nesting behavior

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If you feel as though you haven’t heard from me in awhile, it is because I’ve been in the weeds. But my time in the weeds has been fruitful.

I have been analyzing loon reproductive patterns. Specifically, I am investigating what about adult loons themselves and their surroundings leads to success in hatching their eggs. The trove of detailed breeding data we have — 100+ territories followed closely for almost 30 years — gives us a unique window onto predictors of hatching success. I gave a hint of this analysis some months ago. But more data and a refinement of earlier statistics have clarified our understanding and led to new findings.

HATCHING SUCCESS OF MALE LOONS VERSUS NUMBER OF YEARS ON THE SAME TERRITORY. BARS SHOW ACTUAL DATA. CURVE SHOWS RELATIONSHIP BASED ON STATISTICAL ANALYSIS.

One of these findings might be familiar. Males, which choose the nest location, improve in hatching success as their familiarity with a territory grows. This is easy to understand. They use trial and error to find a good nest location. During the first few years on a new territory, they blunder about and lose many nests to predators. Over time, they improve their hatching success by avoiding sites where predators took the eggs. As the predictive curve shows, males make rapid progress in their first year or two on a new territory. Once three to four years have passed, their year-to-year improvement is small.

However, males continue to show incremental — but statistically significant — improvement in hatching success from year to year, even after 15 years on the same territory. Although the annual improvement is slight, it adds up. For example, a male with 20 years on a territory has a 27% better chance of hatching his eggs than a male in his second year. It’s a little hard to imagine the reason for this sustained improvement. Does a male only notice certain biological aspects of his breeding territory through prolonged exposure to them? For example, might a male observe over the course of many years that mammalian shoreline activity is higher at the south end of the lake than the north end and avoid placing a nest at the south end if — say, in his 17th year — circumstances should force him to relocate it? Maybe. In any event, the finding is exciting for the study of animals generally. It suggests that, for an intelligent animal inhabiting a spatially complex territory, adaptive learning — that is, learning that increases reproductive fitness — never really ends.

What about females? Like males, females improve in hatching success over time, but the pattern is completely different. Females have low hatching success in their first year on a territory but improve markedly in their second year. However, they do not continue to improve. Hatching success for females “tops out” after their second year on a territory. The female improvement pattern seems less difficult to explain than the male pattern. Since females do not control nest placement, they do not have any obvious means to get better at hatching eggs as males do. But they must learn something vital about the territory in that first year in order to enjoy greater hatching success in all other years. Perhaps they quickly learn where to forage in that first crucial year and retain that knowledge from the second year on. If so, that important knowledge might allow them to nest more often or better sustain the incubation effort.

HATCHING SUCCESS OF FEMALE LOONS ACCORDING TO NUMBER OF YEARS ON THE SAME TERRITORY. FEMALES HATCH EGGS LESS THAN HALF OF THE TIME IN THEIR FIRST YEAR ON A NEW TERRITORY BUT ENJOY MUCH BETTER SUCCESS IN THEIR SECOND YEAR AND ALL YEARS THEREAFTER.

Comparison of the two figures shows the contrast between males’ steady improvement in hatching success over their entire residency on a territory and females’ one-time jump up to higher hatching success after their initial year.

How can we be sure that the effects I have described are attributable to females’ and males’ experience on a territory instead of merely age? It seems reasonable to hypothesize that adult loons might get better at hatching eggs simply as they gain experience with nesting, regardless of the territory they are on. But this is not the case. I included age as a variable in the statistical analysis, and age turned out to be a poor predictor of hatching success. Thus, a 29-year-old male who loses his territory and settles with a new female on a new territory is right back to square one. He must learn again by trial and error on the new territory where to nest and where not to nest. He is no better off than a five-year-old male that has just settled on his first territory.

What about duration of the pair bond? Since males and females both incubate the eggs equally, they must coordinate their incubation schedules in order to hatch the eggs. So one might have expected that many years of breeding together would translate into greater hatching success for a male and female. But I included duration of the pair bond as a variable in the statistical analysis. Like age, pair bond duration is inconsequential. Yes, males and females that have been together for many years tend to hatch their eggs more successfully than new pairs, but they do so because: 1) the female is past her first difficult year of low hatching success, and 2) the male continues to improve his nest placement over many years on a familiar territory.

