parental care

on

The hatch is underway. 2023 was a miserable year for black flies. But loon pairs that laid eggs in mid-May and kept incubating them despite fly harassment are getting their reward this week. Granted, this reward comes in the form of one or two tiny puffballs that need continual warming, must be protected from a host of predators from above and below, require gentle handling, and can only consume tiny food items offered patiently and gingerly. But such is the reward.

We are especially excited about the two chicks hatched in the past few days on Little Bearskin Lake in the Wisconsin Study Area. Why? Two reasons. First, with an estimated age of 34 years, the mother of these chicks is our oldest study animal. Second, our team happened to capture this female two years ago and discover that she was injured and ailing after getting entangled in fishing line. Our rescue of this ancient female allowed her to rear a chick with her mate in that year and another last year. If she is able to raise the two chicks just hatched with the same male, she will — in our view — have produced four “bonus chicks” above what would have been possible without her disentanglement. The ability to witness several of our study animals resume breeding after cheating death with our help is one of the joys of our work.

“Two chicks on Little Bearskin?”, you say. “I just see one in the photo.” Indeed, Linda Grenzer captured this picture of the single chick that just hatched on Crystal Lake near Tomahawk. I like the way the chick seems baffled at the distance between itself and its nearby parent. (“Why have you left me here?”)

Do not despair if your loons are still on eggs or not nesting. Little Bearskin, Crystal, and a handful of other pairs are among the intrepid few in Wisconsin that survived the barrage of flies undaunted and will hatch this week.

While most of our Wisconsin loon pairs were forced to abandon their first nesting attempts, Minnesota loons in Crow Wing and Cass Counties tended to respond to fly harassment by postponing their first breeding efforts. Hence, the bulk of our Minnesota pairs began incubating during the last few days of May or first week of June. However, a small number of stalwart pairs in Minnesota laid in mid- to late-May and hung in there during the fly weeks. Those pairs — which include Kimball-East, Kimball-West, Little Star, Big Trout-West, Ossie-Boozer’s, and Ossie-Timberlane — should hatch in the next week to ten days, if all goes well.

We are still hopeful for a solid breeding year for both study areas. Fortunately, loons follow the same philosophy that humans do when it comes to setbacks. When you get punched in the mouth, you don’t stay down. You get back up and see to your business.

on

A week or so ago I gave a talk to the Northeast Loon Study Working Group. Inauspiciously-named and -initialled, NELSWG comprises loon conservationists from New England, the Upper Midwest, and a smattering of other regions within the loon’s breeding range. At present, NELSWG is the only group that attempts to pull together data on loon populations and brainstorm strategies for protecting the species. During my talk I shared our data showing that masses of adult loons and chicks decline as water clarity declines. I then updated the group on my analysis of male and female traits that lead to breeding success of pairs.

Impact of male (blue) or female (red) pair member on a pair’s hatching success. Both males and females are a drag on hatching success in their initial year on a territory. Females have a slight positive impact thereafter. Male experience on a territory continues to improve hatching success even after 10 years.

To remind you, a male’s knowledge of the territory makes a huge impact on the breeding success of a pair. Since males choose the nest location, males are a drag on nesting success in their first few years on a territory because they place the nest in lots of dangerous places. (Note the low blue bars for years 0 to 3 above.) On the other hand, males that have been on a territory for seven or more years are a boon to pair nesting success, because they have learned the safest spots to place nests. (Note the blue bars from 8 to 20 years on territory.) Females have an impact too. In their first year on a territory, females cause low hatching success for their pair. In later years, female territory experience boosts hatching success slightly.

It is almost more interesting to see the factors that do not affect breeding success. A male’s age does not affect his pair’s ability to fledge chicks at all. At first glance, this seems confusing. How can the male’s age have no positive impact on breeding success of a pair, when a male’s breeding experience on a territory is hugely important? The answer relates to cause. It is true that old males tend to have very high breeding success, but this is not because of their age but because, in most cases, they have been on a territory for many years. We know that age itself is not causing high breeding success because old males that nest on new territories have no greater breeding success than young males on new territories. It is familiarity with the territory and not age that is the salient factor.

