A few days ago, I was conducting observations on the lake where Linda Grenzer lives when she snapped this photo of me, the two pair members on the lake (the two loons closest to my canoe), and five territorial intruders. Linda’s picture captures beautifully what transpired during my visit of an hour and a half. Better yet, the photo shines a spotlight on an enduring question that drives much of my research on loon territoriality: what is the purpose of territorial intrusion. This is a question that we have only half-answered.

We do know that many intruders visit in order to learn whether or not a territory has chicks; if they detect chicks, they are more likely to intrude in the following year and attempt to evict the breeding pair member of their sex. (This makes some sense, as the presence of chicks is an indication of good nesting sites and plentiful food for young.) We also know, from a recent analysis, that loons practice natal-site matching; that is, they attempt to settle as a breeder on a lake that is similar in physical size and in pH to the lake they were reared on. So undoubtedly some intruders must be learning about lakes where they intrude so that they can settle on one similar to their natal lake.

But there must be more motivation for intruders to visit lakes defended by breeding pairs. Among the 5 intruders pictured in the photo, for example, were: a banded 9 year-old loon not known to be settled on a territory yet (i.e. a probable “floater”), an unbanded loon whose status is wholly unknown, and three banded loons known to be members of breeding pairs from neighboring territories. While the first two birds could plausibly be shopping for territories through chick detection or natal-site matching, the neighbors are unlikely to be doing so.

What, in fact, could neighbors stand to gain from intruding next door? Several hypotheses are possible here. Neighbors might gain by becoming familiar with other loons with which they might become mated in the future, if they both lose their current breeding positions and must settle elsewhere in the general area. Neighbors might also be trying to learn about the territory, which they might occupy in later years, providing one of the current pair members dies. Another possibility is that neighbors have no particular interest in the territory where they intrude but, rather, are intruding in order to draw attention away from their own territory (since loons on the water tend to attract flying loons to land and investigate). That is, intruders might be attempting to “decoy” loons away from their own territory so that others do not learn about it and attempt to settle there. Finally, neighbors might simply visit to forage in someone else’s territory, depleting the food supply there instead of at home.

An additional question raised by the photo is: what is the breeding pair’s response to intruders? One might expect that the breeders would react aggressively towards intruders, driving them out immediately so that they cannot learn about their chicks (of which there were two in this case, hiding our near shore and far from the intruders) or harm them. Yet this pair — typical of breeders — was tolerant of the intruders and allowed them to roam freely throughout the territory for over an hour. Are the breeders feigning nonchalance to reduce the likelihood that intruders will look closely for and detect the chicks? Or do large numbers of intruders pose a severe threat to territory ownership such that territory owners must tolerate them or risk losing their positions?

I hate to raise so many questions that we cannot answer immediately. Testing of most of these hypotheses for intrusion and defense towards intrusion is feasible in our population. For example, we can look statistically at the “decoy” hypothesis by seeing whether pairs that vacate their territories and intrude next door experience a lower rate of territorial eviction than pairs that remain on their territories faithfully throughout the season. And we can test whether pairs that attack and stalk intruders, rather than tolerating their intrusions peacefully, suffer a higher rate of territorial eviction, because they betray the presence of their chicks and place a target on their own backs.

Such statistical analysis requires large samples of lakes and intrusions, so it will take time. Meanwhile, we will have to enjoy the experience of tracking intruders and breeders around territories by canoe and wondering what peculiar combination of evolutionary interests of breeders and intruders could produce such flotillas.


After the black fly debacle in recent weeks, we were all ready for some good news. Indeed, most territorial pairs had shaken off the flies and gotten back to the business of reproduction. Good tidings seemed the order of the day. Yesterday, Al Schwoegler of West Horsehead Lake called with a thrilling and unexpected report: the eggs laid by the pair, which they had left unattended for many long hours on several days because of the torment of black flies, had begun to hatch! At first neither Al nor I could believe that the eggs were viable. As Al described the behavior of the female on the nest — who has reared a whopping 19 chicks to fledging since 1996, when she was first banded — we gradually let ourselves believe that the impossible had occurred.