Why do I spend so much time making what must seem like fine distinctions? Does it really matter whether old males and females hatch a lot of chicks because they are old or because of experience on a territory? Who cares that old males and females hatch more young not because of more time spent together but because each of them has gained experience with a territory as individuals? We should all care. Since territory familiarity, not age or pair-bond duration, confers reproductive ability upon adult males and females, we should redouble our efforts to ensure not merely that loons themselves live long lives, but that we minimize disturbance to the familiar nesting areas on which their breeding success depends.

Beautiful photo of Clune sitting on his nest by Linda Grenzer.

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The nest in the featured photo has that forlorn, unloved look that many do at the beginning. It seems, at first glance, that the female just crept up on shore and dropped an egg there — as if she had to put it somewhere. Closer scrutiny reveals that the pair had gathered a substantial bed of pine needles from the surrounding area and formed a crude bowl shape around the lone egg. Still, it does not inspire confidence that a portion of rotted oak leaf is draped over the egg. Yet the male approached me as I quickly snapped a few photos, registered the nest’s GPS coordinates, and skedaddled to see if he would incubate. He did not.

I can understand the Thunder Lake pair’s inability to adjust to changing conditions. Only an hour before seeing them, I had stumbled out of my guest room (a wonderful, secluded home by a small lake where our friends permit us to lodge in May and June). My trip to Rhinelander on the previous day had not been the relaxing, sumptuous journey I had hoped for. After a last-minute cancellation of our flight, I buddied up with two other shell-shocked passengers near the gate — one a sales director for a pharmaceutical company from Minocqua, the other a Colombian fellow from Tomahawk who sells farm machinery for a Finnish company — and we rented a car for the 3 1/2 hour drive instead of waiting overnight to be rescheduled. (In truth, Steve rented the car, since his company would cover the cost.) By the time our rental rolled into Rhinelander, it was 2 a.m. Luckily, the engine turned over in the 2007 Toyota Corolla that we leave for nine months in our storage box (the “Loonmobile”). I was able to grab three hours of shut-eye before some passing Canada Geese awakened me. Two — okay, three — donuts and a cup of coffee later, I had seen my first loon pair on Thunder.

My first lake visit in 2022.

We have learned over the years that loon pairs take a day or more to “accept” that they have laid an egg and must incubate it. On these initial days, pairs sometimes wander far from their new nest, leaving the egg dreadfully exposed. I find this curious. The egg is, of course, in danger of being found and eaten from the moment it is laid. The embryo inside it cannot begin developing rapidly until it becomes optimally warmed by the parents. Every moment spent off the eggs seems time wasted and needless risk taken. I suppose, though, that I must defer to my study animals, who have a pretty good record of turning eggs into chicks.

The Thunder pair can be forgiven more easily than most for their reluctance to incubate. The male is new to the territory and unmarked. Probably a 5- or 6-year-old, this is likely the first egg that a mate of his has ever laid. One hopes that he and his well-seasoned mate — whom we banded as a chick on Currie Lake in 2003 — can restore Thunder Lake to productivity, after a six-year chickless slump. To do so, they will have to shake off this egg-denial and get serious about breeding!

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I have touched upon this theme before. A peril of longitudinal investigation is that one decides, after a period of time, that one understands the system. So it has been with the Loon Project.

For many years I have thought I had a good handle on territorial behavior. Indeed many aspects of the loon territorial system have become clear during the course of my work and are not in doubt. Both sexes usually fight to claim their territories and face the constant threat of eviction. Males, which establish strong ties to a territory through controlling nest placement and learning where the best nest sites are, fight harder than females, and sometimes die during territorial battles. Early senescence among males sets the stage for them to become very territorial and aggressive as they reach their declining years (their mid-teens in many cases), which seems a means to help them eke out another year or two on a familiar territory.

