Juvenile loons are in a race against time. While their parents seem to relax following the breeding season — wandering from lake to lake as if on a goodwill tour — juveniles, like the three-month-old in Linda Grenzer’s photo, face a ticking clock. After hatching in June or July, juvies must reach near-adult size by ten weeks of age, practice takeoffs and landings, and become strong enough to make flights of hundreds of miles on their southward migration in early November.

They are racing the ice. Temperatures cool in September, become unpleasantly chilly in October, and truly plummet in November — and lake temperatures follow suit. Ice-up can occur anytime between mid-November and mid-December in northern Wisconsin, and ice-up is the end of the line for juveniles. Opportunistic bald eagles await juveniles that are not prepared to migrate and become trapped in the ice. Apparently sensing the desperate task that will confront their offspring in the fall, parents stuff them with fish for eight long weeks in July and August. Chicks grow explosively during mid-summer. But they face their most challenging task in autumn, when parental support wanes and they must learn to feed themselves, improve their body condition, and prepare for their southward journey.

In general, scientists have paid little attention to the juvenile period in birds. Our neglect is natural enough. The breeding season is chock full of interesting behavioral and ecological events: pairing of mates, defense of breeding territories, selection of nest sites, and relentless territorial intrusions by nonbreeding adults seeking to settle. Perhaps ecologists can be forgiven for focusing their attention on breeding behavior and trusting that juveniles will take care of themselves.

But we wondered. If young adults settle on breeding lakes that closely resemble their natal lakes, might juveniles — which must fight for their lives just to become adults — also exhibit clear preferences for certain kinds of lakes over others? Constrained by flightlessness to forage only within the lake where they hatched, we might expect juveniles to become highly specialized to hunt and consume the species of prey found on the natal lake. So once they become capable of flight, we might expect them to visit and forage on other lakes very similar to their natal one. That is, juveniles reared on a diet of bluegill sunfish and used to hunting that species should spend most of the pre-migratory period visiting lakes full of bluegill that they can catch and consume efficiently. And juveniles accustomed to eating snails and leeches should find lakes full of those invertebrates on which they can feast.

Our interest in lake visitation patterns of juveniles during fall inspired us to plot the local movements of youngsters between lakes in the fall of 2012, 2013, and 2014. Kristin, Gabby, and Nathan used their band-spotting skills to locate juvies in September and October of these years. They found close to 200 cases where a juvenile we had marked had flown to forage on a lake other than its own. Using these data, Brian, who joined us this summer, asked, “Do juveniles forage on lakes at random, or do they prefer to forage on lakes like the one that hatched them?”. As the figure below shows, the mean difference in pH between a juvenile’s natal lake and the lake where we spotted it foraging (red vertical line) was far less than the distribution of differences we would have expected, if juvies had foraged randomly (grey bell-shaped curve).

Z_pH_Randomization

Although Brian has a few statistical checks to complete, the pattern seems clear. Juveniles exhibit strong preference for lakes that resemble their natal one in two respects: 1) pH and 2) water clarity (data not shown). Brian’s analysis is ongoing, and he is trying to learn how closely these chemical and physical attributes predict the food available to loons in a lake. But we are betting that the stark preference of three-month-old juveniles for lakes that remind them of home occurs for a simple reason. Juvies try to spend their time hunting prey in familiar conditions to build themselves up for their most dangerous first southward journey.

2016-07-31 01.32.29

Loon capture is a blur. We set out from our house at 8:45 pm, launch our small motorboat on the first lake, wait for nearly complete darkness, and catch any loon chicks and parents that we can net easily. By the time we have repeated the process four more times, we are rubbing our eyes, our weariness justified somehow by the presence of the sun lurking just below the horizon.

As an essentially negative person, what I often recall after a night of capture and banding are the physical demands of the process and my complete exhaustion. But there are dimensions of the work that are exciting and rewarding. Each loon is unique, and one never knows whether an individual will permit itself to be approached closely and netted or will be wary and elude us. So we experience many disappointments, but they are tempered by the occasional thrill of capturing an individual that, at first glance, appeared too skittish to catch.

