Dispersal is a behavioral process having consequences for individual fitness and population dynamics. Recent advances in technology have spawned new theoretical examinations and empirical studies of the dispersal process in birds, providing opportunities for examining how this information may be applied to studies of the dispersal process in raptors. Many raptors are the focus of conservation efforts; thus, reliable data on all aspects of a species' population dynamics, including dispersal distances, movement rates, and mortality rates of dispersers, are required for population viability analyses that are increasingly used to inform management. Here, we address emerging issues and novel approaches used in the study of avian dispersal, and provide suggestions to consider when developing and implementing studies of dispersal in raptors. Clarifying study objectives is essential for selection of an appropriate methodology and sample size needed to obtain accurate estimates of movement distances and rates. Identifying an appropriate study-area size will allow investigators to avoid underestimating population connectivity and important population parameters. Because nomadic individuals of some species use temporary settling areas or home ranges before breeding, identification of these areas is critical for conservation efforts focusing on habitats other than breeding sites. Study designs for investigating raptor dispersal also should include analysis of environmental and social factors influencing dispersal, to improve our understanding of condition-dependent dispersal strategies. Finally, we propose a terminology for use in describing the variety of movements associated with dispersal behavior in raptors, and we suggest this terminology could be used consistently to facilitate comparisons among studies.

Natal dispersal (i.e., movement of wandering individuals from their birthplaces to their first breeding locations) is one of the most important yet least understood features of ecology, population biology, and evolution. The analysis of the dispersal process from the perspective of the unusual life-histories of raptors (typically less-studied species, because of the difficulties inherent in working with them) may enhance our understanding of some additional aspects of dispersal. Here we comment on the potential offered by studies on raptor natal dispersal, in particular: (a) the importance of floaters in the dynamics, stability, and persistence of breeding populations; (b) the complexity of dispersal as a multistep process characterized by several shifts in behavior; (c) the importance of studying patterns of animal movement, which influence the patterns of population spatial structure; (d) the crucial conservation implications of dispersal studies.

Juvenile raptors often travel thousands of kilometers from the time they leave their natal areas to the time they enter a breeding population. Documenting movements and identifying areas used by raptors before they enter a breeding population is important for understanding the factors that influence their survival. In North America, juvenile Gyrfalcons (Falco rusticolus) are routinely observed outside the species' breeding range during the nonbreeding season, but the natal origins of these birds are rarely known. We used satellite telemetry to track the movements of juvenile Gyrfalcons during their first months of independence. We instrumented nestlings with lightweight satellite transmitters within 10 d of estimated fledging dates on the Seward Peninsula in western Alaska and in Denali National Park (Denali) in interior Alaska. Gyrfalcons spent an average of 41.4 ± 6.1 d (range  =  30–50 d) in their natal areas after fledging. The mean departure date from natal areas was 27 August ± 6.4 d. We tracked 15 individuals for an average of 70.5 ± 28.1 d post-departure; Gyrfalcons moved from 105 to 4299 km during this period and tended to move greater distances earlier in the tracking period than later in the tracking period. Gyrfalcons did not establish temporary winter ranges within the tracking period. We identified several movement patterns among Gyrfalcons, including unidirectional long-distance movements, multidirectional long- and short-distance movements, and shorter movements within a local region. Gyrfalcons from the Seward Peninsula remained in western Alaska or flew to eastern Russia with no movements into interior Alaska. In contrast, Gyrfalcons from Denali remained in interior Alaska, flew to northern and western Alaska, or flew to northern Alberta. Gyrfalcons from both study areas tended to move to coastal, riparian, and wetland areas during autumn and early winter. Because juvenile Gyrfalcons dispersed over a large geographic area and across three international boundaries, conservation efforts should focus on both regional and international scales.

In long-lived raptors with delayed maturity, the period between fledging and settlement on a breeding site may take several years and is poorly understood. In our study of a reintroduced population of White-tailed Eagles (Haliaeetus albicilla), we investigated movements during this transient (juvenile dispersal) phase of natal dispersal using records of wing-tagged birds. Maximum juvenile dispersal distance (JDD), as measured from natal (or release) sites to locations recorded prior to birds settling on a breeding site ranged from 18 to 200 km in different individuals. Observation records suggested that (1) the most extensive movements occurred in the first two years after fledging, (2) males initially dispersed further than females, but, in their second year, females tended to be further from their natal site than males, and (3) as breeding age approached, males were closer to their natal sites than were females. Maximum JDD did not differ between wild-bred and released (reintroduced) birds and there was no indication that it was correlated with subsequent recruitment to the breeding population. We also found no evidence that a bird's fledging date, body size, or the brood size in which it was fledged affected maximum JDD. There was no suggestion that Scottish White-tailed Eagles showed seasonally dependent returns to their natal areas. Maximum JDD during the first two years after fledging and natal dispersal distance (i.e., straight-line distance between natal and first breeding sites) were positively correlated in both females and males. The implication of these results was that, well before reaching maturity, White-tailed Eagles used dispersal movements, at least in part, to assess potential breeding sites, and thus dispersal behaviors were not simply directed toward survival alone.

