John Bielefeldt

VARIATION IN HYDROGEN STABLE-ISOTOPE RATIOS BETWEEN ADULT AND NESTLING COOPER'S HAWKS

Abstract Hydrogen stable-isotope analysis of feathers is an increasingly popular method for estimating the origins of migrating and wintering birds. Use of this method requires that investigators know which feathers are grown on breeding grounds and how the hydrogen stable-isotope ratios of feathers (δDf) relate to those of local precipitation (δDp). In this study, we measured δDf of adult (primaries 1, 3, and 10) and nestling Coopers Hawks (Accipiter cooperii) in Wisconsin, North Dakota, and British Columbia, Canada. As previously shown, δDf of nestling feathers were related to δDp. In contrast, the δDf of adult feathers grown on the breeding grounds were substantially greater than those of their nestlings, and varied significantly across primary feathers and study areas. Our findings suggest that it is not possible to use hydrogen stable-isotope analysis of feathers to learn the origins of migrating adult Coopers Hawks (or possibly adults of other large-bodied species with extended molting periods) until more is learned about the physiological or ecological mechanisms underlying these isotopic discrepancies. Variación en las Proporciones de Isótopos Estables de Hidrógeno entre Adultos y Polluelos de Accipiter cooperii Resumen. El análisis de isótopos estables de hidrógeno en las plumas es un método cada vez más popular para determinar el origen de aves migratorias e invernantes. El uso de esta técnica requiere que el investigador conozca qué plumas crecen en los lugares de nidificación y cómo las proporciones de isótopos estables de hidrógeno de las plumas (δDf) se relacionan con aquellas de la precipitación local (δDp). En este estudio, medimos las δDf en adultos (primarias 1, 3 y 10) y en polluelos de Accipiter cooperii en Wisconsin, North Dakota y British Columbia, Canadá. Como se ha mostrado previamente, las δDf de plumas de polluelos se encontraban relacionadas con las δDp. Por el contrario, las δDf de plumas de adultos que crecieron en los lugares de nidificación fueron substancialmente mayores que aquellas de sus polluelos, y variaron significativamente entre plumas primarias y áreas de estudio. Nuestros resultados sugieren que no es posible utilizar el análisis de isótopos estables de hidrógeno de plumas para determinar el origen de adultos migratorios de A. cooperii (o de adultos de otras especies de aves con tamaños corporales grandes y con períodos de muda extendidos) hasta que los mecanismos ecológicos y fisiológicos que subyacen a estas discrepancias isotópicas sean mejor entendidos.

Nesting Biology of Urban Cooper's Hawks in Milwaukee, Wisconsin

Abstract Urban landscapes vary greatly across North America and long-term data on the nesting biology of Coopers hawks (Accipiter cooperii) from a variety of urban environments will improve our understanding of these poorly studied populations. We studied Coopers hawks nesting in the metropolitan Milwaukee area, Wisconsin, USA, over a 12-year period, 1993–2004. Nesting success for 254 first nesting attempts averaged 64.6% with means of 2.27 young per laying pair and 3.53 young per successful pair. For 8 second nesting attempts (i.e., re-nests), nesting success averaged 87.5% with means of 2.57 young per laying pair and 3.00 young per successful pair. Productivity for first nesting attempts did not vary over the 12-year period, and productivity for re-nests did not differ from first nesting attempts. We documented evidence of nest predation by raccoons (Procyon lotor) and red-tailed hawks (Buteo jamaicensis). On average, second year (SY [i.e., 1-yr-old]) Coopers hawks comprised 14.6% (43 of 295 breeding birds; 21.5% [37 of 172] of F and 4.9% [6 of 123] of M) of the known breeding population. The percentage of SY breeders within this population declined over the 12-year period, suggesting a relatively young population. Coopers hawks consistently reoccupied nest sites annually after initial discovery over an estimated 2 generations of breeding adults, suggesting that population density for our study was at least stable. We trapped 105 breeding adults, including 5 natal dispersal birds. Based on long-term, relatively high reproduction, repeated re-occupancy of nest sites, and confirmed recruitment from within this population, we suggest that these nesting areas were not marginal or inferior habitats and that urban Coopers hawks in this study area were not a sink population. We recommend no active management of this population at this time; however, additional information for nesting Coopers hawks from other urban environs will expand our knowledge base for these populations.