The fruits of loon capture are obvious. By marking individuals and resighting them year after year, we learn about survival rates of adults and juveniles, territory fidelity, natal dispersal, and habitat preference. We glean a good deal of important information from these data. For example, survival rates of young and adults allow us to learn whether the  local population is increasing, decreasing, or remaining stable. And tracking of young loons from egg to first territory has revealed that loons develop strong preferences for breeding lakes that closely resemble their natal lake. Finally, capture is essential as a means to disentangle loons that have been run afoul of angler’s lines or lures.

This year’s capture exposed another distinctive pattern in loon ecology: the presence of ecological traps. An ecological trap is a breeding habitat that appears at first glance to be a good one but ends up being poor for reproduction. For example, a field might experience a burst of insect activity during early spring, enticing songbirds to settle there for breeding, but a crash in insect levels after eggs hatch might occur that suppresses the number of young birds produced. Two nights ago, we captured two chicks from two different lakes back to back. The first territory was a shallow 11-hectare portion of Wind Pudding Lake (my favorite lake name). The chick captured there was a five-week-old that weighed a scant 0.92 kg — less than half what we would expect from a chick of that age. Our daytime observations show that the chick’s parents are no slouches; they respond to its constant begging by making frequent dives and retrieving what food they can to feed it. Moreover, the chick itself dives often to forage. But this shallow lake, covered almost entirely by lily pads (which impair loon foraging), offers scant sustenance. I am afraid that the emaciated Wind Pudding chick will ultimately starve to death, as did the chick on nearby Liege Lake, another shallow lake choked by vegetation. Loon parents on small, acidic lakes struggle to rear even a single chick, whereas those on large lakes of neutral pH often raise two. This stark contrast was highlighted for us, as the lake we visited following Wind Pudding was 1373-hectare Lake Tomahawk. To be sure, loon parents on Tomahawk must steer their chicks through countless jet skis, water-skiers, anglers, and speed boats at all times of day. But vigilant parents are rewarded with abundant food for themselves and their chicks. The Tomahawk-Sunflower Bay chick held by Mina in the photo weighed 3.02 kg, yet it was only a few days older than the chick on Wind Pudding. Clearly the strapping youngster in the photo is heading for a healthy future and likely fledging.

Why on Earth would loons settle to breed on lakes that often provide too little food for their chicks? The answer might relate to the disconnect between nesting and foraging requirements. Alas, large lakes that contain many fish for loons often lack the islands, emergent marshy bays, and bogs that allow loons to avoid egg predators like raccoons. So loons looking to breed seem to be lured onto small, marshy lakes that yield successful hatches but doom their offspring to starvation.

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.

 

It is a struggle for chicks to get enough food to reach adult size. Jack Barr estimated in his 1995 paper that a breeding pair with 2 chicks would remove over 400 kg of food from a nesting lake during a summer. It is difficult to discern how that amount of biomass is related to the productivity of lakes and their ability to support loon breeding efforts, but Barr’s estimate makes the point that families  face challenges in finding enough fish and aquatic invertebrates to sustain their chicks.

The situation is especially dire on small lakes. As I have noted in the past, pairs using small lakes for chick-rearing tend to raise fewer and smaller chicks, which, in addition, are less likely to survive to adulthood. So it stands to reason that such pairs might avail themselves of any opportunity to make use of large lakes near their small breeding lake for foraging. Indeed, one or both pair members often leave their chicks for hours at a time to fly to and forage on larger lakes nearby. This behavior — while it seems like child abandonment! — probably makes sense in that it reduces the amount of food removed from the breeding lake, leaving more for the chicks, which do not have the capacity to fly elsewhere to feed.

If the situation gets very desperate indeed, then shifting of adult foraging to neighboring lakes cannot save the chicks. In such cases, the chicks themselves must be moved. In fact, an uncommon but regular occurrence is for loon pairs to move their chicks from small lakes with vanishing food supplies to adjacent, larger lakes that offer untapped reserves of food. Recently, both the Fox and McGrath pairs have shifted singleton chicks to large neighboring lakes (McNaughton and Little Tomahawk, respectively). The move offers the chicks a far more hopeful future.

