survival

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My mother-in-law came to visit us in California last week. She is an avid follower of my blog (!), so I was excited to learn what she thought of my most recent post. She hated it. That is, she said of my report that loons get old:”I could have told you that!”. Naturally, I was deflated. To think that 23 years in the field had produced a result deemed pedestrian by my mother-in-law!

While one might argue that she is family and should have been blindly supportive of my work, Joanne is right, in a sense. As humans, we are accustomed to old age and deterioration of the elderly. But, as I tried to explain, senescence is not the rule in all animals. Birds are unusual, in fact, as they exhibit relatively late and gradual senescence compared to mammals of similar body size. So the striking and rather sudden senescence that I reported recently is mildly surprising for the taxonomic Class Aves. Still, I think I agree with Joanne that it is not terribly shocking!

But there is more. The blog where I reported senescence in adult loons was based on an analysis that pooled male and female individuals. Since then, I have analyzed the sexes separately. The results are striking. As the figures below show, the senescence that I reported for the species as a whole (measured by decreased survival) is driven purely by males. While males and females that have been on territory from 1 to 14 years survive at a rate of 95% annually, males with 15 or more years on territory only survive at a rate of 58%. (Old females show a very modest decline in survival to about 91%.) Since males and females that settle on territories are almost always 5 years old or older, we can say with confidence that territorial males in their twenties drop like flies; females, in contrast, are survivors.

young males and females do not differ in survival rate

old males die at a much higher rate then females

I don’t know if my mother-in-law will be impressed by these data. For the moment, I must be content in the knowledge that I have found a strong and highly unusual survival pattern. As a behavioral ecologist, this stunning disparity leads to several other questions. Among them are: 1) Do older males exhibit any other evidence of deterioration such as in territory defense, chick production or body condition? and 2) Does the high mortality of older males cause the adult sex ratio to swing towards females such that females are forced to wait years before finding a mate? Rest assured that I am exploring these possibilities with great enthusiasm.

 

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I have been back in California for some months now, my research in Wisconsin a distant memory. A stroll to the end of the pier at Newport Beach changed that. Several of my study species — unbanded animals that probably belong to breeding populations from Alaska, British Columbia, or Alberta — were foraging contentedly off the pier’s end, as fishermen cast their lines all about them. The fishermen appeared to avoid casting near loons, so I was not alarmed by what I saw. Loons are usually adept at avoiding fishing lines (though not always). Furthermore, an ecologist would not be surprised to see loons sharing a fishing hole with anglers, since they are competitors for the same small fish — mostly smelt and mackerel.

Common loons were not the only species diving and pursuing small fishes among the sea lions and occasional pod of common dolphins. It was a treat to find this juvenile red-throated loon out in the waves as well.

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Back home in Irvine I am crunching data, as I do habitually in the off-season. I feel some urgency at the moment, as I am about a week away from having to give a talk on my findings at the Winter Animal Behavior Conference in Colorado.  I will be more positive in a week or so, but I have already confirmed senescence in two different respects. First territorial common loons older than 20 abruptly begin to show much higher year-to-year mortality (roughly 20% annual mortality from 20 on; only 6% mortality up to 20 years). Second, territory holders 20 and older also stand a much greater chance of losing their territory through eviction (again a 20% rate of loss) than do those younger than 20 (12% rate of loss). In the coming days I will explore whether the sexes differ in these respects and whether old territory holders make any behavioral adjustments to this apparent decline in health and fighting ability.

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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.

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LMG_2826 Three Month Old Chick Wing Span

I just got a report from Kristin, who is still in the study area. The chick hatched and reared on little Buck Lake popped over to Crescent Lake for a visit. Kristin was excited, because she had seen many cases of juveniles flapping their wings and practicing takeoffs, but this was the first flight of a juvenile she had recorded to a nearby lake. Thus begins the phase of juvenile wandering that characterizes the fall months. If this year proves typical, trips of maturing juveniles to neighboring lakes — sometimes up to 10-15 miles from their natal lake — will abound in the next 9 to 10 weeks.

But why do juveniles abandon the comforts and familiarity of their home lake to venture to lakes unknown? This is a difficult question with many possible answers. First, let’s think about the downside of such movements. In deserting its natal lake, a juvenile will be faced with new food sources to which it will have to adjust. Since it must feed itself and mature rapidly to put on weight for fall migration, leaving home seems a gamble. A new lake will also contain new risks in the form of predators or aggressive territory holders (although aggression is generally low in the fall, to be sure).

In order for wandering to new lakes to be a sound behavioral strategy, we would expect there to be advantages that more than offset these risks. One can imagine a number of potential benefits to shifting to a new lake. In the case of the Buck juvenile observed by Kristin, the bird was moving from a small lake with limited food resources to a much larger lake with a more abundant prey base. So the short-term benefit of being able to capture more food and put on weight for migration might be a large part of the explanation. Catching and consuming new food items might pay benefits down the road for young loons, as well, as this forces them to practice new modes of foraging that might be useful during migration and winter, when they will be faced with vastly different prey.

