Too Early to Panic…But Brood Size is Falling!

Recently, I made the kind of finding that gives scientists fits. It came about in the same manner that findings often do initially: a hunch.

Since I spend much of my life either working closely with loons or poring over data that describes their breeding success, I am in a good position to notice subtle changes that occur over time. Occasionally when out on a lake, I observe a breeding event and think, “Wow…..that did not happen in the old days!” Then I retreat to my computer, look at data from years past, and see if I am correct. I have to confess: in many cases, I am wrong.


This past August, I noticed what I thought might be a growing pattern in breeding ecology. Mated pairs, it seemed to me, were less often rearing two chicks to fledging. That is, they were either hatching one chick and rearing the singleton only or hatching two chicks and losing one. At least that was my impression. In this case, field data confirmed my suspicion, as the above graph shows. The proportion of singleton broods has risen during the study. In three of the past five years, in fact, two-chick broods were quite uncommon, making up only 1/5 of all broods. Most of the pattern, moreover, appears to result from failure of one of two eggs to hatch, rather than loss of the second chick after hatching.

Faced with a puzzling and unexpected finding, I looked immediately at the usual suspects. Black flies, which have also been worse in recent years, are an obvious possibility. Flies harass incubating loons, reduce incubation times, and might reduce hatching success. In fact, I was convinced when I wrote our recent paper that black flies were the culprits. But then 2018 happened. This season featured a warm spring, a rapid die-off of flies, and very few fly-induced abandonments. So we could not blame flies for the low hatching rate of eggs in 2018.

What about the changing climate? As I have emphasized in a recent post, warm weather is projected to drive loons northwards; could it also be the root cause of the lower hatching rate? I looked to see if warmer May temperatures are correlated with reduced hatching, but they are not. (In fact, warmer temperatures are associated with a slightly higher hatching rate.) Likewise, precipitation might, in theory, reduce hatching rate. Again, years of higher May rainfall were not years of lower hatching success. I breathed a sigh of relief to learn that the lower rate of hatching does not (preliminarily) appear to represent the harmful leading edge of climate’s impact on loons.

Two possibilities remain. First of all, there is a small chance that the pattern is a statistical anomaly — that hatching rate is not actually falling, but that the result occurred by sheer chance. Scientists must always be circumspect about their results, and the statistical test says that there is a 0.6% chance that the finding does not represent a true pattern. (That is roughly the likelihood that you toss a fair coin 9 times in a row and get “heads” every time.) Second and more likely, some unknown factor is at work here. Might there be an environmental contaminant, picked up by loons, that increases developmental abnormalities in embryos or perhaps causes adults to cease incubation of the second egg prematurely? Might disturbance of incubation by humans be the cause of lower hatching success? These possibilities — and numerous others — generate testable predictions, and I will test them. In the meantime, let’s all keep our fingers crossed that the distinct decline in loon hatching success over the past 20 years is, after all, just a blip.