What is Killing Chicks?

Today brought more bad news. As I reviewed yesterday’s lake visits, I saw that Bear and Woodcock had been whittled down from two chicks to one. I objected briefly. “Brian”, I asked, “are you sure Woodcock has lost its second chick?” He was certain.

In the old days (the 90s and early 00s), about half of all loon broods in Oneida County had two chicks, like the 9-day-olds in Linda’s photo. I recall that we used this as a rule of thumb, when gauging how many chicks we would eventually capture and mark. Okay, we thought, half of all broods will have two chicks, and half will have one, so multiply the number of broods by 1.5 to get the total number of chicks. But it has been some years since half of all broods contained two chicks. In fact, we have to go back to 2005 to find a year of parity between one- and two-chick broods. Since then, 68.5% of all broods have been singletons. From 2017 to 2019, 78% of all families had only one chick in them.

This year will only strengthen that trend. After loss of one of two chicks on Woodcock and Bear, 28 of 36 focal pairs with chicks this year (78%) are caring for only one. By the way, chick loss is not just the whittling down of two chick broods to singletons. Indeed, eight of our focal pairs that hatched one or two chicks initially are now without chicks. So the massive increase in chick mortality that began during the past decade or so has wiped out entire broods as well as cutting many down by half. Since the trend of increased chick mortality long ago reached statistical significance, I have begun to fixate on it. What is killing loon chicks?

We cannot blame my favorite scapegoat, black flies, for chick loss. True, the flies had a devastating impact on nesting behavior in May and have reduced breeding more than any other single factor this year. Poor overall loon breeding success in the past five years can also be laid at least partly the tiny feet of Simulium annulus. That is, the flies suppress overall breeding success by wiping out many early nests. But it is late July now. The flies are a distant memory, and chicks are still dying.

Naturally, we look at what has changed in loons’ habitat during the period when chick mortality has been increasing. There are myriad possibilities. (1) Bald eagles are undoubtedly the most despised of all loon enemies. The eagle population has soared over the past four decades, and their impact on loon breeding success has been documented already. We have observed and have had reported numerous cases of loon chicks being taken by eagles — and loons seem to spend most of their waking hours on the lookout for eagles — so we must consider bald eagles a likely cause of increased chick loss. This year we have added eagle counts to our observation protocol. We will soon know whether eagles can be blamed for the increased mortality of chicks. (2) Declining small fish populations are another likely culprit. Small panfish, unfortunately, are not monitored as closely as are large gamefish, but the possibility that less food might be available now than before for loon chicks dovetails nicely with the fact that they are now 10% lighter than they were 25 years ago. We will explore the “decline in small fish” hypothesis in coming years. (3) There are far more humans on Oneida County lakes than there were 25 years ago. Indeed, a collaborator at Michigan State University has already documented that human population density is a strong correlate of adult mortality in our study area. It is quite plausible that human impacts — chiefly boat strikes, accidental hookings, and line entanglements — are the root cause of the decline in chick survival too. Our lakes vary enormously in the amount of human activity they support; this will make it straightforward to test the “human impacts” hypothesis.

Of course, multiple factors might have conspired to reduce the survival rate of loon chicks, including those just mentioned and others. If so, the task of detecting those that are most significant — and devising some means of mitigating them in an effort to restore loon breeding success to what it once was — will be daunting. Naturally, I am hoping that there is a single discrete cause. For example, if we learn that bald eagles are starting to have an unacceptably high impact on loon chick mortality, we would simply have to…….. well….okay…… Let’s hope eagles are not the cause!