But our positive feelings were dashed suddenly by the cruel realities of loon territorial behavior. You see, the last few weeks at West Horsehead have been marked by frequent territorial intrusion. At the very time that the pair was trying to recover from the onslaught of biting insects, the male owner was facing repeated challenges for his position. Finally, by yesterday, both Al and Sally Yannuzzi of my team confirmed that male ownership had passed from the 14 year-old male hatched on Alva Lake who had resided on the lake for most of the past decade to a 9 year-old upstart from Harrison Lake in Lincoln County. The new male, confident in his new position, spent much of the morning resting and foraging near the nest, while the female patiently sat on the eggs. Finally, the female slid off of the nest into the water, revealing a newly hatched chick and second egg, which was on the brink of hatching. Alas, the new male behaved as animals typically do when confronted with helpless young that are not their genetic offspring: he quickly pecked the chick to death as its mother looked on helplessly. The celebration of an unexpected hatch gave way to a wake for a young loon doomed by territorial usurpation. Al took this photo of the female, still mildly protective of her nest containing the dead and unhatched chicks. (Shortly after the photo was taken, the female left the nest to forage with her new mate, with whom she might still renest.) IMG_20140616_134217_724Sorry for the unpleasant photo and description. But there is a valuable lesson here. Loons, like lions and langurs and mice and water bugs, behave so as to promote their own reproduction. Despite the ugliness of this episode, we can hardly hold it against the 9 year-old that he is looking to produce his own biological offspring — before a new usurper comes along and shows him the exit.

To be sure, 2014 is shaping up to be a dismal breeding season. Ice-out occurred weeks later than usual, owing to thick ice and cool weather in March and April, and delayed breeding for all loon pairs. The ensuing warmup in May caused the black fly population to explode to higher levels that we had ever seen. Pairs that had just laid eggs were blanketed by the relentless bloodsuckers and incubating males and females driven off their nests at rates never seen before. In short, almost all loons abandoned their first nests.

Although it is counterintuitive, loons are probably more vulnerable to nest abandonment than many other birds because males and females incubate the eggs equally. Consider the plight of loons trying to incubate their eggs to hatching for 27 days in the presence of black flies. Each pair member must incubate for stretches of several hours before being relieved by its mate. The rotation system must be efficient enough that eggs are incubated over 99% of the time. When not sitting on eggs, a breeder rests, preens and forages to replenish its energy reserves. If either pair member fails to incubate, the nest is doomed to failure, because its mate cannot compensate for missing incubation by remaining on the eggs at all times without long breaks for foraging. So we have a case where the weakest link breaks the chain. Even a tough, determined male incubator is destined to lose a clutch if his mate is less determined than he is and refuses to sit on the eggs and tolerate the torment of biting insects.

We have learned about the necessity of dual incubation from past observations. Nest abandonment commonly occurs following territorial eviction of a pair member during incubation. If its mate is evicted by an intruder, the remaining pair member usually continues to sit on the eggs for a time. With rare exceptions, though, its new mate (the usurper) does not incubate — why should it sit on eggs containing young to which it is unrelated? — the cycle of shared incubation duties breaks down, and the nest fails within a day. (If it is early enough in the season, such pairs will lay a new set of eggs that both will incubate.) A rarer cause of nest failure during incubation is death of a pair member. In 2005, the female on Alva Lake was killed by an eagle while sitting on the eggs. The male valiantly sat on the eggs, taking breaks to forage from time to time, even as female floaters competed in front of him to fill the breeding vacancy. Despite being within a week of hatching, this male could not complete incubation on his own, and the nest was lost.

Enough talk of failure! I will end on an up note by showing you the sweet photo that Linda Grenzer took of a breeding male sitting on a nest. This bird and his mate both tolerated the record outbreak of blackflies earlier in the season and were rewarded with a little fuzz-ball — a much-needed reminder that all is not lost this year. Let’s cross our fingers that other pairs, many of which are incubating a new clutch of eggs after abandoning the first, will be able to duplicate this effort.



We just got a report that a pair of our loons is back, and this is the first pair of the year. Up to now, our pairs have had to content themselves with overflights of their territories to look for open water, which would allow them to land. At other times, they wait (mostly along the Wisconsin River) for their lake to open up. But according to Linda Grenzer, who is super hawk-eyed and on the ball, about 20% of her lake (Muskellunge in Lincoln County) was ice-free by this afternoon, and that was enough to permit the banded pair there to land and begin to defend the territory anew.

The Muskellunge pair that Linda found today is well known to us. The male there is a very tame bird whom we banded on as a chick on Manson Lake in 1998 and who produced chicks with a first female on Deer Lake in 2003 and then bred successfully with her again there in 2004 and 2005. In 2006 and 2007, Deer yielded no chicks. As we have seen with numerous other males, this male then gave up on Deer, evicted the male on Muskellunge (right next door!) in 2008 and paired with a new female there. He has twice produced chicks on Muskellunge and seems firmly ensconced there now. The Muskellunge female is also an interesting individual, as Linda caught her spending some of last summer hanging out with and feeding a chick from Clear Lake (also next door to Muskellunge) when her own chick had shifted to a new lake! Furthermore, Linda photographed this female while she was molting, and I posted the photos on Nov. 28 of last year.) Kudos to Linda for all of the sleuthing. I do not believe her breeding loons do anything interesting without her recording it from her kayak nearby.

