701 Interspecific Killing of a Branta bernicla (Brant) by a Male Branta canadensis (Canada Goose) David A. Krauss1,* and Issa E. Salame2 Abstract - Interspecific killing dissociated from predation is rare in birds. An instance of this behavior episode in which a male Branta canadensis (Canada Goose) killed a Branta bernicla (Brant) was observed and photographed on Long Island, NY in April 2011. The Canada Goose exhibited aggressive nest-defense behavior that is well known for this species, and we suspect that this aggression combined with the Brant’s inability to defend itself due to its smaller size led to this unusual result. The male Canada Goose maintained its attack until the Brant die d. Introduction. Interpretations of interspecific aggression in birds are varied (Murray 1986) and include extroversions of intraspecific aggression (Horn 1988) possibly related to hormone levels during the breeding season, competition for food (Alexander 1987, Conover and Kania 1994, Kalinosky 1975) and competition for nest sites (Kalinosky 1975). Such encounters, while significant, rarely produce fatal results although interspecific aggression at nest sites has been known to cause mortality in some species (Merilä and Wiggins 1995). Records of such interspecific killing in birds are rare, but those that exist most often relate to nesting activity. While predation could be considered an aggressive behavior, it is always intended to result in the death (and consumption) of one of the individuals involved. In our discussion of aggressive behavior below, only agonistic behaviors unrelated to predation are considered. Anseriforms exhibit high levels of nest-site-associated aggression (Black and Owen 1987, Burgess and Stickney 1994, Ciaranka et al. 1997, Ely et al. 1987, Livezey and Huphrey 1985). Steamer ducks present an extreme example of nest-site aggression. Being significantly larger and more heavily built than other ducks, aggressive behavior in steamer ducks often results in interspecific killing (Nuechterlein and Storer 1985). Although many other species are highly aggressive, we can find no other references to interspecific killing by Anseriformes. Swans also are highly aggressive at nest sites, a nd their large size makes them particularly dangerous to smaller waterfowl that they sometimes kill (Ciaranka et al. 1997). Large body size in the aggressive individual seems to be an important factor in the occurrence of interspecific killin g among Anseriformes. Geese are known to be highly aggressive at their nest sites, and much of their social behavior is related to agonistic interactions on (Akesson and Raveling 1982, Hanson 1953, Gauthier and Tardif 1991, Sedinger and Raveling 1990, ) and off (Gregoire and Ankney 1990, McLandress and Raveling 1981, Raveling 1970) their breeding grounds. Branta canadensis (L.) (Canada Goose) males are the primary defenders of their nests, especially during incubation (Sedinger and Raveling 1990). Although the aggression of these geese at nest sites is well known (the Oceanside Nature Center where this incident occurred has warning signs alerting humans to potential goose aggression), this is the first record of such behavior in the literature. Increases in hormone levels (Akesson and Raveling 1981, Hirschenhauser et al. 2000) leading up to the breeding season are related to changes in goose behavior, including increased aggression. Such interspecific aggression seems to have the evolutionary advantage of protecting the nest site and offspring from competitors as well as predators. 1Science Department, Borough of Manhattan Community College, 199 Chambers Street, New York, NY 10007. 2Chemistry Department, City College of New York, 160 Convent Avenue, New York, NY 10031. *Corresponding Author - dkrauss@bmcc.cuny.ed. Notes of the Northeastern Naturalist, Issue 19/4, 2012 702 Northeastern Naturalist Vol. 19, No. 4 Observations. We report here an incident in which an aggressive male Canada Goose attacked and killed a Branta bernicla (L.) (Brant) in defense of its nest 23 April 2011 at the Oceanside Marine Nature Study Area in Nassau County, Long Island, NY. The majority of the Canada Geese nesting on Long Island are part of a non-migratory population (USFWS 2012). The observers were approximately 25–30 m from the birds during the entire episode. A flock of approximately 35–40 Brant was swimming through a channel in the salt marsh roughly 30 m from the Canada Goose nest. At approximately 11:30 AM, an individual Brant, apparently a first-summer bird based on the minor white neck band, broke from the flock and swam towards the Canada Goose nest. It was unclear why the Brant moved away from the flock. The birds were not actively feeding, and there did not appear to be anything that specifically drew the Brant away from the group. Possible explanations may have been a desire to feed in shallower water or to preen on the shore. There was no obvious sign of injury or illness in the Brant. The male Canada Goose rapidly descended from his alert