LMG_2823 Three Month Old Chick

There is a third and more nebulous hypothesis that might explain wandering by juveniles in fall. Three to four month-old individuals might be laying the groundwork for their lives as adults by exposing themselves to many different lakes and learning which are most suitable for them. That is, juveniles might try out lakes of different sizes and shapes containing different fish and invertebrates as a means of finding out what sort of lake allows them to feed most efficiently. If so, this period of exploration might allow them to target lakes more effectively when they return to northern Wisconsin (as they do) to look for a breeding territory in 2 to 5 years. It is even possible that young loons — especially males, which often settle to breed within a few short miles of their natal lake — move from lake to lake in order to create an internal map of the local area, which they will use in a few years when they return in adult plumage and seek a breeding territory.

Like many behavioral hypotheses concerning free-living animals, these 4 hypotheses for fall wandering are vexing to test. But they are not wholly untestable. For example, if juveniles simply move for better foraging, those that move should have higher masses in the fall than those that remain on their natal lakes. And if shifting between lakes prepares juveniles for foraging during migration and winter, frequent-shifting juveniles should enjoy higher survival than infrequent shifters. Even the hypothesis that juveniles begin to map out the local area for their later use can be tested to a degree, as it predicts more rapid settlement by frequent lake-shifters than infrequent ones. So we may, in time, begin to understand juvenile shifting. (This will come as a great relief to Kristin, no doubt, as she is braving the cool, rainy weather to document lake shifts by juveniles that we banded in July and early August.)

As I have noted before, the more immediate reason for tracking local movements of young juveniles has to do with learning about the development of natal-site matching behavior that we found recently. (Natal-site matching refers to the striking tendency of young loons to settle as adults on lakes that are similar in size and water chemistry to their natal lake.) We are curious to see whether loons show preferences for natal-like lakes even in their juvenile movements. If so, short trips during the fall to natal-like lakes will, of course, reinforce natal-site matching and yield young adults that have very strong natal preferences indeed!

Thanks, as so many times before, to Linda Grenzer, who provided these nice shots of the 2014 chicks on her lake. Although it is bittersweet for her, Linda hopes that these two juvies will soon themselves fly off to nearby lakes to gain valuable experience — or maybe just weight — that will prepare them for migration and beyond.

 

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We were sad to learn yesterday that one of our long-term resident birds has died. We are not sure what the cause of death is, but the DNR will do a necropsy at some point and share the results. When a vigorous and healthy adult suddenly succumbs to an unknown cause, as happened here, I always fear that a boat-strike or angling casualty might have occurred, but it is too early to know. All we know, thanks to the sleuthing of Georgia Eusebio, is that the male dragged himself onto shore, moribund, on August 23rd and died shortly afterwards at the Northwoods Wildlife Center. This male was a favorite of ours on the study, as he was  tame and relaxed during our behavioral observations, yet fiercely protective of his territory. He was also a long-distance disperser, having hatched in 1994 on Snipe Lake in Vilas County, which is about 5 miles WNW of Eagle River. His passing leaves his mate, who was banded on South Two in 1997, to care for the two strapping chicks, which are just learning to fly. We think they are old enough to survive losing their father. Gabby and Kristin will continue to follow them this fall, so we shall see.

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I have related several stories about the adventures and misadventures about loons in the study area, and I want to update followers on how those loons are faring. As one might expect, some loons that encountered difficulties were not able to recover, while other loons beat the odds and remain healthy and vigorous.

2 August Post

Two chicks strayed from their parents’ territories to other territories on the same big lake, mingling with other families that were already raising larger and older chicks. In one case, the stray chick was apparently lost, as we have not been able to relocate the Pickerel-South chick that joined the Pickerel-West family. But the Thunder chick, which was considerably smaller than the two Boom-Hodag chicks, joined them and has gained full acceptance by the adults and its larger foster sibs, according to Kristin’s recent observations. (Today she reported that the adults are feeding the interloper!) While we do not understand why it left its parents to join another family, the Thunder chick has continued to thrive despite its dubious decision.

25 July Post

The orphan that we placed with the new pair to complete a family of two adults and two chicks has been fully accepted and is growing and being fed by its foster parents. Its acceptance is so complete, in fact, that we are having to use a simple genetic test to determine which is the biological chick and which is the orphan!

9 July Post

The territorial male that became entangled in fishing line in late June has died. We had thought that he would bounce back after Wild Instincts fed him and treated his wounds and we placed him back on his territory. Alas, he was unable to regain the weight he lost, and he did not survive for more than a few days after release. Since we used a similar protocol to treat and release another male in 2012 that survived, we are a bit puzzled by this male’s rapid demise. Obviously, the details of a loon’s injuries and its precise condition when released dictate its chances of recovery. We are sad to lose this male, who was a vigorous defender of his territory and produced three big healthy chicks during his life: two in 2009 and one in 2010.

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It has been a tough year for loons and fishing. One of our territorial males was hooked in the leg about 12 days ago, and the wound became infected. He was so badly impaired that he had trouble diving and “beached” himself on an island in the lake, as severely injured loons do. About a week ago, we captured him, and Wild Instincts nursed him back to health, treating the wound and feeding him to help him recover lost weight. Although a new male took over his territory, the recovering male is now back on his lake. (The lake is huge, so he will easily be able to use areas not frequented by the territorial pair.) We are hoping that he bounces back just like the male on Lower Kaubashine, who became hooked badly on a lure but was freed by Wild Instincts last year. That male too lost his territory but was strong enough to migrate, return, reclaim his old territory, and produce a chick this year. So happy endings are possible!