But I might have been off in my understanding of the role of lake size and body size in territorial behavior. I have always thought of breeding territories on large lakes as much sought-after, because large lakes have ample food for rearing chicks. (Small lakes, you might recall, run low on food for chicks, resulting in lower fledging success.) If large lakes produce more young, I reasoned, large-lake territories must be highly desirable. Competition must be fierce, then, for these territories. A recent analysis of territorial tenure — how long a male or female can hold onto their territory before getting evicted from it — has forced me to rethink the effect of lake size on territorial competition. The figure below is a plot of territorial tenure versus body mass for males on lakes smaller than 20 hectares (50 acres) in size (like Langley, whose current pair is pictured). As you can see, small males — especially those below 4600 grams — have very short stays on small lakes, in most cases, while large males — notably those heavier than 5000 grams — often enjoy very long territorial tenure. This pattern suggests that, contrary to my expectation, territorial competition is fiercer on small lakes than large.

impact of lk size, male body size, on terr tenure

Let’s look at the same pattern on medium-sized lakes (20 to 80 hectares; or 50 to about 200 acres). You can see that the overall pattern is still evident, although it is weaker here, because a number of very large males (5400 to 5800 grams) have anomalously short tenure.

i2 mpact of lk size, male body size, on terr tenure

Finally, let’s inspect the data only for males on lakes larger than 80 hectares (200 acres). In contrast to my earlier hypothesis, large males are not holding their territories any longer on large lakes than are small males, as you can see from the plot below. Males of all sizes may enjoy long tenure on large lakes.

3impact of lk size, male body size, on terr tenure

How on Earth do small males hold their territories much longer on large lakes — which  seem much in demand, get more intrusions, and appear difficult to defend — than on small lakes, which get fewer intrusions and should be more easily held? I don’t know exactly how males hide in plain sight on large lakes, but it might have to do with the difficulty that territorial intruders have in simply finding a nesting pair and identifying nesting habitat on large lakes. Consider the Lake Tomahawk-Little Carr pair. This pair nests in a marsh at a well-hidden location. When one bird is incubating, its mate is usually far off in the wide open portion of Lake Tomahawk, which is many kilometers long and has an area of 1400 hectares (about 3500 acres). A male intruder might well find and socialize with the off-nest pair member on on the big water, but it would have no way of knowing that the mate of this loner was on a nest hidden far away in a marsh. Similarly, when the eggs hatch, the pair quickly leads the chicks to the main bay of the lake, far from the critical nesting area. Pairs with chicks provide an enticing cue to young males seeking territories, because the presence of chicks tells of the availability of nesting habitat. But a male intruder that encounters the Tomahawk-Little Carr pair and their chicks on the main bay of the lake would face the needle in the haystack problem in locating the precious nesting area that yielded the chicks. A dangerous battle might win the territory, but the knowledge of how to use the territory (that is, where to place the nest) would vanish with the old male’s departure. Hence, large lakes appear to be less valuable to males.

A male intruder bent on taking a territory likely to yield chicks in the future would be better-served by evicting a chick-rearing male on a small lake. Such an intruder would have a much smaller set of nesting areas to inspect and would likely find and use the nesting area that produced the chicks. Thus, we might expect stronger competition among males for small, easy-to-learn territories — a pattern that dovetails with the longer tenure that large, competitive males enjoy on small lakes, compared with small, easily-evicted males.

What about females, you might ask? Do large females on small lakes, like large males, have an advantage in holding their territories when compared with large females on large lakes? If my hypothesis is correct, and the value of a territory depends upon knowledge of safe nesting areas, then large female size should not be especially beneficial on small lakes. Indeed, any impact of female body mass on territorial tenure should be equal across all lake sizes. Why? Because females do not control nest placement in this species. An intruding female that evicts a breeding female with chicks and pairs with the breeding male would have access to that male’s knowledge of nesting sites on a lake of any size. As predicted, large size is no more beneficial to small-lake females than large-lake females. (Indeed, size has an overall weaker effect on competitive ability in females.)

So my post hoc hypothesis for the fierce territorial competition on small lakes holds for the time being. The explanation I have given is not the only one consistent with these data, by the way. In fact, the entire complex pattern described above might be explained by a completely different scenario. Large males might hold small territories longer simply because they are in better body condition. This is highly plausible, because mass is a good predictor of health and condition. Thus, only large males might be able to hold onto small lakes for a long time, because they are in good enough condition to survive in a habitat with limited food. Small males, by this logic, are already in sub-prime condition, so they are destined to have short territorial stays on lakes with limited food. In contrast, medium-sized and large lakes do not show the pattern, because food is not limited on them.