Female age has only a weak negative impact on breeding success. In other words, older females lose chicks at a slightly higher rate than young females. This pattern is a bit difficult to make sense of, because the effect is so steady and gradual. Why would a 15-year-old female lose chicks at a higher rate than a 10-year-old female parent? Both females are in the prime of life, in the loon sense.

Effect of female age on a pair’s fledging success. Females cause a gradual decline in fledging success as they grow older.

To the listeners at NELSWG, though, the pattern that was most remarkable was the lack of a strong effect of mate familiarity. While pairs that know each other nest a few days earlier than pairs that are in their first year together, the pattern is weak (see below). Furthermore, the slightly earlier hatch date among pairs that know each other does not translate into a detectable advantage in overall breeding success. In short, pairs benefit only slightly from knowing their mate well.

Effect of pair-bond duration on hatching date. Pairs in their first year together nest later, on average, than pairs that have been together for at least one year.

How can this be? How can a male and female remain together year after year, raise young cooperatively — and still not benefit from this lengthy association? That was the question asked by Lee Attix at the NELSWG meeting. I don’t have a good answer for Lee. As a male in a 38-year relationship who has raised young cooperatively, I am well aware of the benefits that a long-term partnership can bring in the human species. But loons are different.

on

It is one thing to lose your own young very early and then — at a weak moment — adopt young of another species that resemble your lost young. Adoption after hatching is an inherently risky move, and one likely to saddle you with the responsibility of rearing young very unlike yourself.

But how can things go sideways when you spend four long weeks on a nest and observe carefully as young emerge from the eggs you warmed with your own body? Surely any creature that fights its way out of an egg that you have incubated lovingly beneath you for so long must be worthy of your protection.

So it must have seemed to a breeding loon pair in northwestern Montana two days ago when three fluffy youngsters hatched and dutifully followed their parents onto the water. Yet the fluffy youngsters that popped out of the eggs the loon parents had spent countless hours incubating were not loon chicks, but Canada Goose goslings. So those cold nights, scorching afternoons, and relentless flying pests that the parents endured for four long weeks have produced a surprising outcome.

The loon parents do not seem disappointed. They have spent the past two days attending untiringly to their adorable, if unexpected, brood. And the goslings themselves betray no hint of alarm or discomfort — even when their parents vanish suddenly beneath the lake’s surface.

But Bob LeBlanc, the photographer who discovered this mismatched family, was left scratching his head. After hearing him recount the story of how a loon pair ended up with three goslings, I could shed no new light on the situation.

Certain facts seem clear. First, a goose pair nested on one of Bob’s three carefully constructed (for loons) nesting platforms but then inexplicably abandoned a clutch of eggs soon after incubation began. Finding an attractive nesting location, a later-arriving pair of loons apparently skipped the step where they deposit their own eggs there and instead simply decided to warm the eggs already on the platform.

I’ll be honest. As someone who has devoted his last three decades to learning about common loons and promoting their conservation, I have decidedly mixed feelings about loons raising goslings. The Canada Goose population in the Upper Midwest does not need loons to raise more of them. In fact, according to a recent study, there are about five times as many geese in North America now than in 1970. Goose numbers continue to rise in the Upper Midwest, where I do my research, leading me to worry that they are seizing good nest spots and keeping loons from using them. But for the moment, and in this one loon family, I have to admit that the fuzzy misfits are awfully cute.

on

Three days ago, Allison and I had only one car, so we covered a double circuit of lakes. We loaded two solo canoes precariously on top of our ’07 Toyota Corolla — “That seems safe”, I said, tugging on one of the straps we had used to lash the boats to the roof rack and smiling reassuringly at my dubious daughter — and headed to a northern tier of lakes. I dropped her and her canoe at Brandy, and scurried across Highway 51 to Arrowhead. An hour and half later I covered Kawaguesaga-North, while she observed at Bullhead, and so forth throughout the day. It was tiring, and Allison inevitably had to wait ten minutes or so for me to drive back from my lake to hers, but we visited four sets of lakes this way. Covering many lakes with limited personnel is central to the ethos of the Loon Project, and I was delighted to walk the walk on Sunday.

