migration

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Ensconced as I am in the endless summer of southern California, it is easy for me to forget what loons are facing. As we know from Kevin Kenow’s excellent work, about half of all adults have now left their breeding lakes in Minnesota and Wisconsin and are on their way southwards. Many of these birds are hanging out in the Great Lakes before making the long overland journey to Florida. Some adults remain faithfully with their chicks, hoping to stuff a few extra fish into them before abandoning them to their own devices.

Adults’ departure leaves only chicks on the breeding grounds. Thanks to the the work of our fall observation teams and Brian Hoover, who pulled the data together and wrote it up, we know that most juveniles leave their natal lakes in the fall but hang out nearby. They search diligently for large, food-rich lakes, especially favoring those that resemble their natal one in pH. Their strategy is clear. First, stuff your face with fish where they are abundant and similar to the ones you first learned to hunt. Next, wait until the last possible minute to build up your energy stores. Finally, bolt for Florida before the ice makes it impossible to take off.

The juveniles’ plan has a touching pragmatism to it. There is no subtlety. Birds of the year are not burdened with territorial responsibilities or pangs of parental guilt. They just wish to survive long enough to reach the wintering grounds. And, generally speaking, they do.

But a few get left behind. Thus it happened with the Lake Thompson juvenile this fall. A great strapping youngster when we caught him in late July at five weeks of age, he continued to grow and thrive in the 12 weeks since we last saw him. Ultimately, he had no more need of his parents and moved five miles west to Boom Lake in Rhinelander to fatten up for migration. There, however, he ran afoul of a reckless hunter. Linda and Kevin Grenzer caught him last night and quickly saw that his left wing was fractured. X-rays at REGI confirmed the break — caused by goose shot visible in the x-ray.

I cannot think what else to say.

Top photo by Linda Grenzer. X-ray by REGI staff.

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It seems ages ago when my team was out on the lakes, finding pairs with chicks, capturing and banding them — then hastily storing our canoes and the Loonmobile for the winter. So much has occurred since early August that those frantic days linger in my brain only as a hazy memory.

But the loons stayed on. While I was vacationing in the East, attending faculty retreats, and lecturing with a mask and fogged glasses so that we could keep classes in-person, loon parents continued to stuff their young with food, chicks matured and learned to fly, and most adults abandoned their territories for the Great Lakes. Even now though, as I brace myself for that peculiar mixture of joy, renewal, and mortification that only Thanksgiving with family can bring, loons remain on many small lakes in the Upper Midwest. Most are juveniles, like the one in Linda Grenzer’s photo, which often postpone migration until the very last second. This strategy probably permits them to maximize their energy reserves for their first long overland journey to Florida.

So hold a good thought for the stragglers. The Brandy Lake chick, which a lake resident told me about in an anxious e-mail today, is among them. Perhaps by the time we are all sitting down for our Thanksgiving meal, that strapping juvenile will have begun winging his way anxiously in a south-south-easterly direction. Though he cannot have a clear picture of his destination, he will cease his migration off of Cedar Key, Panama City, Tampa, Fort Myers, or Apalachicola. For a time he will be that nondescript brownish diving “duck” that some young family from Nebraska watches distractedly while building their sand castle. Let us hope that we see him back up north in a few years — and in more striking attire.

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Science, of course, is cumulative. Solving scientific riddles generally requires multiple studies by diverse authors using a broad range of scientific techniques. In fact, scientific conclusions are more compelling when they rest upon findings from many scientists using different techniques and with different backgrounds.

So I was quite anxious to learn what Kevin Kenow had found. Kevin, a scientist with the U.S. Geological Survey and also a member of the Scientific Loon Council at the National Loon Center, has studied common loons for about as long as I have. But while I have focused on the behavioral ecology of loons exclusively and established two fixed study populations for this purpose, Kevin has collected data on many species of migratory water birds, tackled questions related to human impacts and conservation, and worked across a variety of states and waterbodies in the Upper Midwest and beyond.

Kevin’s just-published article pulled together data from loons in Michigan, Wisconsin, and Minnesota. In contrast to my low-tech methods, Kevin’s team used satellite transmitters and geolocator tags to either: 1) track loons’ migratory movements in real time or 2) reconstruct their movements, following recovery of the geotags.