stance near the nest, while the female remained alert on the nest throughout the entire incident. The male goose immediately attacked the Brant, vocalizing and charging simultaneously. After a few initial vocalizations, the attack was silent. The goose pecked and bit the Brant, knocking it down onto its breast as it reached shallow water (Fig. 1A). In this position, the Brant was essentially helpless and became mired in the intertidal mud. Figure 1. A sequence of photographs taken during a Canada Goose’s attack on a Brant on 23 April 2010 at Oceanside Nature Center, Long Island, NY. A) 11:32 AM: in the early stages of the Canada Goose’s attack on the Brant, the Brant actively tried to escape as seen here. B) 11:42 AM: the Canada Goose is forcing the Brant’s head under the water. The goose engaged in this activity repeatedly limiting the Brant’s access to air, accelerating its exhaustion. C) 11:47 AM: The Brant periodically lay dormant, appearing unconscious, during the attack. Here the Canada Goose is holding the limp neck of the unresponsive Brant. D) 12:42 PM: the Canada Goose is forcing the Brant’s head into the mud. The goose didn’t just push, but hammered the Brant’s head into the mud. Ultimately the Brant died due to suffocation by mud that clogged its nares. 2012 Northeastern Naturalist Notes 703 By 11:40, the Brant was stuck in the mud and the Canada Goose continued to peck and bite it, concentrating on the head and neck. The blows caused injury (visible blood) and disorientation in the Brant. The goose frequently bit the Brant’s head and forced it under water (Fig. 1B) and/or into the mud. When the Brant lay quiescent in the mud, the goose would stop pecking. However, as soon as the Brant tried to move away, the goose always resumed its attack. When the Brant tried to flap its way free, the goose bit its wings. After approximately 15 minutes from the start of the attack, the Brant seemed exhausted and frequently lay still in the mud. The Brant appeared to lose consciousness on numerous occasions (Fig. 1C). If the Brant moved again, the goose resumed its attack. When the Brant was still, the goose would wait for 20–30 seconds. If the Brant did not move again within that time span, the goose would peck and bite the Brant hard enough to rip pieces of skin from the Brant. Whenever the Brant tried to flee, the goose resumed its attack on the Brant’s head and neck. Eventually (12:30 PM), the Brant became exhausted and seemed to lack the energy to pull itself out of the mud. The Canada Goose repeatedly pecked the Brant’s head into the mud (Fig. 1D). Several times the goose grabbed the Brant’s head and pushed it, beak first, as far into the mud as it could. The male goose continued to peck at the Brant carcass for several minutes after it had apparently died of suffocation/drowning. When the goose could no longer elicit a response from the Brant (12:50 PM), it returned to its mate on the nest. The Brant’s role in the entire event was passive and it was apparently unaware of the goose nest. The only response it made to the goose was to attempt to flee. The Brant never engaged in any form of agonistic behavior and was never observed attempting to bite the goose. The remainder of the flock of Brant immediately swam away from the attack towards deeper water as soon as it began and remained passive at a distance of 30–50 m. They remained calm and displayed no behavioral signs of agitation after the first few minutes. Due to the rules of the nature center, it was not possible to retrieve the carcass of the Brant for study. Discussion. Interspecific killing dissociated from predation among birds is rarely reported. Such incidents may occur more commonly than they are reported among aggressive species like the Canada Goose. Canada Geese aggressively defend their nests against predators and conspecific rivals as was observed here. Cody (1969) suggested that similar physical characteristics among closely related species could be associated with interspecific aggression and this may have been a factor in this incident. There is no evidence that Canada Geese and Brant hybridize, and it is possible that aggression at this level may be an important isolating mechanism between these similar species. We interpret this event as the result of high hormone levels in the male goose resulting in hyper- aggressive behavior directed towards a Brant that was unable to defend itself against the larger and more powerful attacker. The Brant may have triggered attack behavior in the male goose that continued to increase as the Brant was unable to leave the vicinity of the nest. Early in the nesting season, aggressive behavior may become exaggerated due to high hormone levels (Akesson and Raveling 1981, Hirschenhaus er et al. 2000). Literature Cited Akesson, T.R., and D.G. Raveling. 1981. Endocrine and body weight changes in nesting and nonnesting Canada Geese. Biology of Reproduction 25:792–804. Akesson, T.R., and D.G. Raveling. 1982. 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