While our highly fuel-efficient observations on our last day in the field were very cool, the portrait of reproductive success that emerged from the lakes I visited was decidedly ambiguous. The Hodstradt pair has two thriving, five-week-old chicks. During my visit, the ten-year-old female (hatched on Butternut Lake in Forest County) was struggling to provide enough food for her large family. The alpha chick begged her mercilessly and received 14 feedings. In contrast, the beta chick, which only got two food items, was on the receiving end of three harsh pecks from his larger sibling. Still, Hodstradt has a history of producing two-chick broods, so they appear to stand a good chance of fledging both young.

In contrast to the thriving family at Hodstradt, the Arrowhead breeding pair has been impacted heavily by a wing injury to the male. Even as I began to pull the canoe off of the Loonmobile, I saw a large loon preening awkwardly forty meters off the Arrowhead boat landing. “Uh-oh”, I thought. The telltale drooping of his right wing revealed the male’s identity long before I observed his plastic leg bands. He was alert and responsive to his environment, but he looked worse than ten days before, when we had captured him at night and inspected his injured right wing. I sighed and shook my head; we had hoped he would recover and rejoin his mate and chicks. As I took note of his struggle to preen without stretching his damaged wing, his sodden plumage (which occurs when loons fail to cover themselves with protective oil from a special gland near their tail), and his willingness to permit a fisherman to drift to within ten meters, a grim realization hit me. This male is going downhill rapidly and is not going to recover. (Marge Gibson, a veterinarian with REGI, has inspected a series of photos taken by Linda, and is confident that the right wing is broken — probably at the humerus — an injury she has seen often after severe blunt-force trauma such as a strike by a motorboat or jetski.) Despite the male’s injury, I wondered why he was confining himself to the small, protected cove off of the boat ramp, instead of remaining in the main body of the lake.

I quickly learned why the wounded male was hiding. A pair of loons rested confidently on the southwestern end of the main bay. Unlike the injured bird, these two sat up high on the lake surface. A short time later, they foraged in plain view in the middle of the lake. In other words, they acted like they owned the place. Clearly the male had taken refuge in a protected cove in order to hide from these two new adults that, in the absence of territory defense, had laid claim to the lake. Indeed, the new pair swam east to the mouth of the male’s cove as I observed them, as if hunting an intruder they wished to drive from the lake. I was relieved that neither pair member gave any sign that they detected the injured male in the cove. Somehow — either by diving often, hiding under a dock, or perhaps pulling himself up onto the shore — he eluded them and spared himself their attacks.

The wounded male was not the only loon systematically avoiding the new breeding pair at Arrowhead. As I patrolled the shoreline of the lake, I found his mate foraging madly for one of their two chicks in the northeastern section. Though her territory has slipped away because of her mate’s and her own inability to defend it, the female has been unwilling to desert her seven-week-old chicks. In order to avoid the watchful eyes of the new pair, she and the banded chick I found her with always remained within ten meters of shore and foraged among a stretch of long docks that jut out from the northeastern shoreline. As my video above shows, the chick begged his mother relentlessly for food, while she captured what few small fishes and insect larvae she could find along this sandy stretch. This brief set of observations provided a window onto the female’s plight. In order to fledge her two chicks, she must provision them surreptitiously for at least another month, wait for them to learn to fly, and then hope that they can move to nearby undefended lakes (which chicks naturally do at this age), where they can complete the growth process. The series of practice runs, aborted takeoffs, and awkward landings necessary for a chick to become adept at flight are sure to draw the attention of and aggression from the new breeding pair. If, by some quirk or miracle, the female manages to keep the chicks safe and healthy until they can fly, she will be the first adult we have ever seen to lose her mate when the chicks were younger than five weeks, have a new breeding pair take possession of the territory, yet still manage to fledge them. As much as I respect her determination, I do not like her odds.