Kevin’s team confirmed several migratory pathways that we had known about or suspected through recoveries of dead birds. First, adult loons from the three Upper Midwest states typically “stage” on Lake Michigan in the late summer and early fall, before departing southwards to the Florida Gulf Coast. Second, juvenile loons (those only a few months of age) do not visit the Great Lakes prior to migrating south. Third, first- and second-year loons that are too young to return to the breeding grounds instead migrate northwards in the Atlantic, summering off of the Canadian Maritime provinces. The quirky patterns in loon migration and wintering behavior are important. They make it clear that a loon’s survival to the breeding stage requires that it survive and remain healthy across a period of many months, a variety of water bodies, and a number of geographic areas.

But one of the patterns that Kevin’s team identified loomed above the rest. They found a high rate of mortality among first- and second-year loons, especially in the spring. If you are following this blog and have an excellent memory, you recall that this finding appears to dove-tail with a recent one of ours. We found that “floaters” — the segment of adult loons that are two to five years old and are looking for breeding territories — have been disappearing at a high rate. Floaters alone, you might recall, account for most of the decline we have detected in the Wisconsin breeding population.

Do Kevin’s findings of high first- and second-year loon mortality solve the riddle of what is ailing the loon floater population in Wisconsin? Unfortunately not. In fact, we had long suspected that young adults would die at a higher rate than older adults, because they are less experienced. High mortality among young adults is a common feature of avian populations everywhere. But these new findings might help narrow down the “period of vulnerability” in the life history of loons. And our findings of high young adult mortality in the last decade combined with those of Kevin’s team might tell us where to look. Perhaps conditions along the Florida Gulf Coast and/or the Atlantic have deteriorated recently, causing higher than normal mortality during this trying time of life for young loons. This is only one hypothesis, and it will require rigorous testing. Thanks to Kevin’s team’s cool recent findings, though, the wheels of science are turning.

By the way, follow us on Instagram, if you like, at @loonproject. The whole LP team works together to post cool, informative “loonstagrams”!

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No, I am not talking about the Buccaneers. Kansas City and Tampa are both far from anywhere I have spent meaningful time. Besides, KC won last year, and Tom Brady has won countless times. Enough already!

I am talking about Tampa Bay the place — the large, protected inlet halfway down the west coast of the Florida peninsula. Loon enthusiasts should love Tampa Bay because it serves as the wintering grounds for a good many loons from the Upper Midwest. In fact, so far 33 of 53 recoveries of loons banded by the Loon Project and others in the Upper Midwest — sadly, these are mostly loons found dead on the beach or elsewhere and reported to the Bird Banding Lab in Maryland — have come from Florida’s Gulf Coast, between Pensacola and the Keys. By chance, or more likely just because it is an area of dense population, a good percentage of these birds come from the Tampa Bay area.

This is not a brand new finding. Kevin Kenow of USGS has used satellite transmitters to track loons from Minnesota, Wisconsin, and Michigan and reached the conclusion that Florida’s Gulf Coast is a vital wintering area for our birds. He puts the percentage of Upper Midwest loons that winter there at above 70%. So we have two separate lines of evidence that tell the same story about the wintering location of Upper Midwest loons. Well, okay, you must be thinking, our loons have to winter somewhere. Why does it matter where exactly? It matters because, while we only see them in the summer and tend to focus only on their trials and tribulations during summer, our loons must also survive on their southward migration, sustain themselves on the wintering grounds, and then make it through another northward migratory journey in order to get back to us each year. Of the period during which it is out of our sight, a typical loon from our area spends four to five months on Florida’s Gulf Coast.

When you learn where breeding animals are spending their winters, conditions on the wintering ground suddenly get very real. During the past two decades, ecologists have improved in the ability to track individual animals (usually birds) from breeding to wintering locations, using such tools as satellite transmitters and geolocators. They have also learned that migratory trips do not somehow “reset” an individual so that all individuals that have migrated to the breeding ground start on equal footing. Instead, a difficult (or easy) migration or wintering period leaves a lasting imprint on an animal, placing it at a disadvantage (or giving it an advantage) during the next phase of its life history. Such impacts are called “carryover effects” and have become hot topics for investigation. Surges or declines in breeding populations, we now see, can be as easily explained by events on the wintering grounds as those during the breeding season.