During 1987–89, we placed VHF transmitters and colored identification bands on 15 nestling Bald Eagles (Haliaeetus leucocephalus) in central Arizona to determine the direction and extent of their post-fledging migration. Thirteen of the juveniles fledged successfully, and one transmitter failed; 11 eagles survived the post-fledging, premigration period which ranged from 18 to 65 d (mean  =  44 d). One eagle was lost by trackers after departure, but 10 monitored eagles traveled north from 925 km to 1955 km before stopping for extended periods or until weather prevented further tracking. Habitats varied among stopping locations and included interior reservoirs and lakes, open ranchland, and the Pacific marine coast. Food at inland destinations included spawning fish (cutthroat trout [Salmo clarki]) and fish carrion (common carp [Cyprinus carpio] and white suckers [Catostomus commersoni]). At least nine of 13 fledged juveniles survived ≥1 yr (69%), and a minimum of six (46%) survived to breeding age.

I used band recovery data to examine distances between banding and recovery locations for 154 nestling Florida Bald Eagles and discuss the implications for understanding natal dispersal and philopatry in this species. Band recoveries occurred in 23 U.S. states and five Canadian provinces between 1931–2005. Recovery distance from the natal nest averaged longer for the youngest age classes (ANOVA: F  =  3.59; df  =  5, 153; P  =  0.005), for individuals banded in earlier decades (F  =  1.94; df  =  5, 153; P  =  0.093), and for the months of May through October (F  =  3.10; df  =  12, 153; P < 0.001). Of 35 individuals classed as mature (≥3.9 yr old when recovered; range 3.9–36.5 yr), 31 were located within Florida, which suggested a strong degree of philopatry to the natal state. Among 21 mature eagles of known sex with known banding and recovery locations in Florida, females, particularly younger birds, had longer recovery distances (N  =  9, mean  =  93 km, SE  =  22.4) than did males (N  =  12, mean  =  31 km, SE  =  5.3; t  =  2.67, df  =  19, P  =  0.026). The records examined here suggest a high degree of philopatry and relatively short natal dispersal distances, particularly in male Bald Eagles.

In 2004–05, we evaluated seasonal variation in the diet of White-tailed Kites in a suburban area of Concepción, southern Chile. Kites' diet was determined based on pellets (N  =  209) and consisted of rodents, insects and birds (95.6%, 13.8% and 0.6% of all consumed individual prey [N  =  343]). Olivaceous field mice (Abrothrix olivaceus) and long-haired mice (A. longipilis) were the most common prey (47.5% and 32.4% of all prey by frequency, respectively; 40.3% and 38.9% by biomass, respectively). Rodents were important in the diet in all seasons (frequency  =  88.6–100%, biomass  =  98.8–100%), and the ranking of rodent prey species in the diet was virtually the same for all seasons (χ²  =  4.6, P > 0.05). The geometric mean weight of vertebrate prey was also similar among seasons (27.0–28.6 g). The frequency of rodent prey species in the pellets was positively correlated to rodent prey abundance in the environment, as measured using live-trapping surveys (r_s  =  0.83, P < 0.05). Our results corroborate previous findings that the White-tailed Kite is a rodent-specialist predator and suggest that in our study area it behaves as an opportunistic predator (i.e., capturing rodent prey in proportion to their availability).

La filopatría y la capacidad de dispersión representan aspectos interrelacionados, variables y determinantes de las dinámicas poblacionales de una especie. No obstante, la información disponible sobre estos procesos es escasa para especies amenazadas como Neophron percnopterus debido a las dificultades logísticas que implican estos estudios. Esta especie es considerada altamente filopátrica, para la cual se han descrito distancias medias de dispersión de aproximadamente 20 km. Presentamos el primer caso documentado de dispersión efectiva a gran distancia (550 km) que implica el movimiento de un individuo a una población distante. Una hembra, anillada como pichón en el norte de España, fue encontrada en Francia, 550 km al norte de su área natal, en dos años consecutivos (2006 y 2007). Esta hembra fue presumiblemente reclutada por la población reproductiva distante de su pareja. Proponemos que la tendencia de la especie de congregarse en dormideros comunales, donde se reúnen individuos de diferentes poblaciones, puede propiciar la formación de parejas de individuos originarios de distintas poblaciones. La combinación de la atracción social y la filopatría natal de ambos individuos parece haber determinado el patrón espacial de dispersión y la localización del territorio de asentamiento de la pareja.

