Testing the Legends of Loons
by Walter Piper
I first heard the echo of loons' voices across Lake Temagami, a 91-square-mile watery expanse some 300 miles north of Toronto, where my family had a string of rustic cabins. The loons usually called at night, and in the morning my mother, her eyes bright with excitement, would ask my brother and me, "Did you hear the loons last night? Wasn't it wonderful?" Although he and I were both heavy sleepers, especially during those cool July nights when we lay nestled in flannel sheets and thick wool blankets, we too were often awakened--and enchanted--by the birds' calls.
The loons' resonant laughter and long, mournful wails were sounds that made me, a product of big city suburbs, glad I was safely indoors with the door firmly latched. And yet they were magical sounds that filled me with wonder and respect for loons. During long summer days on the lake, we saw many of these birds, often in pairs, diving, remaining underwater for what seemed an eternity, and then finally surfacing a hundred yards away. We seldom succeeded in keeping these wary creatures in sight for more than a few minutes, despite heroic efforts to follow them by canoe. Their skittishness only increased my fascination.
Thirty years have passed since I first saw and heard loons on Temagami, and in many ways I find their behavior more vexing now than ever. I have grown up and devoted my life to the study of animal behavior. I am not sure the loons inspired my career, but it excites me that now I have the training, after years of research with more mundane species, to unlock the mysteries of loon behavior. In a sense, I have come full circle.
I am not alone in my reverence for loons. Residents on lakes throughout the loon's breeding range--which stretches from Alaska to Iceland and includes northerly portions of 14 northern states--have come to adore these creatures because of their ghostly nocturnal calls, stylish black and white plumage, and beady orange eyes. The common loon, the species I watched at Temagami, is the most familiar of the five loon species in its family. It breeds on lakes ranging from a few acres in size to Lake Superior; wherever it can find fish to eat and a protected area in which to nest and raise its one or two young.
The appearance of common loons on countless stamps, coins, billboards, mailboxes, and license plates attests to the popularity of the species. Loon researchers too have come to venerate their study animal--to the point where a considerable body of legend has grown up around them. The most widespread myth among loon enthusiasts is that the birds mate for life; that is, each bird has only one mate during its lifetime and refrains from breeding after its mate dies. A related myth is that each loon returns, year after year, to the same breeding lake. In examining the loons' mating system and the behavior of marked individuals, I was in a position to put such legends to the test.
The first inkling that I might finally learn about the mysterious aquatic creatures that haunted my youth came during the summer of 1992, when I met David Evers. Beginning in 1989, Evers, a wildlife biologist working with the Whitefish Point Bird Observatory in northern Michigan, had begun refining the onerous and previously ineffective technique used to capture adult loons. By the time I met him, he had become the world's expert. As one might imagine, safely catching and restraining a powerful 10 to 12-pound aquatic creature with a daggerlike bill is no small task. But Evers had found that, armed with a small motorboat, a spotlight, tapes of loon calls, and a net used for landing large sportfish, four courageous souls working at night could accomplish this feat with a high rate of success.
Evers' breakthrough had allowed him to place distinctive combinations of colored leg bands on 536 adult loons and 267 chicks since 1989, mostly in Wisconsin, Michigan, and Minnesota. By censusing the birds in subsequent years, Evers then began to determine the rates at which they returned to their territories and learn about the life histories of individual loons. His main interest was loon conservation, especially the threat posed by exposure of loons to methylmercury, a neurotoxin and environmental contaminant that accumulates in the small fish eaten by loons and occurs in high enough levels in loon tissues to pose a potential threat to survival and reproduction.
What intrigued me, as a behavioral ecologist, were the frequent instances that Evers observed in which mated pairs appeared to share their territories with third, unbanded individuals that were often males. If mated loons commonly spent time with other males, I reasoned, what was to stop females from mating with them? Many songbirds engage in so-called extrapair matings (see Fingering Infidelity). Why should loons be any different?
Actually, I expected loons to be different. In most birds, females can raise several young to independence without male assistance. This offers males the opportunity to increase their reproductive success by copulating with females other than their mates. For their part, females might participate in such copulations as insurance against their own mates being sterile or to improve the genetic quality of their offspring. Loons, however, depend almost entirely upon alert and rapidly-swimming prey: fish and crayfish. Moreover, chicks eat prodigious amounts and are unable to feed themselves for three months. These constraints seem to force male loons to assist females in caring for the one or two chicks that hatch. The enormous expenditure of time and energy males put into raising chicks raises the stakes in the paternity game. We would expect a male loon to try to ensure that he is the genetic father of the young he raises, perhaps by guarding his mate during her fertile period.
