Ferruginous hawk (Buteo regalis) COSEWIC assessment and status report: chapter 6

Biology

Life cycle and reproduction

Ferruginous Hawks are thought to breed for the first time at 2 years of age (Bechard and Schmutz 1995). The proportion of adults that breed is determined by prey availability, particularly in populations dependent on jackrabbits (west of the Rockies) and ground squirrels (east of the Rockies). It is also believed that breeding pairs may exhibit a nomadic tendency in relation to prey availability; this could explain ‘’the dynamic nature of breeding populations’’ (Bechard and Schmutz 1995).

Clutch size is variable but clutches of 2-4 eggs are common (range 1-8; see Bechard and Schmutz 1995 for more details). In a recent study, Houston and Zazelenchuk (2006) found that, based on a survey of 1,433 nests, mean brood size ranged from <2.6 young per nest in very poor years to >3.2 young per nest in very good years. Little is known about lifetime reproductive success, but one banded male in Alberta contributed to the fledging of 20 young or more over a period of seven years (Bechard and Schmutz 1995).

In Alberta and Saskatchewan, annual adult survival rate was estimated to be 0.708 (SE = 0.024) while first year survival for nestlings was 0.545 (SE = 0.147) (Schmutz et al. 2008).

Predation

Adult Ferruginous Hawks are probably not predated by any species other than Great Horned Owls (Bubo virginianus) and humans; relatively few documented cases of nest predation exist. The vulnerability of hawk nests to predation varies depending on nest location. For example, Bechard and Schmutz (1995) considered coyotes (Canis latrans), American badgers (Taxidea taxus) and foxes (Vulpes spp.) as serious threats to ground nesting pairs and their young. Both crows (Corvus sp.) and Common Ravens (Corvus corax) prey on eggs and nestlings, whereas Great Horned Owls are the main predator of nestlings; Golden Eagles (Aquilachrysaetos) also prey on young (Bechard and Schmutz 1995).

Physiology

No information.

Dispersal/migration

Unlike the relatively sedentary or short-distance southern breeding populations, northern populations of Ferruginous Hawks (from Washington, Montana, North Dakota, Alberta and Saskatchewan) are migratory (Bechard and Schmutz 1995). Hawks from the north and central Great Plains generally stay to the east of the continental divide, follow grasslands during their migration, and prey on ground squirrels and prairie dogs. Their overwintering destinations are Oklahoma, Texas and northern Mexico (Bechard and Schmutz 1995, Houston and Zazelenchuk 2006). A Tri-National investigation of migration and winter ranges of the Ferruginous Hawk is currently underway (Watson and Banasch 2005); preliminary results demonstrate that Canadian hawks winter as far south as Oklahoma.

Interspecific interactions

Ferruginous Hawks are generally harassed by other Buteos more often than they perpetrate harassment; in turn they harass Golden Eagles and Great Horned Owls because both of these species prey on young Ferruginous Hawks. On the overwintering grounds in Texas, both Golden and Bald Eagles (Haliaeetus leucocephalus) dominate in disputes over captured prey caught by Ferruginous Hawks. Swainson’s Hawks are also dominant over Ferruginous Hawks in breeding areas (Bechard and Schmutz 1995).

The Ferruginous Hawk nests in habitats used extensively by Swainson’s Hawks and, less frequently, by Red-tailed Hawks (Buteo jamaicensis). Joint defence of overlapping territories between Ferruginous Hawks and other Buteos can be either beneficial (increasing nesting success) or detrimental (reduced reproductive success) if interspecific agonistic interactions occur (agonistic interactions also occur against tethered intruders) (references in Bechard and Schmutz 1995). This is of interest because competition from other Buteos has been offered as one explanation as to why Ferruginous Hawks have declined in some areas (Schmutz 1984).

Adaptability

Ferruginous Hawks appear to show poor adaptability, since they are a native grassland specialist; unlike the Swainson’s Hawk they do not adapt well to agriculture.

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