Sometimes, as a scientist, I get tunnel vision. I get so locked-in while running statistical tests to verify simple behavioral patterns that I cannot see beyond those patterns.

This past week, I got another illustration of the problem. I was asking a basic question: “Do territorial loons show stronger territory defense when they have chicks than at other times?”. We might expect such a pattern for two reasons: 1) young chicks are sometimes killed by “rogue” intruders, and 2) intruders learn that a territory is of high quality — and possibly worth fighting for later — from seeing a chick or chicks in it; so territorial pairs should hide their chicks from intruders. As stated, this question is binary; I am just asking if the chick-rearing phase is characterized by more intense territorial behavior than other phases of the breeding cycle (like the pre-nesting and and incubation periods). And there is nothing unsound about asking that question. It is just a bit narrow.

I didn’t see the limitations of the question until I plotted the data on territorial defense against stage of the breeding cycle. Here are those data.

At first glance, I suppose, the graph looks a little busy. “Why did he have to plot TWO lines on a single graph?”, you might ask. My goal was to allow the viewer to compare two kinds of territorial behavior towards intruders at once: 1) territorial yodels by males and 2) outright attacks of intruders and other forms of physical aggression — and to look at how those behaviors vary throughout the breeding cycle. The graph allows us to see not merely how territory defense varies when territory owners have chicks or do not have them, but how territory defense changes throughout the breeding season — from 20 days before the eggs hatch to 50 days after.

What do we see? Whereas the statistical analysis I did simply told me that both yodels and aggression are more likely during chick-rearing than at other times, the graph paints a more nuanced picture. First, we see that yodels spike sharply at hatching and are rather infrequent at other times (red line). That is, males yodel with surgical precision during the period when their chicks are less than two weeks old and seldom at any other time. In contrast, aggression (blue line) by male and female parents peaks much later — when chicks are three to six weeks old. In short, territorial pairs seem to employ yodels and aggression for different purposes.

Here is my interpretation. As a grad student of mine showed experimentally, yodels are effective tools for discouraging landing by intruders that have entered the airspace above a territory. By yodelling, a male can cause an intruder bent on making a territorial visit to change its mind and visit elsewhere. Frequent yodels by males with tiny chicks, then, keep intruders away from chicks when they are small and most vulnerable to being killed by territorial intruders. But if intruders are so dangerous to young chicks, shouldn’t territorial aggression also be very frequent at this time? No, it should not. Yodels are so effective at driving intruders away at this time that few intruders approach pairs with young chicks closely enough for aggression to be necessary! Instead, it seems, parents only need to defend their chicks with physical aggression when this critical stage has passed, fathers have stopped yodeling their heads off, and intruders are comfortable enough to land in a defended territory and engage in social behavior with territorial pairs.

As I was strutting about the house and congratulating myself for solving this small puzzle, I presented the idea to my wife. She inspected the graph and asked a very reasonable question: “Why do males stop yodelling?”. After making a mental note never to share my ideas in the future, I puzzled over my wife’s vexing but insightful observation.

Here is my tentative response. We know from Jay Mager’s work that yodels are costly. Why? Because, by yodeling, a male is telling young nonbreeders about its identity, size, and body condition. Such information might allow those young adults to decide whether or not to try to evict the male from his territory immediately, to do so at some point in the future, or never to try. So yodels betray valuable information about the yodeller that might best be hidden. In particular, a small old male that has fallen into poor condition is placing a great big target on its back by yodeling to protect his chicks. Since yodeling males keep intruders at bay — a short-term benefit — but are also giving away valuable information — a long-term cost — males should use this potent vocal weapon only in time of greatest need.

Now……that is just so much arm-waving. I have conveniently tailored my hypothesis about the laser-targeting of yodels to the observations we have made. But at least, by developing a testable hypothesis, I have laid the groundwork for future progress. One day experimental playbacks of yodels at different stages of the breeding phase — not just at the young chick stage, where they normally occur — will determine whether the hypothesis has merit. So it goes with science.

Three days ago, Allison and I had only one car, so we covered a double circuit of lakes. We loaded two solo canoes precariously on top of our ’07 Toyota Corolla — “That seems safe”, I said, tugging on one of the straps we had used to lash the boats to the roof rack and smiling reassuringly at my dubious daughter — and headed to a northern tier of lakes. I dropped her and her canoe at Brandy, and scurried across Highway 51 to Arrowhead. An hour and half later I covered Kawaguesaga-North, while she observed at Bullhead, and so forth throughout the day. It was tiring, and Allison inevitably had to wait ten minutes or so for me to drive back from my lake to hers, but we visited four sets of lakes this way. Covering many lakes with limited personnel is central to the ethos of the Loon Project, and I was delighted to walk the walk on Sunday.

While our highly fuel-efficient observations on our last day in the field were very cool, the portrait of reproductive success that emerged from the lakes I visited was decidedly ambiguous. The Hodstradt pair has two thriving, five-week-old chicks. During my visit, the ten-year-old female (hatched on Butternut Lake in Forest County) was struggling to provide enough food for her large family. The alpha chick begged her mercilessly and received 14 feedings. In contrast, the beta chick, which only got two food items, was on the receiving end of three harsh pecks from his larger sibling. Still, Hodstradt has a history of producing two-chick broods, so they appear to stand a good chance of fledging both young. 