We are at an early stage in our analysis of breeding season impacts on wintering loons and vice-versa. In fact, we know nothing about carryover effects in loons. But now that we are zeroing in on wintering locations of loons from different breeding populations, the stage is set to look for such patterns. At the moment, I have set my sights on a less lofty goal. You might recall my post in June 2020, when I pointed out how many adult loons had failed to return to Wisconsin in spring of 2020 after leaving in fall 2019. The simplest explanation for this very low return rate of Wisconsin breeding adults is that some event occurred during the winter of 2019-2020 along the Florida Gulf Coast that killed many loons there. More broadly, I have begun to explore data that Florida wildlife officials collect annually on red tides and other environmental events that threaten ocean-dwelling animals. Could fluctuations in annual survival rates of loons in Wisconsin be explained by mortality events recorded along the Florida Gulf Coast?

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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.

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Last year I wrote a blog post in which I concluded that late-hatching chicks returned at a rate no different from early-hatching chicks. I found the result surprising, as one would expect early hatchlings to have a head start in learning to feed themselves, honing their flight skills, and preparing for their first migratory journey. The photo and story I got from Linda Grenzer a few days ago has forced me to wonder if I need to collect more data on this question.

The breeding pair on Squaw Lake had an eventful year in 2018. Delayed, like all other pairs, by the late thaw, they initially nested along the shoreline near the boat landing. After a predator snatched both eggs off of the nest, they nested again not far away. This time they were more fortunate; the eggs hatched, but not until about July 22. When we captured the family on August 3rd, we found the chicks almost comically small — two little puffballs that did not approach the size of the many other juveniles we had encountered. Chicks are cute in their first few weeks, and we enjoyed observing them and handling them cautiously while giving the female a new set of bands.

Our delight at seeing the adorable chicks was tempered by the fear that chicks hatched so late would not mature in time to complete the southward migration. The fear is justified; parents must balance the energetic demands of their demanding offspring against their own need to maintain good body condition and prep for their autumn journey. Inevitably, adult loons spend progressively less time on their home lake in September as they forage intensively, molt into drab winter plumage, build up fat levels, and, in late October or early November, head south. This goes for parents and non-parents alike.

So it was not surprising to get a report from Linda that the Squaw adults had left their breeding lake, leaving their late-hatched chicks to fend for themselves. What was alarming was that one chick had chased someone’s jig, managed to hook itself above the base of the bill, and was no longer diving or foraging normally. Further evidence of its desperate condition was that it was not difficult to capture and weighed a mere 1750 grams — roughly 1 kg less than it should have at 9 weeks. Following an X-ray at Raptor Education Group, Inc. in Antigo, the chick was found to have swallowed a second hook from a separate encounter with an angler.

Since we have long since ceased our routine visits to study lakes, we can only speculate about the series of events that put the chick in this bind. Marge Gibson of REGI suspects that, without parents to help it satisfy its foraging needs, the chick was struggling to feed itself. In its desperation, the chick began to attack fishing lures until the hook in its cheek and weakness conspired to incapacitate it.

If Marge is right, and late-hatched chicks are sometimes left with too little feeding capacity to maintain themselves, then this pattern should show up in our data. Specifically, we should see fewer very-late-hatched chicks return as adults to the study area. This plausible scenario will fuel another round of data analysis…when I find time!

To end on a positive note, the angling victim is bouncing back at REGI and feeding voraciously. If you do not believe me, look at this video from the REGI website.

https://www.facebook.com/RaptorEducationGroupInc/videos/470615703434171/

If it continues to thrive, the REGI folks will face another challenge: what to do with a healthy juvenile, but one whose stay in captivity and recovery made flight practice impossible.