Since this post has become mostly a tribute to Linda’s great field work, it is fitting to end up with a photo she sent me a few weeks back. She took this photo on the Wisconsin River near Tomahawk on 14 April. If you look closely, you will notice that one of the colored leg bands has a geolocator tag attached. The tag from this male, who is, by the way, from a territory up in Vilas County, will provide a good deal of information on his past migratory movements and foraging patterns that might help USGS biologists learn about the occurrence of avian botulism, which kills many loons.



Linda Grenzer photo from April 2014 Wisc River

As most of you know, most loons acquire their territories by force. That is, a non-breeding adult intrudes into a territory defended by a breeding pair, challenges the pair member of its sex, battles with that pair member and, if lucky, evicts that owner. (An evicted owner is sometimes killed in battle, especially if it a male, but more often simply leaves its former territory after a defeat and looks elsewhere to settle.) By the way, evictions (also called takeovers or usurpations) are about equally frequent in males and females.

We have now observed or inferred 226 territorial takeovers, and we are at a point where we can begin to look for other patterns in these data. I have been analyzing the data to learn whether one eviction leads to another. For example, if the territorial male on a lake gets evicted, does that place his mate at risk for losing her position as breeder as well? Based on 2207 breeder-years of data, the answer is a resounding “Yes!”. When a male is evicted from his lake, his mate is at substantial risk for losing her position. Female eviction also exposes male breeders to an increased threat of eviction, although the pattern is not as strong.

What can we make of this pattern? It is early, and many analyses remain to be completed, but here is an early read on this finding. Breeders are largely stuck with the danger of eviction and with the problem that their mate’s eviction threatens their own position. Females, which are about 20% smaller than males, are really in a bind, as their size probably prevents them from helping their mate avoid eviction — thus protecting themselves indirectly. On the other hand, males might be expected to chip in and help their mate drive off a would-be usurper, if doing so protected their ownership as well. Yet we have never observed a male teaming up with his besieged mate to drive off a potential usurper. If this happens, it must be rare. Indeed, males appear to be dispassionate observers of female battles for territorial ownership, despite the risk it poses to them. Why? Perhaps males are better off with a mate that fights strongly (such as a proven usurper), since having a vigorous mate ensures future years of eviction-free breeding. It is a bit sad and selfish to say it, but if your mate is vulnerable to eviction, it is probably best to let her fend for herself than to intercede and save her.

On Manson Lake, the long-time male breeder was displaced in May of this year by a younger male. After seizing control of the territory, the usurper proceeded to nest with the long-time resident female.

But the original territory owner did not give up on his territory easily. He remained in the vicinity and was a frequent intruder into his old domain. In fact, between the 13th and 19th of June, the old male drove off the usurper and regained ownership of his territory. At this point, incubation by the female and the usurper was well underway, and the eggs were within 10 days of hatching. The female continued to incubate the eggs, and the old male — though he had played no part in the reproductive effort and had not fertilized the eggs — defended the nest. (We did not observe him to incubate the eggs, however.) When the eggs hatched, he treated the chicks as his own.

Territorial usurpation creates some awkward reproductive decisions for loons. It is common for a breeder to lose ownership of the territory during a breeding attempt. In most cases, incubation ceases, and eggs are abandoned. If there are chicks present, they are often killed by the usurper, while the remaining pair member looks on helplessly. Manson is not the only lake where a usurper has seized control of a territory but continued to care for offspring that do not belong to him. In fact, we have observed at least 5 other cases of such behavior — all among males. That is, we now have 6 cases where a usurping male has wrested territorial ownership away from the male breeder but either permitted the young to survive and be reared by the female breeder (2 cases) or seamlessly stepped in to assume paternal responsibilities (4 cases). I am beginning to see a pattern!

Evolutionarily speaking, animals should behave so as to maximize the number of their own young they contribute to the next generation. It is generally unexpected to see animals care for young that are not their biological offspring.  I will continue to be on the lookout for more examples of such behavior in loons — and will puzzle over the apparent tendency of male usurpers, but not females, to be willing to adopt the offspring of their predecessors.