In contrast to the thriving family at Hodstradt, the Arrowhead breeding pair has been impacted heavily by a wing injury to the male. Even as I began to pull the canoe off of the Loonmobile, I saw a large loon preening awkwardly forty meters off the Arrowhead boat landing. “Uh-oh”, I thought. The telltale drooping of his right wing revealed the male’s identity long before I observed his plastic leg bands. He was alert and responsive to his environment, but he looked worse than ten days before, when we had captured him at night and inspected his injured right wing. I sighed and shook my head; we had hoped he would recover and rejoin his mate and chicks. As I took note of his struggle to preen without stretching his damaged wing, his sodden plumage (which occurs when loons fail to cover themselves with protective oil from a special gland near their tail), and his willingness to permit a fisherman to drift to within ten meters, a grim realization hit me. This male is going downhill rapidly and is not going to recover. (Marge Gibson, a veterinarian with REGI, has inspected a series of photos taken by Linda, and is confident that the right wing is broken — probably at the humerus — an injury she has seen often after severe blunt-force trauma such as a strike by a motorboat or jetski.) Despite the male’s injury, I wondered why he was confining himself to the small, protected cove off of the boat ramp, instead of remaining in the main body of the lake.

I quickly learned why the wounded male was hiding. A pair of loons rested confidently on the southwestern end of the main bay. Unlike the injured bird, these two sat up high on the lake surface. A short time later, they foraged in plain view in the middle of the lake. In other words, they acted like they owned the place. Clearly the male had taken refuge in a protected cove in order to hide from these two new adults that, in the absence of territory defense, had laid claim to the lake. Indeed, the new pair swam east to the mouth of the male’s cove as I observed them, as if hunting an intruder they wished to drive from the lake. I was relieved that neither pair member gave any sign that they detected the injured male in the cove. Somehow — either by diving often, hiding under a dock, or perhaps pulling himself up onto the shore — he eluded them and spared himself their attacks.

The wounded male was not the only loon systematically avoiding the new breeding pair at Arrowhead. As I patrolled the shoreline of the lake, I found his mate foraging madly for one of their two chicks in the northeastern section. Though her territory has slipped away because of her mate’s and her own inability to defend it, the female has been unwilling to desert her seven-week-old chicks. In order to avoid the watchful eyes of the new pair, she and the banded chick I found her with always remained within ten meters of shore and foraged among a stretch of long docks that jut out from the northeastern shoreline. As my video above shows, the chick begged his mother relentlessly for food, while she captured what few small fishes and insect larvae she could find along this sandy stretch. This brief set of observations provided a window onto the female’s plight. In order to fledge her two chicks, she must provision them surreptitiously for at least another month, wait for them to learn to fly, and then hope that they can move to nearby undefended lakes (which chicks naturally do at this age), where they can complete the growth process. The series of practice runs, aborted takeoffs, and awkward landings necessary for a chick to become adept at flight are sure to draw the attention of and aggression from the new breeding pair. If, by some quirk or miracle, the female manages to keep the chicks safe and healthy until they can fly, she will be the first adult we have ever seen to lose her mate when the chicks were younger than five weeks, have a new breeding pair take possession of the territory, yet still manage to fledge them. As much as I respect her determination, I do not like her odds.  

After my report of continued decline of the former breeding male from three days ago, Linda and Kevin Grenzer visited Arrowhead yesterday. They found the same cast of characters that I had seen two days before — the skulking, incapacitated male, the confident new pair, the plucky old female, and the banded chick that she had been feeding — but, incredibly, Linda also turned up the unbanded chick that we had not seen on two previous visits and had given up for dead. In fact, Linda got a series of photos of this chick as it followed its wounded father onto shore (see featured photo at top). It is touching to observe the chick’s dilemma — sitting awkwardly and reluctantly on land, yet refusing to abandon its fading father. I guess if we are looking for a positive from the recent events at Arrowhead, it is that the family is doggedly sticking together in the wake of a gut-wrenching calamity. 

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!

It is usually no fun to be wrong, but maybe this is an exception. In my blog post yesterday, I surmised that the sudden appearance in flight of the male from Little Bearskin meant that he and his mate had failed in their second nesting attempt. This seemed a safe presumption; I knew from many years of experience that males do not often leave females alone with small chicks. Yet I was mistaken. A lake resident (thanks, Nancy!) corrected me by pointing out that at least one chick had hatched on Little Bearskin this year, and Martha found two chicks on the lake during her early-morning visit today.

As we have explained in an earlier publication, there are three reasons why males tend not to leave their breeding lakes when their chicks are in their first two weeks of life. First, females cannot yodel, and therefore they are unable to discourage intruders from landing in the lake and approaching chicks by means of this aggressive vocal signal. Second, by virtue of their greater size, males are better equipped to intimidate and drive away intruders that do approach chicks. Third, having two parents guarding chicks when they are small permits breeding pairs to cover two bases — they can send one parent out to engage intruders and leave the other to protect the chicks, in case an intruder should come close.

In fact, years ago on Langley Lake we witnessed the danger that parents face if one of them ventures off territory when their chicks are small. In this case, two intruders landed when the male was off the lake, forcing the female to choose between: 1) staying beside its week-old chick, and 2) leaving its chick to interact with the intruders. She chose the latter course, but that strategy backfired when the intruders dove and split up. At this most inopportune moment, the chick happened to give an alarm call that one of the intruders heard. The intruder quickly found the calling chick and, with no parent nearby to intervene, killed the chick in a matter of seconds.

With that horrid incident seared into my brain (and a good deal of quantitative data on chick attendance to back it up), I was fairly confident that the appearance of a breeding male on a lake not his own meant that he had failed in his breeding attempt at home. In fact, I am still scratching my head over the Little Bearskin male’s decision to leave his mate, his two helpless chicks, and his home lake with its abundant food supply, in order to visit a neighboring lake that held nothing but failed and displaced conspecifics. I guess I will have to continue my research for a few more years to make sense of that odd bit of behavior.