Journal of Field Ornithology J. Field Ornithol. 84(3):223–233, 2013
DOI: 10.1111/jofo.12022
Range sizes and habitat use of non-breeding Crested Caracaras in Florida James F. Dwyer,1,3,4 James D. Fraser,1 and Joan L. Morrison2 1
Department of Fish and Wildlife Conservation, Virginia Tech, 100 Cheatham Hall, Blacksburg, Virginia 24061, USA 2 Department of Biology, Trinity College, 300 Summit Street, Hartford, Connecticut 06106, USA 3 Current address: EDM International, Inc., 4001 Automation Way, Fort Collins, Colorado 80525, USA Received 2 December 2011; accepted 25 April 2013
ABSTRACT. In Florida, habitats that include the breeding territories of Crested Caracaras (Caracara cheriway) are protected, but non-breeding individuals may be vulnerable because they may occupy different areas and habitats. We captured and radio-tagged 58 non-breeding caracaras in Florida from July 2006–March 2009, determined their locations during weekly flights, and used GIS and compositional analysis to evaluate range sizes and habitat use. Non-breeding caracaras (N = 58) ranged five times more widely during breeding seasons (N = 573 locations) than during non-breeding seasons (N = 592 locations), and ranged >250 times more widely than breeding caracaras that defend territories year-round. The large ranges of non-breeders suggest they may be searching for and evaluating prospective territories or breeding opportunities (territory prospecting). Pasture occupied by cattle was the most used habitat relative to availability and was used more than pasture without cattle, likely because insects associated with cattle are an important food source for caracaras. Cattle numbers in Florida are declining and, because both breeding and non-breeding caracaras primarily occupy pasture, this may present difficulties for long-term management. Citrus groves were also used more than expected given availability by non-breeding caracaras, but are rarely included in nesting territories. Because pasture and citrus were often adjacent, we suggest that citrus groves may function as refugia from socially dominant breeding caracaras. Conservation and recovery efforts for Florida’s caracara population are needed throughout the range of non-breeders, and should include management that ensures availability of habitat matrices of cattle pasture and citrus groves. ´ de a´ reas utilizadas por individuos no-reproductivos RESUMEN. Uso de h´abitat y extension de Caracara cheriway en Florida En Florida, el h´abitat que incluye el territorio reproductivo del Caracara crestado (Caracara cheriway) est´a protegido, pero los individuos no-reproductivos, pudieran ser vulnerables, debido a que estos ocupan h´abitats y a´reas variadas. De julio del 2006 a marzo del 2009, capturamos y le colocamos radiotransmisores a 58 individuos noreproductivos de caracaras en Florida, y determinamos sus localizaciones durante sus vuelos semanales. Utilizamos GIS y an´alisis composicional para evaluar la amplitud de su distribuci´on y uso de h´abitat. Los individuos noreproductivos (N = 58) se encontraron cinco veces m´as ampliamente distribuidos durante la e´poca de reproducci´on (N = 573 localidades) que fuera esta (N = 592 localidades), y sus movimientos fueron m´as amplios que los caracaras reproductivos los cuales defienden el territorio a trav´es de todo el a˜no. El mayor uso de espacio por los no-reproductores sugiere que estos muy bien pudieran estar buscando o evaluando territorios prospectivos y/o oportunidades para reproducirse. Los pastizales utilizados por ganado fue el h´abitat disponible m´as utilizado y fue usado en mayor grado que los pastizales sin ganado, tal vez debido a que hay insectos asociados al ganado que son fuentes importantes de alimento para los aves. La cantidad de ganado en Florida se est´a reduciendo, y dado el caso de que tanto las caracaras reproductivos como los no-reproductivo est´an utilizando pastizales con ganado, esto muy bien pudiera representar dificultades, a largo plazo, para el manejo de los caracaras. A´ereas con c´ıtricos tambi´en fueron usados, en mayor grado que lo esperado, pero raras veces utilizado como territorio de anidamiento. Dado el caso de que los pastizales y las a´reas con c´ıtricos con frecuencia est´an adyacentes, sugerimos que las siembras de c´ıtricos sirven de refugio de individuos reproductivos socialmente dominantes. Esfuerzos por conservar y recobrar la poblaci´on de caracaras de Florida son necesarios a todo lo largo de las a´reas utilizadas por los no-reproductivos, y debe incluir manejo que asegure la disponibilidad de h´abitat de pastizales con ganado y sembrados de c´ıtricos. Key words: Caracara cheriway, floater, Florida, raptor, seasonal range
Delayed maturation may evolve when the probability of successful breeding is higher after the first year of life, when there is intense com4
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petition for resources, and when the penalty for losing in resource competition is high (Hawkins et al. 2012). These criteria apply to many raptors, and raptor life cycles commonly include one or more years between independence and establishment of breeding territories (Newton
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1998, Kenward et al. 2000). During this period, juveniles disperse from natal territories, immatures develop adult plumage, and adults search for breeding territories as floaters (Morrison and Wood 2009, Penteriani and Delgado 2009, Blas et al. 2011). Such floaters would presumably enter the breeding population if breeding sites were available (Newton and Rothery 2001, Penteriani et al. 2008). Non-breeders are often secretive and found in habitats not used by breeding birds (Newton 1992, Sergio et al. 2009, Schindler et al. 2012), perhaps because territory holders exclude non-breeders from high-quality habitats (e.g., Schindler et al. 2012). If habitats used by non-breeders and breeders differ, non-breeding individuals may not be fully protected by conservation measures focused on maintaining habitat for breeders (Dwyer 2010). Because survival of non-breeders can affect long-term population persistence (Rohner 1996, Penteriani et al. 2005, 2011), data concerning the ranges and habitats used by non-breeders should be incorporated into recovery and conservation