The Lasting Impact of Black Flies

black fly impact


Since 2014 marked by far the most intense explosion of black flies in recent memory, I wanted to look at how loons are faring reproductively this year compared to last. It has now been over a month since the peak of “black fly season”, which was miserable for loons but lasted only about a week. Yet the flies were thick enough and their outbreak coincided so closely with the start of incubation for most pairs that about 70% of all loon pairs in our population abandoned their nests.

It seemed certain that the epic abandonment of early nests in 2014 would negatively impact the number of chicks produced by our population. And it has. Based on my analysis of 96 territories covered in 2013 and 2014, I have the following results. Last year, 41 pairs  had hatched chicks by 30 June, while 14 had nests close to hatching, 10 had early nests whose outcome was uncertain, 21 pairs had failed for the year, and 10 pairs had never nested (to our knowledge). In 2014, the numbers were 6 chicks, 30 promising nests, 15 early nests, 21 failed pairs, and 14 pairs without no nest during the season.

The disparity in chicks produced is alarming, of course. This is the most important number. The picture gets a bit rosier when you crunch the numbers and recognize that many pairs currently on nests will eventually hatch chicks. Based on an assumption of 70% hatch rate among pairs in the “likely hatch” group and 40% in the early nest stage — perhaps slightly optimistic — the total estimated chicks produced in 2013 and 2014 are 54.8 and 33.0, respectively. To put it another way, about 57% of all pairs reared chicks in 2013; 34% will in 2014. This is a decline of 40% in the number of territorial pairs producing chicks.

Now, we need to step back here. Loons are long-lived animals, and most pairs that failed to raise chicks, owing in large part to black flies, will try again next year. So, in the big picture, the decrease in rate of chick production is not so awful. In a few more years, we will look back on 2014 as just a worse-than-average year for loon chicks, not a year that threatened to end loon life as we know it. But it is hard to be very upbeat at the moment.