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Survival rates and lifetime reproduction of breeding male Cooper's Hawks in Wisconsin, 1980-2005.

Article Excerpt
Survival is a fundamental attribute in understanding population ecology of avian species as well as trade-offs that may exist among other life history traits, including increased reproduction versus decreased survival (Ricklefs 1973). Information on survival is difficult to obtain (Newton 1989) and there are few estimates of survivorship for Cooper's Hawks (Accipiter cooperii) (Rosenfield and Bielefeldt 1993). Early studies of survival of Cooper's Hawks were based on banding data in which geographic affiliations and breeding status of marked individuals were vague or unknown (Henny and Wight 1972). More recently, using telemetry, Mannan et al. (2004) reported 6-month survival rates for post-fledgling Cooper's Hawks in an urban environment in Arizona, and Roth et al. (2005) described winter survival for both immature and adult Cooper's Hawks in rural and urban settings in Indiana and Illinois. Only one study in an urban setting in Arizona has reported survival rates of breeding adult Cooper's Hawks (Mannan et al. 2008).

Urban environments are relatively new habitats for Cooper's Hawks and other breeding raptors, and these populations are poorly studied (Love and Bird 2000, Stout et al. 2007, Rutz 2008). Raptors in urban landscapes may have increased mortality rates because of collisions with anthropogenic obstacles (e.g., Sweeney et al. 1997) or diseases (Mannan et al. 2008). There are no reports of relative survivorship of breeding Cooper's Hawks in urban versus rural habitats.

We conducted cross-generational, mark-recapture studies of nesting Cooper's Hawks in both urban and rural settings in Wisconsin starting in 1980 and have shown that age of adult males appears to be unrelated to brood size (Rosenfield and Bielefeldt 1997). Other aspects of reproduction, including brood size, are correlated with body mass of males and females (Rosenfield and Bielefeldt 1999). Adult males on our Wisconsin study areas have lifetime nesting area fidelity (Rosenfield and Bielefeldt 1996), and new males in nesting areas indicate deaths of previous occupants, in turn allowing for estimates of replacement male survivorship (Jenkins and Jackman 2006).

We used a 26-year data set to: (1) report annual survival rates of breeding male Cooper's Hawks, (2) examine the possibility of temporal variation in annual male survival, (3) investigate the possibility of differences in male survival between rural and urban breeding habitats, (4) explore the possibility that male survival is related to body mass or size, and (5) report lifetime reproduction of adult male Cooper's Hawks.

METHODS

Study Areas.--We studied breeding Cooper's Hawks during 1980-2005 at two principal areas in central and southeastern Wisconsin as described by Murphy et al. (1988), Rosenfield et al. (1995), and Rosenfield and Bielefeldt (1996). Our central Wisconsin area included the abutting municipalities of Stevens Point, Whiting, and Plover with a predominately urban human population of ~38,000, and a human density of ~600/[km.sup.2] (U.S. Department of Commerce 2000). Our southeastern Wisconsin area involved rural environs of the Kettle Moraine State Forest, South Unit. These study sites were chosen without preconceptions about their suitability for nesting Cooper's Hawks (Bielefeldt et al. 1998).

A nesting area was defined as an area 800 m in diameter that was occupied by a breeding adult male in one or more years; a nesting area was considered reoccupied when we found a new nest in a subsequent year within 400 m of the original nest in the area (Rosenfield and Bielefeldt 1996). Most occupied nesting areas (> 90%) were found before egg-laying by listening for dawn vocalizations (Rosenfield and Bielefeldt 1991, Rosenfield et al. 1996) or by searching for partially constructed nests during the pre-incubation stage, ~mid-March through late April in Wisconsin (Bielefeldt et al. 1998).

Field Procedures.--Breeding adult male Cooper's Hawks were trapped and re-captured (or remotely identified via color marks) in later years near their nests during pre-incubation or incubation stages, but mostly during the nestling period using a variety of techniques (Rosenfield and Bielefeldt 1993, 1999; Rosenfield et al. 2007a). We classified age of adult male Cooper's Hawks at initial capture following Rosenfield and Bielefeldt (1997); birds were banded with USGS lock-on aluminum leg bands as well as colored, alpha-numerically coded leg bands (Rosenfield and Bielefeldt 1996). Initial captures of unbanded breeding males usually involved ASY (after second year) birds [greater than or equal to] 2 years...

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