actually impact population reproductive success. This was a matter of studying an animal for 25 years and almost ignoring one aspect of their biology – until you finally look straight at that aspect ... and realise it’s crucially important! Divers get slammed by black flies, but they make a very good response by reusing the nest sites where the flies hounded them, instead of abandoning those sites altogether – as they do when raccoons get their eggs. This makes sense, because the raccoons are their main enemies (that is, egg predation is a more severe problem then black flies), and a safe nesting site from raccoons is a vital resource.”

“This study is one of the first that examines the negative influence of black flies on a population of birds rather than on individuals alone. Perhaps more importantly, by studying a marked population of divers for almost a quarter century, it is possible to determine the stochastic [that is, less predictable] influence of black fly outbreaks on the population as well as understand the nuances of how these mechanisms might work,” added Jeb Barzen, an ecologist with Private Lands Conservation, who was not involved with the study.

“Black flies severely reduce reproductive effort of divers periodically but not every year, a finding that is important to understanding the population dynamics of this iconic species. The subtler examination of individual response to black flies provides insight into differences in how males and females, as well as individuals in different habitats and individuals of different ages, respond to black fly infestations. Collectively these results inform ecologists and managers alike in issues relevant to the conservation of diver populations and other long-lived, territorial bird species.”



Piper, W H, Tischler, K B and Reinke, A. 2018. Common Loons respond adaptively to a black fly that reduces nesting successThe Auk 135: 788-797 https://doi.org/10.1642/AUK-17-239.1.

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