 

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Spring is on its way. As I twiddled with my phone just now to check conditions in the study area, the temperature was nine degrees warmer in northern Wisconsin than in southern California — the result of an unseasonably warm day in the study area and an unseasonably chilly one here in SoCal. At long last, the ice is melting, and loons will soon be back on their lakes. Meanwhile, as Linda’s photo shows, they gather in droves on open water. Being Linda, she also checked out their bands, yesterday and today, to see how many she could identify. We are all the beneficiaries of her tireless efforts, because her IDs provide a window onto the lives of returning breeders that are, for the moment, baffled in their efforts to reclaim their territories.

Who are these birds, and where are their territories? Are they males or females? Are they, in fact, breeders or young floaters, who lack territories and will spend the year challenging territorial residents for ownership? Are they migrants hundreds of miles from their breeding area or neighbors stopping by until their nearby lake becomes ice-free? We cannot answer all of these questions, but we can answer most of them based on Linda’s meticulous observations.

First of all, the loons in these aggregations are almost all males. Of the 12 birds positively ID’d by Linda, 10 are known males, one is a female, and one is of unknown sex. Linda suspected this herself: she reported numerous territorial yodels, sometimes by multiple males at once, as the birds rafted about in ice-free portions of the lake. Females must be only a few days behind; in fact, as I was writing this, Linda reported seeing a breeding pair near last year’s territory on Nokomis.

Second, loons present on the breeding grounds at the cusp of the seasons are mostly breeders that have come to reoccupy territories, rather than young floaters bent on evicting them. Only one ID’d loon from Linda’s list is a possible floater; all others are known breeders.

Third, loons that aggregate on ice-free water in early spring are a geographical smattering — many from neighboring lakes but a good many from farther north, whose lakes might not be open for a week or more. Linda’s sightings show this clearly, as her list includes breeding males from:

1) the adjacent Nokomis-East Central territory,

2) Indian Lake, which is 6 miles to the NE,

3) Soo Lake, 11 miles to the NE,

4) Silver Lake (Lincoln County), 8 miles S,

5) South Blue Lake, 16 miles to the N,

6) Miller Lake, 18 miles N,

7) Blue Lake-West Territory, 20 miles N,

8) Forest Lake (Vilas County), 40 miles NE,

9) Rock Lake (Vilas County), 50 miles N, and

10) Crab Lake (Vilas County), 50 miles N.

and a lone female from Burrows Lake, 10 miles NW.

What can we learn from these sightings? The preponderance of males confirms that males precede females on migration by at least a few days, and shows that males are the main ones bottlenecking on rivers and open lakes near their ice-locked breeding territories. Returning males push hard to return very close to the date that their lake becomes ice-free; females dawdle by a few days. Why it is so crucial for males to return as close to ice-out as possible is unclear, especially since the paucity of young, non-territorial birds present suggests that breeders are not likely to be challenged for territorial ownership in the first few days after settlement. Perhaps males, because they select the nesting site, return as early as possible to take note of any changes in the lake or shoreline over the winter that might require them to move the nest from the prior year’s site.

We can also discern that these early spring aggregations do not comprise neighbors reacquainting themselves after a winter apart, but individuals from far-flung lakes that do not know each other at all and are likely not to encounter each other again. While we should not be surprised by some tense moments between such unfamiliar individuals — it is, after all, spring and hormone levels are high — we should expect mostly peaceful loafing and feeding. These males simply have nothing to gain from battling unfamiliar loons.

In short, the rafts of loons in Linda’s photo are playing a waiting game. Since territory settlement is a time during which owners must defend their lakes vigorously — even if they do get a brief hiatus after settlement, owing the late return of young challengers — we would expect these males to do next to nothing until their lakes open up. They should feed, rest, and expend as little energy as possible before the onerous task of breeding begins.

 

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As many of you know, I am a worry-wart. Normally I get so stressed-out about my kids, my teaching, my research, my health — and many other matters that are going well — that I hardly have time to obsess about loons in the study area. But Linda Grenzer’s bleak photo of conditions on her lake today gave me a jolt. Could the late ice-out that we are facing in 2018 delay the season so much that it damages the breeding prospects of our loon population?

One might think that the later the ice comes off of the lakes, the later the loons nest, and the less time parents have to fatten up their chicks and prepare them for their first southward migration. Thus, a late ice-out might well lead to reduced breeding success for the population. Although there are many “if”s in this string of logic (and a preliminary analysis did not bear out the pattern), I felt concern  gnaw at me.

