It was jarring two days ago to look at the database we use to track our breeding loons. During most years, the first week in June is the peak of incubation. At this time, breeding pairs that laid eggs early are well into the 28-day period that will carry them to hatching; others have lost an early clutch and replaced it. But steady, determined incubation is the rule at this time of year.

Not so this year. As the screen grab from Evelyn and Tarryn’s data entry shows, 2019 is yet another year of severe black flies. I had a sinking feeling that black flies would be a plague when I was in the study area in May.

As our recent paper shows, cool springs are killers. In years when April and May are cooler than average, black flies live longer than average. From the standpoint of a female fly, cold weather makes flight and dispersal more difficult, so a female in a cold spring is likely to delay her quest for the blood meal she needs to nourish her eggs. Not all females postpone reproductive activity, however, so cold springs reduce synchrony between female flies. The result, from a loon’s perspective, is a longer period when flies are around to harass them on nests. In contrast, warm springs cause a well-synchronized explosion of fly biting activity. During warm years, Simulium annulus blackens the skies for a few days and exacts an awful toll on incubating pairs during that brief period. Many pairs, though, are able to weather the onslaught, maintain viable eggs (no doubt aided by warm temperatures), and incubate the eggs to hatching despite the interruption.

Elaina’s photo of the nest on Hilts Lake is typical of what we see in a cool spring. Flies are abundant, to be sure. In the photo, you can see many in the air above the nest but also scores hanging onto the arched roof on this platform. There are enough flies around the nest to keep the pair from incubating the eggs, but few enough so that this infestation does not reflect an explosion, such as would occur in a warm year.

Now that I have got you worried — sorry — you must be wondering where 2019 ranks relative to other years. What kind of productivity can we expect? I ran a quick back-of-the-envelope analysis by comparing rate of chick production in each year with the proportion of pairs sitting on nests during the first week of June. Even with the one “outlier” (2013), there is a tight correlation. That is, we can usually tell pretty well how productive a year our loons are going to have by looking at the proportion of all pairs on eggs in early June. If 2019 falls near the line, then the 0.45 proportion of sitting pairs in early June of this year predicts 0.42 chicks per territorial pair. This is not terrible — look at 2011, for some perspective — but not what we had hoped.