After my report of continued decline of the former breeding male from three days ago, Linda and Kevin Grenzer visited Arrowhead yesterday. They found the same cast of characters that I had seen two days before — the skulking, incapacitated male, the confident new pair, the plucky old female, and the banded chick that she had been feeding — but, incredibly, Linda also turned up the unbanded chick that we had not seen on two previous visits and had given up for dead. In fact, Linda got a series of photos of this chick as it followed its wounded father onto shore (see featured photo at top). It is touching to observe the chick’s dilemma — sitting awkwardly and reluctantly on land, yet refusing to abandon its fading father. I guess if we are looking for a positive from the recent events at Arrowhead, it is that the family is doggedly sticking together in the wake of a gut-wrenching calamity.

on

Raising of a child by one parent alone is common enough in humans that we have words to describe the phenomenon. Since humans are highly social, rearing of a child by a single parent — and a support network of friends and relatives — can be effective. Not so in loons. The loon breeding system could be called “obligate biparental care”, because both male and female are usually required to fledge even a single chick. When one parent is lost during chick-rearing, the typical result is rapid death of the chick or chicks, either because the loon that replaces the dead parent actively attacks them or because they receive much less food and protection from a single parent and perish from other causes. In fact, sustained single-parent loon families only occur when one parent dies or is evicted and the remaining parent somehow manages to sequester the chick from the new adult that fills the breeding vacancy. The occurrence is rare enough that I remember all of the cases in our study area. 

When the Washburn female was injured in 2000, she deserted her family and turned her attention to her own survival. Though it seemed heartless at the time, this decision made sense. Even though helping to rear the chick she left in the male’s care would have increased her reproductive fitness, she was right not to risk her life for the chick. An adult that sacrifices itself for a chick is throwing away many future years of breeding success. After the female left the family, the male spent his time at the northern end of the lake, feeding the chick vigorously when he could. Somehow he managed to fledge it. 

In 2006, the Garth male vanished just after the chick hatched, leaving the female alone to raise it. A single loon mother is in a bind; she lacks the size to intimidate and drive off other loons and ability to give the territorial yodel that could prevent many intruders from landing on her lake in the first place. Instead, single females must tolerate visits by many intruders and hope to keep them away from the chick. A new male soon took over Garth and paired with the female. The female and her new mate seemed to reach some sort of uneasy agreement; she spent time with him, while he neither fed nor attacked his step-chick. The chick, which we banded late in the year and affectionately call “Stripe Hell” because his bands are blue-stripe over taupe-stripe, red-stripe over silver, ultimately survived to adulthood. In fact, as an adult, this male claimed the breeding territory on Lee Lake in 2012 and produced chicks there in 2016 and 2017. Following his eviction from Lee in 2018, this product of a one-parent family settled on South Blue in 2019 and then resettled on Miller Lake this year, where he is now raising a chick with his mate. Apparently having been raised in a single-parent home does not prevent a loon from leading a successful adult life. 

Things do not always go smoothly for step-chicks. When the male succumbed to some unknown ailment on Flannery in 2015, he left his mate alone to fend for their 2-week-old chicks. In this case, the female led the chicks down to Velvet Lake, which attaches to Flannery at its southern end. When a neighboring male took over, he found the chicks and pecked them viciously, killing one and forcing the survivor to hide underneath docks in Velvet to escape his marauding stepfather. This chick never received as much food as a chick normally would; we are not certain if it survived the ordeal.  