As you see, we have a long way to go to figure out exactly what the above graphs are showing us. Distinguishing between the “small-lakes-are-highly-competitive” and the “holding-small-territories-requires-good-body-condition” hypotheses will take some years. The difficulty of using what seem like beautiful, clear data to reach a firm conclusion provides a nice window on what it is to be a scientist.

 

 

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Behavioral ecologists are human. Although we try hard to view biological events critically – to look for confounded factors, biased samples, untested assumptions – we miss a lot. So it is when we look at the nesting behavior of birds.

Ecologists around the world have made a simple, elegant discovery about how birds respond to nest failure. Once they have settled on a breeding territory and reared young successfully once, breeding birds get conservative. They reuse the same nesting site again and again. On the other hand, if they try to nest in one location and the nest fails, they shift to a new location. We call this simple strategy the “win-stay, lose switch” rule.

Let’s think a bit more about the win-stay, lose-switch (”WSLS”) nesting rule. What is it about a nest’s location that links it so critically to success or failure? The main answer is predation. Most predators are long-lived mammals (raccoons, squirrels, foxes), reptiles (mainly snakes), or birds (crows, jays, hawks, or gulls) that travel within fixed small ranges looking for food throughout their lives. If a bird’s first nesting attempt is not found and gobbled up by a vertebrate predator, a second one at the same site will likely escape predation as well. On the other hand, a raccoon, blue jay, or rat snake that found and ate your eggs at one site in mid-May will likely do so again in June, if you reuse the nest site. By moving away from the site of a failed nest, you might find a new site that does not fall within the predator’s home range – or is better hidden or otherwise inaccessible to the animal – and the prospect of successful breeding is renewed. That is the simple beauty of the WSLS rule.

While predation is the most obvious and important reason for using the WSLS rule, there are other reasons why moving a nest might be beneficial following failure. A species like the cliff swallow, whose nests become infested with swallow bugs – blood-sucking insects that attack and kill nestlings – should (and does) respond to infestations by moving the nest. The key point: vertebrate predators and tenacious parasites are persistent and location-specific nesting threats. To place a new nest at the same spot shortly after losing a first one to such a threat is to court disaster.

The WSLS rule has been confirmed as a logical and successful nesting strategy by ecologists around the world. Numerous theoretical papers have been written about it (including one by me). The rule is so widespread that scientists often think of it as “the way” that birds respond to nest failure. But closer inspection of nest failures shows that we have oversimplified the picture.

Nest failure can also occur owing to threats that are fleeting and non-location-specific. Fleeting, non‑location-based threats are those that occur at a brief moment in time, are not likely to recur soon, and are no more likely at one location than another. Examples are “freak” weather events, like early spring snowstorms or heat waves. Fleeting threats of this kind usually end quickly – so quickly that they abate before the nesting pair can even lay a new clutch of eggs. Fleeting threats make very different demands on nesting birds than do persistent threats and should be countered with a different strategy. Why? Think of a pair of loons whose nest has been flooded by a 6-inch rainstorm. If the pair were to use the WSLS rule to respond to this fleeting threat, they might move their nest away from a traditional nesting location (say, a favorite island) that they had used to produce many fledglings in years past and choose a new, untested nesting location. In so doing, the pair would discard years of accumulated knowledge about their territory and  dim their breeding prospects.

What is the proper response to a fleeting threat of nest failure? Nothing! That is, the logical and adaptive response (i.e. that which maximizes the chance of breeding success) is to ignore fleeting causes of nest failure and consider the next nesting attempt a “do over”. Do birds have the capacity to respond differently to different causes of nesting failures? It is too soon for a general statement, but loons can do so. If a predator gets their eggs, loons use the WSLS rule (i.e. they move the nest). If a fleeting threat causes them to abandon their eggs, loons ignore that nesting attempt, often placing a new set of eggs right back in the same nest they started and abandoned a week or so before.

Followers of the blog will know that loons face a fleeting (but very severe) threat to nesting that most other birds do not: black flies. Perhaps their vulnerability to black flies — which typically only cause nest failure for a week or so in late May —  has caused loons to evolve a more sophisticated response to nesting setbacks than other birds. I have begun combing through the literature on avian renesting behavior in order to determine if, indeed, the nuanced renesting behavior of loons is unique. Since I have just started, we can bask for the moment in the possibility that loons are a cut above the rest.

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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.

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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.