planning (Ferrer and Harte 1997, Morrison and Wood 2009, Penteriani et al. 2011). Crested Caracaras (Caracara cheriway; hereafter caracaras) occur from northern South America through the southwestern United States, with an apparently isolated population in Florida (Morrison and Dwyer 2012). Breeding caracaras have been well studied in Florida (Morrison and Humphrey 2001, Morrison and Dwyer 2012, Dwyer et al. 2012b), less studied in Texas (Dickinson and Arnold 1996, Actkinson et al. 2007) and Baja California (RiveraRodr´ıguez and Rodr´ıguez-Estrella 1998), and little studied elsewhere. Non-breeding caracaras have been studied only recently and only in Florida (Morrison et al. 2008, Dwyer 2010, Dwyer et al. 2012a). Breeding caracaras are nonmigratory, occupying nesting territories year round. Most nesting (96%) is initiated from October to March, with 61% of nesting initiated from December to January (Morrison 1999). Young birds typically depart natal territories 4– 6 mo after fledging (Morrison and Dwyer 2012). Non-breeding caracaras in juvenile, immature, and adult plumage are highly social and form mixed-age groups that regularly forage together and share communal roosts that can contain up to 300 individuals (Dwyer 2010). Age at first breeding can be as early as 3 yr and includes birds
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in immature plumage (Nemeth and Morrison 2002). However, some individuals persist in adult plumage without establishing breeding areas for at least 3 yr (Dwyer et al. 2012a), apparently because all breeding habitat is occupied. Non-breeding birds in adult plumage (floaters) have been described only in the Florida population, but it is not known whether this is because floaters do not occur in other populations or because other populations are less well studied. The Florida population of Crested Caracaras is Federal and State listed as threatened and is believed to be declining due to habitat loss (USFWS 1987, Logan 1997, Morrison and Dwyer 2012). Conservation and recovery planning has focused on their breeding range and habitat because no quantitative data exist describing habitat use by non-breeders. To be as effective as possible, conservation and recovery planning must also include ranges and habitats occupied by non-breeding caracaras (Nemeth and Morrison 2002, Dwyer 2010, Dwyer et al. 2012a). Thus, our objectives were to determine the areas and habitats used by non-breeding caracaras in Florida. METHODS
Our study area was a 43,000-km2 area that included the entire known range of caracaras in Florida (35,000 km2 ; Morrison and Dwyer 2012) and surrounding areas (Fig. 1). From July 2006 to October 2008, we sought nonbreeding caracaras throughout our study area by driving public roads any place caracaras were reported by contacts in the birding and regulatory communities. Much of the habitat occupied by caracaras in Florida is privately owned (Morrison and Humphrey 2001) and birds moved more rapidly between properties than we could secure permission to access them. Thus, because caracaras regularly consume carrion (Morrison et al. 2008), we used a trap baited with carrion to trap caracaras along the sides of public roads wherever we found apparently nonbreeding individuals. Breeding status and age classes. We determined breeding status based on individual range sizes. Morrison and Dwyer (2012) report ∼90% of telemetry locations of breeding adult caracaras (N = 28) occur within 2.5 km of their nest sites, and all locations occur within 9 km. Therefore, we assumed that caracaras
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Ranges and Habitat of Non-breeding Caracaras
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Fig. 1. Locations (N = 1166) of non-breeding Crested Caracaras (N = 58) had a highly clustered distribution, resulting in a more limited and specifically defined range for the species in Florida, July 2006–March 2009. The 1990s range estimate is based on USFWS 1999. Background map layers provided by the Florida Geographic Data Library (www.fgdl.org).
that moved ≥10 km within a season were nonbreeding. We aged captured caracaras based on plumage, with juveniles having streaked breasts, immatures having breasts with a mix of streaking and barring, and adults having barred breasts (Morrison and Dwyer 2012). Caracaras transition from juvenile to immature plumage at about 12 mo, and attain adult plumage in 2 to >4 yr (Voous 1983, USFWS 1999, Nemeth and Morrison 2002). Because our study area was mostly privately owned and individual birds moved widely, we rarely saw birds after we
marked them. Thus, we could not determine when individuals transitioned from juvenile to immature plumage and from immature to adult plumage. Consequently, we combined all non-breeders for analyses. We determined the sex of each captured caracara by using DNA obtained from blood samples (Avian Biotech International, Tallahassee, FL). To collect blood, we inserted a needle into the ulnar vein and withdrew 0.2–0.6 ml. Telemetry. We used VHF-radio tags to track non-breeding caracaras (American Wildlife Enterprises, Monticello, FL; Holohil,
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Carp, ON, Canada; Advanced Telemetry Systems, Isanti, MN; Wildlife Materials, Murphysboro, IL). Tags weighed 30 g (250 times larger than reported previously for breeding caracaras (mean = 15.5 km2 ; Morrison and Dwyer 2012) and were particularly large during the breeding season. We hypothesize that non-breeding caracaras move in search of or to evaluate prospective territories or breeding opportunities (territory prospecting) during the breeding season. Prospecting is expected
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Table 2. Results from compositional analysis comparing proportional use of land cover types by 44 nonbreeding Crested Caracaras (N = 1076 telemetry locations) to proportional availability of those land covers (N = 1040 random locations) in Florida, July 2006–March 2009. Ranks are shown according to randomization tests with 1000 iterations. Used habitatb
Land a
Cover OPAST CITRU UPAST GRASS PALMH ROWCP FORSS URBAN OPH2O WETLD
Proportion (95% CI) 0.40 (0.34–0.45) 0.20 (0.14–0.27) 0.16 (0.13–0.19) 0.07 (0.05–0.11) 0.04 (0.02–0.05) 0.06 (0.04–0.09) 0.04 (0.03–0.06)