Among the several single-parent families we have recorded in 28 years, Squash-Northwest is perhaps the most memorable. The Squash-Northwest male in 2012 hatched a chick with his mate but was injured and died when the chick was two weeks old. Left alone with a small chick, his mate not only protected and raised it to adulthood, she also found a new mate. She was able somehow to balance the demands of her ravenous youngster and the male that she paired with — while keeping them physically separated so that the replacement male did not harm his step-chick. On one visit, we would find her staying close to the chick, feeding it and fending off eagles at the northwestern end; on the next visit, the female would forage and rest with her new mate on southern side of the lake, while the chick hid near shore a kilometer away. While we marveled at the ability of the female to lead this double life and thus to keep the chick alive, we were on pins and needles during the entire chick-rearing period. It seemed inconceivable that the female could sustain her balancing act for the many weeks it would take until the chick was old enough to fend for itself. On each visit to the territory, we expected to find that the chick was dead or severely injured following an attack by its stepfather. Yet the fatherless chick, pictured above and dubbed “Miracle Chick”, not only survived but grew by leaps and bounds.

This year we have a new — and increasingly dark — variation on the single-mother theme playing out on Arrowhead Lake. About a month ago, the right wing of the 12-year-old breeding male became injured. (It is a soft tissue injury, we think, as we did not find a break when we inspected the wing after capture a week ago.) The injured male has engaged less and less with his mate and two chicks over the past month. Instead he spends his time resting and foraging alone on the southern end of the lake. His mate, a seasoned breeder who has produced at least seven sets of chicks with at least three different males on Madeline Lake and Arrowhead, appeared to step up her chick feedings and attendance to compensate for her mate’s absence. Her efforts seemed to pay off; the chick we caught a week ago had achieved a healthy weight, and its sibling (which we did not catch) was of roughly equal size. But the situation has degenerated in the past week. The male, though still alert and feeding himself, shows no signs of recovery from his wing injury and continues to avoid the family. In the past few days, the six-week-old chick that we did not capture was lost (to an eagle, according to lake residents). Today, Lyn reported that intruders landing on the lake roamed about it at will, because neither male nor female showed any semblance of territory defense. It seems only a matter of time before a new breeding pair takes over Arrowhead, and that will likely lead to the demise of the surviving chick. 

Let’s try to be optimistic. If the male recovers and begins to defend the territory again, the chick is in great shape. At six weeks, it has already survived the most difficult early phase of chick-rearing. The veteran female is an attentive mother; perhaps her care can keep the chick healthy in the meantime. And in the sad event that the male does not recover, the female’s efforts might be enough to keep the chick alive and growing even in the presence of a new male. After all, it has been done before. 

In any event, I think I have made my point. Biparental care is almost mandatory in common loons. While a human dad or mom can usually call upon a support network of friends and relatives to help feed and protect their child, a male or female loon that loses its mate during chick-rearing is very much alone. 

on

It is usually no fun to be wrong, but maybe this is an exception. In my blog post yesterday, I surmised that the sudden appearance in flight of the male from Little Bearskin meant that he and his mate had failed in their second nesting attempt. This seemed a safe presumption; I knew from many years of experience that males do not often leave females alone with small chicks. Yet I was mistaken. A lake resident (thanks, Nancy!) corrected me by pointing out that at least one chick had hatched on Little Bearskin this year, and Martha found two chicks on the lake during her early-morning visit today.

As we have explained in an earlier publication, there are three reasons why males tend not to leave their breeding lakes when their chicks are in their first two weeks of life. First, females cannot yodel, and therefore they are unable to discourage intruders from landing in the lake and approaching chicks by means of this aggressive vocal signal. Second, by virtue of their greater size, males are better equipped to intimidate and drive away intruders that do approach chicks. Third, having two parents guarding chicks when they are small permits breeding pairs to cover two bases — they can send one parent out to engage intruders and leave the other to protect the chicks, in case an intruder should come close.

