April 2017

Vol. 45, No. 2

Field Notes: Interspecific Aggression by a Common Loon in Winter

Jeffrey Boone Miller

After work, late on the afternoon of December 12, 2016, I went for a walk at Fresh Pond in Cambridge, Massachusetts. This 155-acre kettle hole pond usually holds good populations of migrating waterfowl from October through December until ice-up (Barton 1995), so my focus was on the waterbirds. When I reached the southernmost cove of the pond, near a region called Weir Meadow, I spotted a loosely associated group of waterfowl about 50 meters from shore that included two male and two female Common Goldeneyes (Bucephala clangula), four male and two female Hooded Mergansers (Lophodytes cucullatus), and two male Greater Scaup (Aythya marila). Shortly afterwards, I noted a Common Loon (Gavia immer) much farther out, perhaps 250 meters from shore. These different species were about to interact in, what was to me at least, an unexpected manner.

Over the next couple of minutes, while the ducks and mergansers were diving regularly for food, the loon swam directly toward them, soon closing to within a few meters of the four goldeneyes. Now alert to the loon, the goldeneyes stopped diving, extended their necks, assembled into a tight group, and began swimming rapidly away from the loon and toward shore. At this point, the loon extended its neck parallel to the water with its bill pointed at the goldeneyes. The loon looked much like a swimming snake, showing only its bill, eyes, and the top of its head and body. Staying in this posture, the loon accelerated toward the four goldeneyes, all of which took the hint and flew off with their characteristic whistling flight.

The loon then turned its attention to the Hooded Mergansers, which were just a few meters away and now also on high alert. Again the loon approached rapidly with its neck outstretched and with little of its body showing. While two of the mergansers tried diving to escape, the remaining four took flight immediately, as did the diving pair when they resurfaced. The loon then made one aggressive move toward the two Greater Scaup which were about 15 meters to the west of the loon. However, the scaup avoided confrontation by swimming around a nearby point and out of sight of the loon, at which time the loon ended its aggressive behavior and resumed a normal swimming posture.

Throughout this episode, the loon did not vocalize, stand upright, or attack from underwater, even though all of these behaviors are part of a loon’s repertoire of aggressive behaviors. The loon’s actions did, however, appear to incorporate the “threat” behavior described by Byrkjedal (2011) in which the aggressor swims toward its target after adopting “a low posture, neck and head stretched forward pointing at the opponent.”

During the breeding season, Common Loons show interspecific aggression toward other species of waterfowl. Observers have noted loons attacking and killing ducklings and even adults of several species including Common Goldeneyes, Red-breasted Mergansers (Mergus serrator), Long-tailed Ducks (Clangula hyemalis), Ring-necked Ducks (Aythya collaris), Common Eiders (Somateria mollissima), Mallards (Anas platyrhynchos), Redheads (Aythya americana), and Canada Geese (Branta canadensis) (Sperry 1987, Kirkham and Johnson 1988, Morton and Pereyra 2011). In addition to attacking other species, breeding loons frequently show intraspecific aggression toward other loons. Such encounters between breeding loons can result in injury or, particularly when fights are between males, death (Piper et al. 2008).

In contrast to their aggressive behavior during the breeding season, observers have reported that Common Loons are seldom aggressive during migration or on wintering grounds. The Birds of North America species account, for example, states that Common Loons “In winter, typically do not show aggression.” (Evers, Paruk, McIntyre, and Barr 2010). Two studies of loons wintering in Rhode Island also noted that the birds did not show aggressive behaviors (Daub 1989, Ford and Gieg 1995). Though examples of intraspecific aggression have been noted among Common Loons wintering in Norway and Virginia (McIntyre 1978, Byrkjedal 2011), I have yet to find another report of interspecific aggression by a Common Loon in the winter. I would welcome information about such sightings from readers.

I have visited Fresh Pond consistently during autumn migration ever since my wife, Kathleen Buckley, first took me there in 1988. In almost every year, I have observed one or two Common Loons on one or more visits. The interspecific aggression I report here, however, was the single instance of this behavior that I have observed. Given the low level of aggression reported for loons in the winter and my previous experience, this behavior was truly unexpected.

Why did this loon carry out an interspecific attack at a migratory stopover on a winter day? There is a lively debate in the scientific literature about the costs and benefits of interspecific aggression by loons. On the breeding grounds, one thought is that such indiscriminate aggression toward other species, including mammals as well as birds, may lessen predation on loon hatchlings, essentially by clearing the nesting territory of all possible predators (Morton and Pereyra 2011). In the winter, however, hatchling predation is not an issue and loons typically become less aggressive, which makes the behavior of the bird at Fresh Pond surprising. Perhaps this bird responded to a specific set of circumstances that were similar to conditions that trigger aggression on the breeding grounds. In particular, Fresh Pond, as a fresh water lake reminiscent of breeding habitat, may provide a stimulus that is lacking at saltwater wintering sites. The loon at Fresh Pond seemed to have gained nothing from its aggression other than a less crowded neighborhood and some practice for the coming breeding season, but perhaps that was sufficient.


  • Barton, J. H. 1995. Ten Years and a Year: The Fall Waterfowl Census at Fresh Pond, Cambridge, 1984-1993, 1994. Bird Observer 23 (1): 11–24.
  • Byrkjedal, I. 2011. Social behaviour of wintering Great Northern Divers Gavia immer in relation to age categories. Ornis Novegica 34: 10–16.
  • Daub, B. C. 1989. Behavior of Common Loons in Winter. Journal of Field Ornithology 60 (3): 305–311.
  • Evers, D. C., J. D. Paruk, J. W. McIntyre and J. F. Barr. 2010. Common Loon (Gavia immer) in The Birds of North America Online (P. G. Rodewald, Ed.). Ithaca: Cornell Lab of Ornithology.
  • Ford, T. B. and J. A. Gieg. 1995. Winter Behavior of the Common Loon. Journal of Field Ornithology 66 (1): 22–29.
  • Kirkham, I. R. and S. R. Johnson. 1988. Interspecific Aggression in Loons. Journal of Field Ornithology 59 (1): 3–6.
  • McIntyre, J. W. 1978. Wintering Behavior of Common Loons. The Auk 95 (2): 396–403.
  • Morton M. L. and M. E. Pereyra. 2011. Additional Data and Perspectives on Interspecific Aggression in the Common Loon, Gavia immer. The Canadian Field-Naturalist 125: 61–62.
  • Piper, W. H., C. Walcott, J. N. Mager, and F. J. Spilker. 2008. Fatal Battles in Common Loons: A Preliminary Analysis. Animal Behaviour 75: 1109–1115.
  • Sperry, M. L. 1987. Common Loon Attacks on Waterfowl. Journal of Field Ornithology 58 (2): 201–205.

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