territorial intruder

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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.

 

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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.

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Dozens of territorial pairs in our study area are now on nests. Several pairs have failed. Early indications are that failures this year will be caused mainly by predation. Raccoons are the most frequent culprit, according to work done by the DNR in our region using nest cameras. In spite of the havoc wreaked by raccoons, we still hope for a high rate of hatch.

Territorial visitors are a common sight on breeding territories these days. They will become increasingly common as the season wears on. Many lake residents wonder who these territorial intruders are and what they are doing. These loons are not, as many folks surmise, chicks that have matured and are returning to visit their parents. They are true territorial intruders — individuals that are young and have never possessed a territory or are older and had one but were booted off of it by another loon. Intruders intrude to learn about the territory and its defenders, primarily because they hope to usurp the territory by force on this or a subsequent visit.

One interesting pattern that we have seen in the last few years among intruders is a higher rate of long-distance intruders. For example, On Big Carr Lake a few days ago, we saw a female from Moon Lake in Vilas County. This is a bird banded by my collaborator, Mike Meyer of the DNR. More recently I identified a 5 year-old intruder from Powell Marsh: again, this was a DNR-banded bird from up in Vilas County. Of course, we value every banded intruder, as each gives us an opportunity to track a loon’s movements and the strategy it uses to gain a breeding slot. But the long-distance intruders are especially valuable, because they are chiefly females, which disperse much longer distances than males, on average, and which, therefore, we have learned less about.