In fact, years ago on Langley Lake we witnessed the danger that parents face if one of them ventures off territory when their chicks are small. In this case, two intruders landed when the male was off the lake, forcing the female to choose between: 1) staying beside its week-old chick, and 2) leaving its chick to interact with the intruders. She chose the latter course, but that strategy backfired when the intruders dove and split up. At this most inopportune moment, the chick happened to give an alarm call that one of the intruders heard. The intruder quickly found the calling chick and, with no parent nearby to intervene, killed the chick in a matter of seconds.

With that horrid incident seared into my brain (and a good deal of quantitative data on chick attendance to back it up), I was fairly confident that the appearance of a breeding male on a lake not his own meant that he had failed in his breeding attempt at home. In fact, I am still scratching my head over the Little Bearskin male’s decision to leave his mate, his two helpless chicks, and his home lake with its abundant food supply, in order to visit a neighboring lake that held nothing but failed and displaced conspecifics. I guess I will have to continue my research for a few more years to make sense of that odd bit of behavior.

on

My new team and I are racing around the study area, still catching up to our banded breeding population. At each lake, we record the bands of the female and male, look quickly for any active or failed nests — only in obvious places — and race to the next lake to repeat the process: (“Ok…the female has a yellow band on right and is red over green on left? Good enough….let’s go!”.) The work is frantic and exhausting, and we are only halfway through. We are all so busy covering lakes that there is little time to reflect on what we have seen. I have trouble remembering what lakes we have even visited at the end of each day, so anxious am I to eat a meal and hit the hay for the next 5am wakeup.

Yet some patterns have emerged from our lake visits that remain lodged in my brain. It has been a dreadful first round of nests for most breeding pairs. Typical pairs in the study area abandoned their first nesting attempt three to four weeks ago because of the clouds of flies that descended upon them and have only just begun to renest or think about doing so. Based on what we have seen, it appears that 70 to 80% of all pairs could not stand to incubate the first clutch of eggs they laid in early to mid-May, making 2020 even slightly more devastating a black fly year than 2014, the previous worst year on record. Our study population has seen a steady slide in chick production over the past quarter century; 2020 will only strengthen that demoralizing pattern.

So you can imagine how it warmed my heart to hear about Linda’s loon pair (“Clune” and “Honey”), who managed to buck the trend and stick it out through all four weeks of incubation. At a time when the population as a whole is reeling, the assiduous parenting on display in Linda’s video below took my mind off of the population’s struggles for a moment and reminded me that good things can still happen.

on

Many of you have e-mailed me to ask, “What became of the duckling reared by loons?” It is a reasonable question. Each passing day during the summer revealed startling new behavioral quirks in the peculiar, touching relationship between these inseparable misfits. Having witnessed well over a thousand loon families — and by this I mean those consisting entirely of loons — I found each of my visits to the Long Lake pair a revelation. Each time I watched male and female loons feed their precious adoptee a fish or warn it about a passing eagle, I involuntarily shook my head. How could two species separated by 70 millions of years of evolution come together into such a tight and successful makeshift family? Every day the family remained together seemed to defy logic.

Yet their familial bond persisted. Following my most recent post on the loon-duckling family, the duckling grew and grew some more. By the end of July (as Linda Grenzer’s photo shows), the duckling was close to adult size, and the only uncertainty was whether or not it would sink its parents by continuing to ride about on their backs. By mid-August, the pair and duckling were spending more time apart. On multiple occasions, we saw the duckling take off and fly around the lake a few times before landing near its anxious guardians. By now fully capable of feeding itself and weeks beyond the normal fledging date for mallards reared by their own species, the duckling seemed to cling to its parents more for their sake than for its own.

By September 4th, the duckling and its loon parents were gone from the lake. We will not ever know where the duckling went or how it lived after leaving Long Lake. Although we could have attempted to mark it in July, as we do loon chicks, I could not bring myself to do so. Even as a scientist fascinated by the behavioral outcome, I was too transfixed by the beauty of the family to capture them and risk disrupting it.

 

on

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.