Ketupa blakistoni es una especie amenazada que alguna vez estuvo ampliamente distribuida en Hokkaido, la isla principal más septentrional de Japón, pero actualmente presenta una población estimada de 35 parejas reproductivas con una distribución muy local. Detecté tres casos de endogamia estrecha durante un estudio de 20 años de duración sobre una familia de esta especie. El primer caso sucedió entre un macho y su cría hembra, después de la pérdida de su pareja anterior. Esta pareja padre-hija produjo siete crías entre 1992 y 2003. La tasa de eclosión de los huevos de esta pareja fue sólo del 41.2%, sustancialmente menor que la de la pareja original (100%). El segundo caso de endogamia sucedió poco después de la pérdida del macho original de la misma pareja en 2003. Su nieto, que había eclosionado en 1997, apareció y se apareó con su abuela. Esta pareja abuela-nieto produjo un volantón hembra en 2005. El último caso sucedió entre dos hermanos en un lugar distante de su territorio natal. El macho y la hembra habían emplumado en 1994 y 2000, respectivamente, del mismo nido y compartían los mismos padres. Ellos produjeron un volantón en 2004. Propongo que la posible reducción en el éxito de eclosión asociada con parejas de individuos cercanamente emparentados podría ser un factor de preocupación en relación con la conservación de esta especie amenazada.

El riesgo potencial de la competencia o la depredación por parte de individuos coespecíficos o congéneres podría influenciar el comportamiento de las aves y moldear la estructura de las comunidades de rapaces. Comparé la tasa de presencia de Falco columbarius en la ensenada de Totten, Washington, en dos períodos de tiempo: en la década de 1980 (cuando F. peregrinus era una especie rara) y entre 1999 y 2005 (cuando la abundancia de F. peregrinus había aumentado). Registré siete veces más a F. columbarius en las visitas realizadas entre 1980 y 1988 que en las realizadas entre 1999 y 2005. En contraste, no vi ningún F. peregrinus entre 1980 y 1988, pero entre 1999 y 2005 observé a esta especie con una frecuencia un poco mayor en comparación con la de F. columbarius en el mismo período. El cambio en la presencia de F. columbarius coincidió con un incremento en la abundancia de F. peregrinus. Propongo la hipótesis de que la presencia de F. peregrinus ha influenciado el comportamiento de F. columbarius, que es una especie de presa potencial para el halcón más grande.

En estudios de aves rapaces, la habilidad para identificar individuos correctamente es facilitada por la utilización de anillas de aluminio. Debido a que las aves rapaces generalmente descansan sobre líneas eléctricas, las anillas de aluminio pueden contribuir al riesgo de electrocución, o pueden servir como indicadoras de electrocución en cadáveres intactos. Para comprobar estas posibilidades, marcamos 89 individuos de Parabuteo unicintus en Tucson, Arizona, U.S.A. con anillas de aluminio o de plástico y comparamos las heridas y causas de muerte en 23 individuos recogidos. No encontramos diferencia en la tasa de electrocución en 41 individuos marcados con anillas de aluminio y 38 individuos marcados con anillas de plástico, y no encontramos quemaduras en las anillas o en las patas alrededor de las anillas. En general, las hembras y los adultos fueron más propensos a ser encontrados muertos por electrocución. Nuestros datos no apoyan la hipótesis de que las anillas de aluminio aumentan el riesgo de electrocución de las rapaces, pero sí concuerdan con otros estudios que identifican al sexo y la edad como factores de riesgo.

Circus pygargus es bastante abundante en el sudoeste de Europa (península ibérica y Francia), pero es raro en la península italiana, donde ha sido clasificado como “amenazado.” En este estudio, documentamos 241 intentos reproductivos de individuos de C. pygargus de la población del este y centro de Italia desde 1987 a 2005. Medimos el tamaño de la nidada, la tasa de eclosión, la tasa de emplumamiento y el éxito reproductivo. También investigamos el hábitat de anidación y la distribución agregada de los nidos. Encontramos que esta población de C. pygargus ha disminuido en términos de parámetros reproductivos y de abundancia, incluyendo el éxito reproductivo y la tasa de emplumamiento. Más aún, observamos una disminución en la fracción de parejas que anidan en las áreas agrícolas (cultivos). Estas disminuciones pueden estar relacionadas a una escasez general de recursos alimenticios y a una alta mortalidad durante la cosecha de los cultivos. Nuestros resultados destacan la importancia de proteger los ambientes naturales para conservar esta especie a escala nacional.

En Taiwán, se conoce poco acerca del comportamiento reproductivo de Spizaetus nipalensis, un ave de bosques que se encuentra amenazada. Descubrimos cinco nidos entre 2005 y 2007. Entre marzo y junio de 2007, hicimos observaciones en los tres nidos activos, que estaban ubicados en montañas de elevaciones bajas en el sur de Taiwán. Los machos trajeron el 76.4% (DE: ±21.5%) de los ítems alimenticios a los nidos. Con base en el color del iris, inferimos que los machos de las tres parejas eran más jóvenes que sus consortes. En términos de frecuencia, las presas llevadas a los nidos fueron mamíferos (76.1 ± 10.8%), aves (15.9 ± 8.2%) y reptiles (7.6 ± 6.1%). De forma inesperada, las ardillas voladoras nocturnas (Petaurista philippensis y P. alborufus) fueron las presas más importantes en los tres nidos. Cuatro macacos Macaca cyclopis jóvenes también fueron llevados a los nidos.