lethal contests

on

If you have been floundering lately, as I have, let’s gain some perspective by considering the plight of the Silver Lake loons. The pair’s struggles began in mid-May, as black flies thwarted their efforts at incubating a first clutch of eggs. According to Pat Schmidt, who watches the pair carefully throughout the breeding season, incubation proceeded normally during the nighttime — cool temperatures kept the relentless pests at bay. But the marked female and male were unable to stay on the eggs during daylight hours, when black flies were active and biting. On again, off again incubation finally gave way to abandonment during the last week of May, but the pair reset themselves quickly, adding two additional eggs to the two they had earlier tried and failed to hatch. Despite the cumbersome task of warming four large eggs simultaneously, the birds produced a chick at the very end of June. Their fortunes seemed to have turned.

The greatest risk faced by a breeding loon pair with a chick is our national bird. Bald eagles nest on tall white pines along lake shores and are a frequent sight over lakes. Indeed, eagles are such a routine part of the scenery on the lakes that loons often deign to wail at them as they pass overhead. Eagle fanciers might try to convince us that these raptors even purposely lull loons into a false sense of security with their constant, mostly innocuous flights nearby so that they can occasionally strike at loons suddenly with deadly purpose. An opportunity for such a surprise attack might occur when an eagle appears just above the tall trees at the lake’s edge as a week-old chick’s parents both happen to be underwater diving for food. Perhaps it was such happenstance that allowed an eagle to carry off the Silver Lake chick on July 2nd. In any event, eagle predation brought the breeding efforts of the pair to an unsuccessful close this year.

The sting felt by lake residents at the loss of the chick had begun to abate by July 18, at which point the territorial female, “Copper” (named for one of her plastic leg bands), found herself in a desperate battle. She was beaten badly, chased across the water, attacked from below as she rested on the lake surface, and finally forced to take refuge on land to escape further damage. By the time the violence had ceased, Copper had to be carried, helpless, to the Northwoods Wildlife Center. She died there a few days later.

As I have made clear in numerous posts, males are the ones that battle dangerously (apparently because of senescence) in most cases. So how do we explain the latest Silver debacle? An oddity concerning contestants might offer a clue in this case. Copper, who had reared chicks on Silver in 2014 and 2015, had battled repeatedly for ownership over the past several years with her bitter rival, “Mint”, the previous Silver female and mother of the chicks in 2010 and 2012. Even after losing the territory to Copper in early 2013, Mint was a frequent intruder into Silver Lake. Hence, both females had raised chicks in multiple years with the male, and both were highly motivated to vie for control of the territory. In addition, banding records indicate that Copper and Mint were of very similar size.

Now to game theory. If an animal encounters a long-lived opponent with which it is very closely matched in fighting ability but happens to get the upper hand at some point, it might then pay for that first animal to press its advantage and even kill the opponent. Why? Because our research has shown us that closely-matched pairs of females, like Copper and Mint, often give each other fits. Two females on Heiress and two others on Oscar-Jenny were so close in fighting ability that they traded off ownership of those territories over many years, each female hindered in her breeding efforts because of the constant interruptions of the other. The result was poor reproductive success for both rivals. Although there is enormous risk involved, it might occasionally pay for females to exhibit the vicious battling we associate with males. Specifically, a lightning strike to finish off your archrival might sometimes be worthwhile to avoid a chronic, destructive feud.

 

on

IMG_4317

“Wow”, Lainey said, “that band number is right next to the female we caught last night on Sherry”. She was right; the adult male from Skunk, which we had just netted and whose band number I was reading aloud to Lainey for data entry during banding, had a number imprinted on his aluminum USGS band that followed immediately after that of the Sherry female from the previous night. The reason for consecutive bands is that the Sherry female is the mother of the Skunk male and was caught and banded with him a decade ago (less two days) on Sherry Lake. I remember July 31st, 2005 on Sherry vividly, because I was equipped with a video recorder on top of a helmet with which we recorded the capture process for research presentations. But the recapture of mother and son on back to back nights ten years after we had first marked them has also caused me to reflect upon several key features of loon biology that have become familiar to me through my work.

First, loons live a long time. In this twenty-third year of my work, I still encounter birds in the study area that were on territory, as adults, when I first started covering them. Females, in particular, are survivors. While males have rather high mortality –partly owing to their proclivity for battling dangerously — females linger. When their mate dies, females find another; when a female is evicted unceremoniously from her territory, she stoically moves to a new lake nearby and awaits a chance to re-insert herself back into the breeding population. So it goes with the Sherry female whose worn-out band from 2005 is pictured on the bottom in the photo. She was “widowed” suddenly in 2009 (a possible eagle kill) but hung onto her territory and was joined by a new male in short order, who has been her mate since.

Second, young males do not disperse far from their natal lakes to breed. The Skunk male, from whom we removed the top band in the photo last night, moved about 15 miles from his natal haunt, Sherry. That dispersal is, actually, a bit longer than average for males, many of whom settle to breed on a lake adjacent to where they hatched years earlier. Short-distance male dispersal is essential to my work; without it, I would not have a large marked cohort of 2 to 5 year-olds of known age and natal origin in the study area at all times whose territory settlement strategies could be investigated. (At last count, we had seen 295 adult loons in the study area that were marked originally as chicks.)

Third, loons vary tremendously in their behavior towards humans. My assistants and I dread the Sherry female, because she is the most skittish individual we know. She cannot be approached easily within 100 meters on the water; she tremolos (i.e. alarm calls) incessantly when a chick is present and a canoe appears on the water. In fact, my assistants tell me that she begins to tremolo in anticipation of a canoe being placed on the water and that they have begun a strategy of hiding behind bushes and trees along the shoreline in order to make observations of her when she is with the chick. (Fortunately, Sherry Lake is tiny, so this observation strategy is workable.) I joked that we should wear camouflaged clothing when visiting Sherry; my assistants did not find this funny. Oddly, the Sherry female’s mate is among the tamest loons we study and never tremolos at us when we collect data there.

