Inter‐ and intra‐specific dominance relationships and feeding behaviour of Golden Eagles Aquila chrysaetos and Sea Eagles Haliatetus albicilla at carcasses

Abstract

The behaviour of Golden Eagles Aquila chrysaetos and Sea Eagles Haliceetus albicilla scavenging on artificially laid out carcasses in coastal Sør‐Trøndelag Province, Norway, was studied during two winters (totalling 640 h of observations) and the intervening summer (430 h). Neither species fed at carcasses in summer. Although smaller, Golden Eagles were strongly dominant over Sea Eagles in direct competition for carcass access. In Sea Eagles, females dominated males, while in Golden Eagles, few conflicts between birds of known sex were observed. Age effects were weak and not statistically significant in both species. Conflicts for carcass access tended to be most escalated between Golden Eagles and least escalated between Sea Eagles, with interspecific conflicts intermediate. Most conflicts were won by the aggressor, suggesting that birds were generally able to assess relative dominance before launching an attack. Young eagles fed longer at a carcass than older individuals in both species, suggesting that young eagles may have been hungrier or less efficient feeders. Sea Eagles waited longer than Golden Eagles between arrival in the immediate carcass area and feeding at the carcass. This effect was greater when the carcass was already occupied but also occurred when no other eagle was already present. While interspecific competition for carrion did not appear to have important consequences for the two species in coastal Norway, in western Scotland, where Sea Eagles are currently re‐establishing, carrion is important in the diet of both species all year round. Interspecific competition for this resource may therefore play a role in determining the ultimate realized niche (and therefore numbers) of the two species in Scotland in the longer term.

Lead and eagles: demographic and pathological characteristics of poisoning, and exposure levels associated with other causes of mortality

“…Our finding that bald eagles were lead poisoned more frequently than golden eagles could be the result of golden eagles being more successful predators and less likely to feed on carrion containing lead fragments, or bald eagle carcasses being noticed more frequently, because of the white plumage on adults, than golden eagle carcasses. Displacement of golden eagles scavenging on lead-containing carcasses by bald eagles seems unlikely because of the aggressive nature of golden eagles (Halley and Gjershaug 1998). We also found that female eagles were diagnosed with lead poisoning more frequently that males and adults more frequently than juveniles.…”

“…We also found that female eagles were diagnosed with lead poisoning more frequently that males and adults more frequently than juveniles. This may be at least partially explained by intraspecific dominance at scavenged carcasses, although there is more evidence for females displacing males than for adults displacing juveniles (Halley and Gjershaug 1998;Garcelon 1990). In combined data from two reports of lead poisoning in bald eagles, 18 of 26 birds were females and 14 of 26 were adults (Kaiser et al 1980;Reichel et al 1984).…”

The history of eagles in Britain and Ireland: an ecological review of placename and documentary evidence from the last 1500 years

“…IUCN guidelines also recommend monitoring the effect, if any, of reintroductions on the established native flora and fauna. Golden Eagles are dominant over White-tailed Eagles in Norway (Halley & Gjershaug 1998). Nevertheless, the possibility that Whitetailed Eagles reintroduced to Scotland might outcompete resident Golden Eagles (Watson 1997, Halley 1998 has not been entirely ruled out, in spite of substantial evidence to the contrary (Whitfield et al 2002, Love 2003, Evans et al 2010.…”

Breeding season diets of sympatric White-tailed Eagles and Golden Eagles in Scotland: no evidence for competitive effects

“…Therefore, if competition occurs in Scotland, and reintroduced White-tailed Eagles are to displace Golden Eagles (Thom 1986, Watson et al 1992, Watson 1997, Halley & Gjershaug 1998, Halley 1998, the limiting resource is most likely to be food, and White-tailed Eagles should be the superior competitor. Halley & Gjershaug (1998) showed that the Golden Eagle is the superior interference competitor over food, based on observations of behavioural interactions at carcasses in Norway. This runs counter to the postulated competitive advantage of reintroduced White-tailed Eagles over Golden Eagles.…”

“…Since their reintroduction to western Scotland in 1975 numbers of White-tailed Eagles have been increasing (Green et al 1996, Evans et al 2003, 2009). Several authors have suggested that the continued spread of the White-tailed Eagle in Scotland will lead to displacement of Golden Eagles through competitive effects as coastal ranges are reclaimed (Thom 1986, Watson et al 1992, Watson 1997, Halley & Gjershaug 1998, Halley 1998.…”