Robert N. Rosenfield

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.

Serologic Evidence of West Nile Virus Infection in Three Wild Raptor Populations

Abstract We assayed for West Nile virus (WNV) antibodies to determine the presence and prevalence of WNV infection in three raptor populations in southeast Wisconsin during 2003–04. This study was conducted in the framework of ongoing population studies that started before WNV was introduced to the study area. For 354 samples, 88% of 42 adult Coopers hawks (Accipiter cooperii), 2.1% of 96 nestling Coopers hawks, 9.2% of 141 nestling red-tailed hawks (Buteo jamaicensis), and 12% of 73 nestling great horned owls (Bubo virginianus) tested positive for WNV antibodies by the constant virus–serum dilution neutralization test. Samples that tested positive for WNV antibodies were collected across a wide variety of habitat types, including urban habitats (both high and low density), roads, parking areas, recreational areas, croplands, pastures, grasslands, woodlands, and wetlands. Based on the increased prevalence and significantly higher WNV antibody titers in adults compared with nestlings, we suggest that nestlings with detectable antibody levels acquired these antibodies through passive transmission from the mother during egg production. Low levels of WNV antibodies in nestlings could serve as a surrogate marker of exposure in adult raptor populations. Based on breeding population densities and reproductive success over the past 15 yr, we found no apparent adverse effects of WNV infections on these wild raptor populations.

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.

Prey of Peregrine Falcons breeding in west Greenland

Previous studies on the diet of Peregrine Falcons (Falco peregrinus) in Greenland were based primarily on prey remains, an indirect technique that may produce biased results. Earlier estimates of prey biomass are too general and speculative to be conclusive. No other study provided data on the age of prey species or showed that Rock Ptarmigan (Lagopus mutus) can be an important component in the diet of Greenland peregrines. We used direct observations of prey deliveries and collection of prey remains to assess prey (in terms of frequency of occurrence and biomass) of breeding Peregrine Falcons in West Greenland. We also determined prey selection in relation to local prey availability. During 492 hr of observation at two eyries we found that four small passerines-Lapland Longspur (Calcarius lapponicus), Northern Wheatear (Oenanthe oenanthe), Common Redpoll (Carduelis flammea), and Snow Bunting (Plectrophenax nivalis)-contributed over 80% of the prey items delivered, with the longspur being the principal prey species in terms of frequency of occurrence and biomass. However, ptarmigan contributed almost as much biomass as longspurs at one eyrie. Fledglings of all these bird species provided the majority of items and biomass at both eyries. Analysis of 676 prey items from prey remains at 159 eyries showed similar frequencies and rankings for the prey species comprising the majority of the diet of Peregrine Falcons as determined by prey deliveries. Lapland Longspur was taken in proportion to its availability near two eyries; ptarmigan and Snow Buntings were taken more frequently than expected. Local differences in prey use (especially ptarmigan) were found.

RELATIONSHIP BETWEEN NEST SUCCESS AND CONCEALMENT IN TWO GROUND-NESTING PASSERINES

Abstract We examined vegetation characteristics around nest sites of the Hermit Thrush (Catharus guttatus) and Ovenbird (Seiurus aurocapillus) for three summers in the northern-and mixed-hardwood forests of northern Wisconsin and the Upper Peninsula of Michigan to test the hypothesis that nests with greater concealment are less vulnerable to depredation. Because these two ground-nesting passerines differ in terms of nest structure and behavior near the nest, they present an opportunity to examine how these two factors influence reproductive success in sympatric species. Depredation was the most common source of nest failure for both species, with Ovenbirds having higher nest success for data pooled across years. Side (but not overhead) concealment was correlated positively with nest success for the Hermit Thrush but not for the Ovenbird. Side and overhead concealment did not differ between the Hermit Thrush and Ovenbird. We found a substantial proportion of nests in ground pine (Lycopodium obscurum). Hermit Thrush, but not Ovenbird nests in ground pine were significantly more concealed than nests in other sites both from the side and overhead. Vegetative concealment at the nest microsite may be more important to the open-cup nest of the Hermit Thrush than to the domed nest of the Ovenbird. Because flushed Ovenbirds perform a distraction display and Hermit Thrushes do not, Ovenbirds may have an incentive to choose nest sites that offer less than maximum concealment.

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

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Colonization, Growth, and Density of a Pioneer Cooper's Hawk Population in a Large Metropolitan Environment

Abstract Coopers Hawks (Accipiter cooperii) have recently colonized many urban landscapes across North America, but data on breeding densities and trends in densities of these populations are lacking. We surveyed for woodland raptors throughout approximately 1000 km2 in the metropolitan Milwaukee, Wisconsin, area over a 21-yr period, 1988–2008. We documented the natural colonization of this urban landscape by a pioneer Coopers Hawk population and its subsequent growth from 1993–2008 (4 to 41 laying pairs, 4 to 55 occupied sites). Nearest-nest distances decreased and the number of Coopers Hawk laying pairs increased while nesting surveys remained consistent temporally and spatially, indicating that density of breeding pairs was increasing in the metropolitan Milwaukee area. Approximately 15 yr after initial colonization, the breeding density of Coopers Hawks in some localized areas averaged one laying pair per 330 ha (range: 68–587 ha). From 1996–2008, as breeding density increased, average annual productivity (number of young/laying pair) for Coopers Hawks in Milwaukee County, a subset of the overall larger metropolitan study area, also increased. During the early years of colonization, a relatively high proportion of individuals or pairs of birds appeared to occupy nest sites but did not breed. Younger birds may have played a role in the colonization of this urban landscape. This population was likely increasing at a relatively rapid rate during the late 1990s and continued to increase throughout the remainder of our study.

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.