The observation of frequent association between third loons and breeding pairs had cast doubt upon my expectation of genetic monogamy in loons. But the data I collected dispelled that doubt. I learned that loon pairs do remain close together. In fact, they spend 82 percent of their time within 20 yards of each other during the female's fertile period, leaving little opportunity for extrapair copulations. Moreover, DNA fingerprinting showed that all 58 chicks from 47 loon families were the genetic offspring of the pair that reared them. Thus, among the hundred or so birds in which genetic analysis of parentage has been carried out, loons fall into the handful--including such species as the black vulture, peregrine falcon, Old World willow warbler and Wilson's storm petrel--wherein males always rear their own offspring.
If the extra loons in breeding territories that David Evers had observed were not males seeking extrapair copulations, then who were they and what were they after? Previous observers had regarded loon intruders as rare visitors of little consequence to the breeding biology of pairs. But I found intrusions to be frequent--occurring two to six times a day in most territories throughout the three-month-long breeding season. Most pairs have to engage in constant confrontations--and the occasional knock-down, drag-out battle--with intruders. I had two remaining hypotheses about the intruders: first, they might simply be looking for food; call this the foraging hypothesis. Second, they might have been trying to take over territories from the resident pair: the takeover hypothesis.
I tested these two hypotheses in 1994 and 1995. It was easy to reject the foraging hypothesis. An intruding loon rarely forages; it spends most of its time approaching, interacting with, or fleeing from the resident pair. Only the takeover hypothesis remained, and, on the face of it, it appeared plausible.
Good breeding territories--those in which chicks are produced almost every year--are a scarce commodity in northern Wisconsin owing to a high rate of egg predation. Some territories are inhabited by pairs or loners of either sex that never attempt to nest. Other territories rarely produce chicks, despite two to three annual attempts by a resident pair. A third class of territories--those with few predators or those with islands that provide loons with nesting sites that predators cannot reach--consistently produce chicks. From a loon's point of view, northern Wisconsin is a swath of poor territories dotted with a few good ones. For this reason, most loons should be looking to move to a better territory. Vacancies do arise through the deaths of residents, but the fastest means of obtaining a good breeding slot is simply to kick someone else out of theirs!
The possibility that loons might usurp each others' territories--and each others' mates--horrifies those who embrace the widespread legend that loons remain on a single territory and mate for life. It is with a heavy heart, but with scientific certainty, that I report that loons do kick each other off of territories--and quite frequently. In fact, of the 36 marked territorial residents I observed from 1993 to 1995, six lost their territorial positions during the breeding season. At least five of these were kicked off their territories by unmarked intruders, three females and two males. Clearly, loons often lose their breeding positions to territorial intruders, and residents of both sexes are vulnerable.
Most damaging to loon legend, however, is the indifference with which each loon seems to view the territorial battles of its mate. The vast majority of battles that occur in breeding territories--which can include vicious pecking, beating with wings, and lengthy chases across the water--occur between single intruders and the pair member of the same sex. Moreover, when a loon is driven off its territory, it departs alone, leaving his or her conqueror and erstwhile mate as the new breeding pair in possession of the territory. Any eggs or chicks produced by the original two birds are lost, although the remaining member of the original pair often attempts to renest with its new mate. Thus, the much-vaunted pair bond of loons does not outlast each pair member's ability to defend its breeding position from intruders of its own sex.
I don't mean to suggest that all breeding loons must fight constantly to maintain their breeding status. Violent fights are, in fact, quite rare. But breeding pairs--especially those with good territories--encounter anywhere from dozens to hundreds of intrusions each year. In most cases, an intruder spends five to 15 minutes engaging in a series of stereotyped interactions with the pair that includes approaching, circling, and head bowing and then it flies away, perhaps to intrude elsewhere. Eventually an intruder will arrive that does not leave quickly, and this leads to an escalated battle that might cost the embattled resident its territory and its mate. We are currently trying to understand the factors that lead to such escalated contests.
What is the fate of breeders that have been driven off of their territories? In the five observed cases, these birds simply moved "next door," taking up residence in the nearest unoccupied lake. One female attempted unsuccessfully to breed in the new lake in the year after losing her territory. A second female became a nonbreeder but made frequent visits back to her original territory and once fought unsuccessfully with the resident female in an attempt to regain her old territory. Because loons can live 20 to 30 years, we will have to wait to learn how losing its territory affects a loon's breeding success in subsequent years.
Just as I have matured, so has the study of loon behavior. We can no longer seriously entertain the engaging notion that loons mate for life, but we can respect loons as tireless parents and paragons of conjugal fidelity. We cannot maintain the myth that loons always treat each other amicably, but we can admire their valiant efforts to defend territories and raise young despite the presence of so many intruders that threaten their status and their chicks' survival. A pleasing, if paradoxical, aspect of my growing knowledge about loon behavior is that as my research has begun to dispel the myths that envelop them, loons are even more fascinating to me now than when I first heard their haunting calls echo across Lake Temagami.
Walter Piper is a research associate in the Molecular Genetics Laboratory and the Department of Zoological Research at the National Zoo. His work has been supported in part by Friends of the National Zoo.
(ZooGoer 25(3) 1996. Copyright 1996 Walter Piper. All rights reserved.)