Lifetime Nesting Area Fidelity in Male Cooper's Hawks in Wisconsin

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Vocalizations of Cooper's hawks during the pre-incubation stage

From 1986 to 1990 we studied the vocalizations of 10 mated pairs of Coopers Hawks (Accipiter cooperii) during the pre-incubation stage. We heard four different vocalizations during nest-building, copulatory, and other pre-laying activities. There was a marked asymmetry in vocalizations between the sexes during the pre-incubation period. Compared to males, female Coopers Hawks (1) had a larger repertoire of calls, (2) called more frequently, and (3) called during a wider array of activities. We suggest that the various calls signal presence, identity, location, dominance, and non-aggressive motivation. We attribute some of the intersexual differences in pre-incubation vocalizations to the pronounced degree of reversed sexual size dimorphism in this species.

Prevalence of Trichomonas gallinae in nestling Cooper's Hawks among three North American populations

Abstract Trichomoniasis, a digestive tract disease of raptors caused by ingesting the protozoan Trichomonas gallinae of infected columbid prey, recently was reported to be common in and an important agent of mortality among urban nestling and fledgling Coopers Hawks (Accipiter cooperii) in Arizona. However, the prevalence of T. gallinae in wild raptor populations is poorly documented. During 2000 we detected T. gallinae in only three (2.7%) among 110 nestling Coopers Hawks at 48 nests in both urban and rural habitats in Wisconsin, North Dakota, and British Columbia. Mortality attributable to trichomoniasis among 5- to 22-month-old Coopers Hawks was zero in the three combined study areas. Prevalence of infection by T. gallinae seems to vary greatly among nesting Coopers Hawks across this raptors continental breeding range.

VARIATION IN REPRODUCTIVE INDICES IN THREE POPULATIONS OF COOPER'S HAWKS

Abstract We investigated several reproductive attributes among three spatially and morphometrically separable northern populations of breeding Coopers Hawks (Accipiter cooperii) in differing ecological settings in British Columbia, North Dakota, and Wisconsin during 1995–2001. We did not detect significant inter-year variation in reproduction within any of our study areas. Cumulative clutch and brood sizes were significantly higher in British Columbia (mean = 4.41 and 3.60, respectively) and Wisconsin (4.26 and 3.73) than in North Dakota (3.5 and 3.0), but not significantly different between British Columbia and Wisconsin. Total nest success rates (91 and 82% in British Columbia and Wisconsin, respectively) varied significantly among all three study sites, but were lowest in North Dakota (68%). We hypothesize that smaller clutch and consequentially lower brood sizes in North Dakota may be the result of the comparatively later nesting phenology in this highly migratory population. Our results underscore the significance of clutch size data, and the need for further research on regional and other large-scale variation in avian demographic parameters to help decide if, when, and where population demographics may warrant a management response.

Survival Rates and Lifetime Reproduction of Breeding Male Cooper's Hawks in Wisconsin, 1980–2005

Abstract There are few published data on annual survival and no reports of lifetime reproduction for breeding Coopers Hawks (Accipiter cooperii). Breeding males (n  =  105) in central and southeastern Wisconsin had an annual mortality rate of 19%, or a survival rate of 81% for birds ≤10 years of age. We did not detect significant differences in mortality rates between urban and rural habitats, nor between the earlier 13 years and later 13 years of this study. Male Coopers Hawks produced from zero to 32 nestlings during their lifetimes. Body mass or size appeared unrelated to annual survivorship and lifetime reproduction, although lifetime reproduction was correlated strongly with longevity of breeding males. Fifteen of 66 males (23%) produced most (53%) of the nestlings. Our studies occurred in an area where breeding populations may be increasing with some of the highest reported productivity indices and nesting densities for this species. Habitat used for nesting on our Wisconsin study areas may be less important for survivorship and lifetime reproduction than acquisition of a nesting area in which a male will breed throughout his life.

