In this study I evaluated six ways in which adult female Columbian ground squirrels (Spermophilus columbianus) potentially alter their overall activity and foraging behaviour to satisfy the increased energetic needs of reproduction, all of which may involve a trade-off with predation risk. A sample of nonparous females was obtained by administering a short-term chemosterilant at the time of breeding. Relative to nonparous females, parous females spent more time above ground, more time foraging, and less time vigilant, particularly during lactation, and had a longer active season. Thus, in absolute terms, parous females spent more time exposed to aboveground predators than did nonparous females. Parous females spent less time at burrow entrances and, during lactation, were active farther from escape burrows than were nonparous females. Parous and nonparous females did not differ in the percentage of time spent vigilant during foraging bouts. These results indicate that females respond to the energetic demands of reproduction by increasing the absolute amount of time spent foraging, which entails an acceptance of an increase in exposure to predators, and not by modifying their behaviour during foraging bouts.
SUMMARY(1) A polygynous population of northern harriers was studied for 4 years in New Brunswick to establish the costs and benefits to females within harems.(2) Polygynous harems of up to five females were observed, and clear hierarchies in clutch size and reproductive success were apparent in each year. Alpha females raised more fledglings than any other harem female.(3) Seasonal declines could explain the tendency of polygynous females to lay smaller clutches but not their reduced reproductive success relative to monogamous females.(4) Later-settling polygynous females suffered higher rates of predation than either monogamous or cr females. This could not be explained by differences in nest site quality; nor could female inexperience be cited, since older females predominated in harems.(5) Reduced success of polygynous females was best explained by male provisioning rates. Alpha females received significantly more prey items per day than any other harem female.(6) We surmise that the increased predation of later (p/, y, and 6) harem nests was also a result of low male provisioning rates; later-settling females were forced to forage by themselves and to leave their nests unattended.(7) We contend that a skewed sex ratio is not the prime reason for the evolution of polygyny, as suspected in other harrier populations, but that differences in male quality or controlled resources contribute to the crossing of the polygyny threshold.
Often seen conspicuously hunting low over the open coastal marshes of New Jersey, the northern harrier is a medium-to large-sized hawk. A white "rump patch," low buoyant flight, and the position of its wings in a shallow "V" dihedral characterize the airborne northern harrier. It also has an owl-like facial disk that aids it in detecting prey in tall grass or low-light conditions by concentrating sound towards the ears. Adult northern harriers are sexually dimorphic in both plumage and size. The smaller males are slate gray above and white below with contrasting black wingtips and a black trailing edge to the wing. The male's white breast has varying amounts of light rufous (rusty) spotting. The larger female harriers are brown above and buff colored below with brown vertical streaking on the chest and belly. Unlike that of the male, the underwing of the female is dark and the black wingtips are obscured. On all harriers, white uppertail coverts that form the white rump patch are a key in-flight field mark. Adult harriers of both sexes have lemon yellow eyes. On all ages, the legs are long and yellow, the cere-the fleshy area behind the base of the bill-is yellow, and the bill is black. Juvenile northern harriers are extremely similar in appearance to adult females. Juveniles are brown above, although slightly darker than adult females, and have a cinnamon wash to the underside that is faintly streaked. As this reddish hue fades during their first winter, juveniles tend to appear much more like adult females. Juvenile males are born with grayish eyes that turn to an adult-like lemon-yellow by their first winter. Juvenile females have dark brown eyes that take at least two years to appear yellow. Northern harriers employ several different calls. A chirp-like call between male and female is given during food exchange or on the ground during copulation. This call is also exchanged between females and nestlings. Harriers reiterate an alarm call consisting of a series of kaks when a predator or intruder disturbs the nesting area. A high-pitched, gull-like whine is emitted, most often by the male, during courtship and territorial display flights. A northern harrier hunting low over a marsh is one of the most distinctive raptors in flight. Reminiscent of a turkey vulture (Cathartes aura), the harrier holds its wings in a dihedral and rocks from side to side. The harrier is a slim, buoyant hawk with narrow wings and a long, slender tail, providing it with maneuverability and agility. On migration, harriers can appear significantly different in flight. When soaring, the harrier looks slender-bodied and cross-shaped and soars on flat wings. In a glide posture, the
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