The great variety in loon tameness is a topic of great interest to us, as I have mentioned. It amazes me that an adult such as the Sherry female could react so strongly (and, it would seem, maladaptively) to humans, which she encounters constantly. Doesn’t she waste energy with her fruitless calls? Shouldn’t skittish birds like her leave fewer offspring and live shorter lives than other adults who tolerate humans without constant complaint? If so, she is not a good example of the pattern, as she has behaved this way for the ten years we have known her while cranking out chicks. Indeed, the Sherry female and the Oneida-East male, another vociferous but fecund individual with whom my staff has to cope, make me wonder if I have got it backwards. Maybe loud-mouthed loons warn humans away, lessening the likelihood of injury to themselves and their brood, and are rewarded with high evolutionary fitness.

on

Graph

There are three main reasons why I do field research. First, I love the outdoors. While it is unpleasant and inconvenient to many, the field is my briar patch. Second, I love observing and being with animals and gain new insights by watching loons up close and not just looking at data in my office. And third — well, I enjoy looking at data in my office. Complex puzzles and contradictions concerning loon behavioral ecology often become comprehensible when I look hard the numbers and run statistical tests.

Yesterday I had one of those Eureka! moments during data analysis. For the past several months, I have been looking at factors that: cause loons to: 1) be evicted from their territories and 2) disappear from their territories (that is, die). Some months ago I shared with folks that having your mate get evicted puts you at risk for eviction yourself. That is an interesting pattern and one that I continue to explore. But yesterday, I conducted the most sensitive and powerful analysis to date on the potential impact of age on rate of eviction and death in territorial residents. The  results were clear: age has a substantial impact on both eviction and death rates. Specifically, old loons are at risk for being booted off of their territories and they are even more strongly at risk for dying. The pattern was a bit tricky to detect, because very young adults are also at risk for eviction and death. So loons — like humans — have a “prime of life”. Loons of 4 to 6 years of age are still developing and improving in condition; at this age, they have a rather high rate of eviction and death. They reach prime condition about age 7 or 8 and remain in good condition until about 20, then condition falls off again, exposing them anew to a high rate of eviction and death.

The figure above shows the shape of the pattern with respect to age and death rate, when other factors are controlled. Sorry for the arcane title of the Y axis, but here is how to interpret the pattern. The curve shows how often territorial loons die, higher values indicating a higher death rate. The values are higher for young ages and old ages. Thus, the “U” shape shows that young and old loons die at a higher rate than do loons of moderate age. If you look closely, you can see that old age has a much stronger, more consistent impact on mortality than does young age. So senescence is very clear from the graph.

If you are a loon aficionado, you know that male loons often die in the course of defending their territories, while females rarely do so. How does the new finding of senescence fit in with lethal contests among males? On the one hand, it fits, because senescence might make male loons “desperadoes”, who fight hard for their territories because they have no future to lose. On the other hand, the pattern of senescence cannot by itself explain male-only fatal battles, because females and males both senesce. Oh well….I am always looking for another excuse to look at data in my office.

on

A lot can happen in a short time, it seems. I have spent only three days in the study area so far, but already we — Joel, Eric Andrews and I, and our incredible citizen scientists Linda Grenzer and Al Schwoegler — have found 8 active nests. Considering the territories we have visited recently and those we have not, I estimate that about 15-20% of all pairs in the study area are already incubating. Clearly the pairs have shortened the window between ice-out and egg-laying in order to compensate for the very late spring this year. I suspect this is possible, in part, because females were able to recoup much of the energy consumed during spring migration by foraging for weeks along rivers in the study area, before their breeding lakes opened up. That is, the extra foraging time near their territorial home apparently compensated for the foraging time that would normally occur on their territory.

Territorial turnovers have been common this spring; many marked pair members from 2013 either failed to return in the spring or did return but were evicted from their territories. The evicted birds include a young “ABJ” (“adult banded as a juvenile”; meaning a loon we banded as a chick) male from Schlect Lake. This ABJ male, hatched on Fox Lake in 2005, produced two chicks, of which one fledged, in 2012. But in early 2013, the Fox ABJ had been replaced by another ABJ male, this one also hatched in 2005, but on McNutt Lake. In late 2013, the Fox ABJ was able to retake his territory (possibly after the McNutt ABJ left it) and lived there the rest of the summer. However, two days ago, Eric and I witnessed a nasty battle between the Fox and McNutt ABJs (now both 9 years old) that culminated in the exhausted and defeated Fox male taking refuge on shore to avoid further attack from the McNutt male. Quite a grim spectacle! It remains to be seen whether the Fox ABJ can recover, drag himself off of this tiny 25-acre lake and get on with his life.

While our problems pale in comparison to the desperate life-and-death struggle that the Fox ABJ is facing, this latest contest is troubling to us as well. You see, we have hypothesized that dangerous contests of this kind likely occur when very old males (with very little reproductive fitness to lose) roll the dice by battling to win a few more years on their territory rather than accepting displacement by a younger, stronger male. (This is termed the “terminal investment hypothesis”.) Naturally, we must use statistical tests on a large body of data before drawing any conclusions. Still, it was unsettling to see a vigorous young male — and one that doubtless would have many potential future years of reproduction ahead of him — suffer a life-threatening encounter that flies in the face of our pet hypothesis.