Life-History Trade-Offs of Breeding in One-Year-Old Male Cooper'S Hawks

Abstract. Life-history theory suggests that delayed maturation is beneficial to birds when costs of breeding early in life are high. We compared selected aspects of the biology of male Coopers Hawks (Accipiter cooperii) that began breeding at 1 year of age (SY males) vs. males that began breeding ≥2 years of age (ASY males) in an effort to elucidate what renders breeding of SY males rare. Of the 732 males of known age whose breeding we studied over 32 years (1980–2011) 13 (2%) were SY and 719 (98%) were ASY During this period, the incidence of breeding of SY males changed markedly, as 12 of the instances of breeding of SY males were recorded in the first half of our study, only 1 in the latter half. We suggest that SY males could acquire nest sites more readily in the first half of the study when the breeding population of ASY males was apparently lower. Males breeding in their second year were on average similar in size to ASY males but were molting more extensively than were ASY males. ASY males lived longer than males breeding in their second year, and maximum lifetime production of young was greater, so SY males may have sacrificed greater longevity for breeding early. A strategy of facultative breeding in response to the constraints of higher numbers of older breeding males and restraints of trade-offs in energy allocation may explain why SY male Coopers Hawks rarely breed.

COMPARATIVE RELATIONSHIPS AMONG EYE COLOR, AGE, AND SEX IN THREE NORTH AMERICAN POPULATIONS OF COOPER'S HAWKS

Abstract Although adult plumage in Coopers Hawks (Accipiter cooperii) exhibits little or no sexual dichromatism, iris color reportedly changes from yellow or light orange in younger birds to shades of orange or red in older birds, especially in males. However, there is little quantitative data on this phenomenon. It has been suggested that male eye color may serve as a signal of age and hence reproductive fitness and thus offer a basis for nonrandom mating in Coopers Hawks. In this study we examine the relationships between eye color and age, sex, and reproductive output for Coopers Hawks in two breeding populations in British Columbia and North Dakota, 1999–2002, and compare these results to those previously published for a breeding population in Wisconsin, 1980–1995. Coopers Hawks in British Columbia and North Dakota appear to acquire darker orange or red irides more frequently and more quickly than their counterparts at known and relative ages in Wisconsin. Females in all study sites are slower and less likely than males to acquire the darkest eye colors. Eye color is not a reliable predictor of age in individual male and female Coopers Hawks, for researchers and perhaps for the birds themselves, because individual hawks of a given eye color displayed variation in known and relative ages in British Columbia and Wisconsin. There was no significant relationship between the eye color of males and their brood sizes in any of these three populations, and therefore no discernable support for the premise that male eye color per se signals male fitness, or functions as a sexual trait for assortative mating in this species.

Body Mass of Female Cooper's Hawks is Unrelated to Longevity and Breeding Dispersal: Implications for the Study of Breeding Dispersal

La Masa Corporal de Hembras de Accipiter cooperii No Esta Relacionada Con la Longevidad y la Dispersion Reproductiva: Implicaciones Para El Estudio de la Dispersion Reproductiva Capturamos, marcamos individualmente y pesamos 271 hembras reproductoras de segundo ano de Accipiter cooperii durante la epoca de cria en dos areas de estudio principales en Wisconsin durante el periodo comprendido entre 1980 y 2007. Luego recapturamos o volvimos a avistar a algunas de las aves utilizando prismaticos o telescopios y las identificamos a traves del color de sus anillas. Para analizar la relacion de la longevidad de la hembra con el tamano, examinamos la masa corporal de adultos reproductores recapturados o avistados nuevamente por categoria de edad relativa durante el ultimo ano de deteccion. Encontramos que la masa corporal (o tamano) en esta especie no estuvo relacionada con la longevidad ni con la dispersion reproductiva entre, o la fidelidad con, las areas de nidificacion en Wisconsin. Aunque previamente demostr...