So I did what scientists often do to stave off despair: I looked at the data. First, I looked to see if loons nest later when the ice goes out later, which almost has to

Screen Shot 2018-04-15 at 8.46.07 PM

be true. It is true, but there is a lot of noise in the data. That is, loons are constrained to nest somewhat later in years when the ice goes out later, but the picture is not simple. The reason for the noise becomes clear when you look at the lag time between when loons settle on their territories and when they hatch their

Screen Shot 2018-04-15 at 8.43.02 PM

young. There is a very strong pattern here. When the ice goes out early (left side of graph), loons dawdle and wait weeks before nesting. But when the ice goes out late, as it will this year, pairs get down to business quickly, nesting within a week or so of territory return. So loon pairs are somehow able to catch up in years of late ice-out so that their breeding schedule does not differ greatly from other years. (Notice also that the orange line in the top graph is flatter than the blue line.)

What accounts for this pleasing pattern? We can make a pretty good guess based on findings in other migrant birds. Spring migration is an energetically costly process. In an early year, the ice is gone so quickly that loons settle on their lakes as soon as they return from the wintering grounds. In such cases, their fat levels are very low from migration when they first occupy their territories, and it takes a good deal of foraging before they return to good condition. In a late year, loons cannot settle on their territories right away but must wait on nearby rivers that have open water. There, they are able to forage and restore their bodies to good condition. As a result, loons hit their territories in prime body condition and fully recovered from the migratory flight in years of late ice-out. Thus, they can get down to breeding quickly.

Although I was heartened by the data I saw above, I had a look at the numbers that most directly addressed my concern about late ice-out and population breeding success. There is a no statistical tendency for the population to produce more loon chicks in years of early ice-out, despite the many years of data we have to look for such a pattern. Indeed, some of our best years for loon breeding (2013, for example) have occurred when the ice goes out late. So those many of you shivering in northern Wisconsin and other frigid regions can relax about one thing; the loons are no worse off in years when spring comes late than when it arrives early.

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I must confess that I had mixed feelings to learn this week from Nelson that the North Nokomis pair had hatched two chicks. Despite having seen scores of newly hatched chicks, I still enjoy watching the tiny fuzzballs bob up and down next to their huge parents while the adults, in turn, move gingerly around their tiny young to avoid injuring or drowning them.

So it was, in one respect, nice to learn that the North Nokomis pair had hatched the eggs from the conspicuous nest I had inspected on 25 July, after following the simple, clear instructions from my research team. But I recall thinking, “Oh geez!” on that date, because only two other breeding pairs of the 120 that we follow were still sitting on eggs. (Both of those, we had determined, were cases of infertile eggs that the pair had incubated for six weeks or more.) I have always presumed that chicks hatching in late July or August have too little time to mature physically, develop foraging skills, and learn to fly in time to make the fall migration.

Now we have the data to look at this question robustly. In other words, having captured and marked 983 chicks since 1991, we can determine whether hatching date is a predictor of survival to adulthood. Logically, there must come a date in late summer beyond which chicks run out of time. However, pairs might face a gradual decline in the likelihood of their chicks surviving migration, or there might be a rather sudden threshold date past which chicks that hatch cannot survive.

If we look at all chicks banded from 1991 on, and calculate how many have made it back to the study area as adults, we learn what the figure

Screen Shot 2017-08-12 at 11.24.21 PM

shows. In short, hatching date does not appear to influence survival to adulthood strongly. At the very least, we can say that chicks hatching in mid-July survive at a rate no lower than those that hatch a month earlier. There is a hint of a decrease in survival from early to late hatches, but it is only a hint.

As usual, our data are not perfect. In fact, we have too few cases of very late hatches to gauge the likelihood of the two North Nokomis fuzzballs (which hatched on about 28 July) making it off the lake this November. On the plus side, 470-acre North Nokomis Lake has one of the highest rates of survival to adulthood in the study area. (The territorial males on Gilmore and Cunard were hatched there.) I would like to think that the gutsy North Nokomis pair will be rewarded this fall with two healthy fledglings. So I am keeping fingers crossed for them.