But what of the move? Unable to fly, chicks must “skootch” on their bellies to cross land. Chicks on land are easy prey for mammals and eagles that happen upon them during land crossings, as they are weaker and smaller than adults. What is more, land crossings are not silent, stealthy events. Loon parents vocalize loudly when urging their young to make land crossings, which would seem to alert predators and increase the danger. Evidently, only loud, persistent vocalizations by adults are sufficient to coax chicks into venturing onto land, so lake shifts must be accompanied by blaring of bugles! Since adults often accompany chicks during land crossings, they are even more conspicuous to observers than chicks alone would be. The extreme danger confronted by loon families that abandon small nesting lakes emphasizes just how depleted food supplies must be in order to justify the move.

Two days ago, two of us — Mari and I — observed two instances of a curious and rare phenomenon in loons: adoption of an additional chick by pair with chicks already. In both cases, chicks on a very large lake shifted from their biological parents who resided in one territory on the lake to another breeding pair with two chicks on an adjacent territory within the same lake. Mari observed the Boom-Hodag Park pair adopt a chick from the Boom-Thunder territory; I saw the Pickerel-West pair accept one of two Pickerel-South chicks into their family. (All chicks had been banded, so we knew their origin.)

We are not the first to observe behavior of this kind. It has been reported occasionally on large lakes that chicks get “scrambled up” and end up with the wrong parents. But we had never observed it in our study area. To see it occur twice in the same day is quite extraordinary and makes us wonder why and how it occurs at all.

Certainly most chicks are best off if they remain with their biological parents, providing sufficient food is available for their parents to feed them. As I noted in a recent post, it is a desperate chick indeed who strikes out on its own in hopes of finding a new family. The odds are against finding a family willing to accept it — and one that has one or more chicks of the similar age as the wanderer, so that the wayward chick is not beaten up by its foster sibs. (In fact, it is a concern in both chick scrambles we saw yesterday that the host chicks are slightly larger than the foster chicks.)

So why does this behavior occur at all? We do not know at present, but I can offer one plausible hypothesis. This time of year is characterized by high rates of territorial intrusions. When intruders enter a territory with chicks, chicks hide out near shore and leave their parents to engage the intruders. Parents and chicks are reunited only after all intruders have left the territory, which might be an hour or more after intruders land. In most territories, which consist of single whole lakes, reuniting with parents is straightforward. Parents search where they left the chicks, and parents and chicks vocalize to find each other. But on large lakes, it is conceivable that chicks might lose track of their parents (and vice-versa) following intrusions. If chicks respond to the inviting calls of neighboring parents (which of course, are intended only for their own chicks), and neighboring parents accept non-biological chicks as their own, then chicks could wind up with the wrong parents. Scrambling of this kind would be most likely, of course, where two pairs with chicks use adjacent parts of a big lake.

It will take more than two instances of chick scrambling for us to be able to discern its precise causes. But the loss of a chick or chicks by a pair could well have a negative impact on their fitness (i.e. if the lost chicks perish). So this behavior pattern seems important and worthy of further thought and study.

Life is risky for loon chicks, especially on small lakes. Our work and that of others has shown that chicks on small, acidic lakes grow more slowly and that the beta (smaller) chick is often much smaller than the alpha chick. In such cases, the alpha chick commonly pecks its sibling, which gets less food from the parents, often falls behind the family group and frequently dies. Faced with certain death if they remain near their family, many beta chicks engage in a desperate act — they leave the family, strike out across land, and try to find another lake with a breeding pair that will accept and feed them. This is not entirely foolhardy, as breeding pairs with chicks will often accept additional chicks that join their families.

Most beta chicks that choose to search for another lake are doomed, of course. It is a longshot that a chick will: 1) be able to find a nearby lake and travel to it without meeting a terrestrial predator, 2) happen to find a breeding loon pair there, and 3) gain acceptance from the pair, in the event that they have a chick or chicks of their own. Still, if you face certain death, a “Hail Mary” such as venturing to a nearby lake might make sense.

In any event, beta chicks can sometimes be found along roads or alone  in lakes without other loons, and they frequently perish there. But recently an orphaned beta chick found its way to a roadside and was picked up and carried safely to Wild Instincts where it was nursed back to health. We then estimated the size and age of the chick, matched it with the chick of one of our breeding pairs, and released it with that family. Now, two weeks later, the foster chick seems to have been fully accepted by the family and is being fed by its foster parents and accepted by its step-sib. So this seems a rare story of a desperate beta chick getting a chance with a new family that might help it reach adulthood.