In Hoffman and Smith (2003), we summarized two decades of raptor migration count data from western North America. McCaffery and McIntyre (2005) offer an extensive critique of the “conclusions” we drew from these data concerning the migration ecology and status of western Golden Eagles (Aquila chrysaetos). Many of their specific points about data limitations are well taken. Contrary to the flavor of their critique, however, we were not offering any definitive conclusions about the status or habits of eagles. Rather, we merely sought to describe the tendencies in the data and offer reasonable speculation about potential underlying causes of the documented patterns and trends. Our primary goal was to challenge colleagues to help us carefully consider our data and help formulate reasonable hypotheses about causal factors. We welcome and applaud McCaffery and McIntyre's (2005) thorough review of our work and genuine concern for guarding against unwarranted speculation. We believe, however that the depth of their critique was unjustified because the precision of the migration count data we presented simply is not sufficient to support detailed inspection of every annual change. Moreover, while we cannot disagree with them concerning the limits of the migration data we presented, we stand firmly behind our contention that there are indeed reasons to be concerned about the status of Golden Eagles in western North America, particularly within the sagebrush-steppe ecoregion.
¿Es el Estatus de Aquila chrysaetos Especialmente Preocupante de Acuerdo a los Conteos Migratorios del Oeste de los Estados Unidos?: Respuesta a McCaffery y McIntyre
Resumen. En un trabajo previo (Hoffman and Smith 2003), resumimos datos sobre conteos de migración de rapaces realizados en el oeste de Norte América. McCaffery y McIntyre (2005) presentan una extensa crítica de las “conclusiones” que sacamos a partir de esos datos con respecto a la ecología de migración y el estatus de la especie Aquila chrysaetos. Muchos de sus argumentos específicos sobre las limitaciones de los datos son acertados. Sin embargo, en contraste con el tono de su crítica, en nuestro estudio no ofrecimos conclusiones definitivas sobre el estatus o los hábitos de A. chrysaetos. En cambio, simplemente quisimos describir las tendencias de los datos y presentar especulaciones razonables sobre las posibles causas de los patrones y tendencias documentados. Nuestro principal objetivo era desafiar a nuestros colegas para que nos ayudaran a considerar nuestros datos cuidadosamente y a formular hipótesis razonables sobre los factores causales. Por tanto, recibimos con beneplácito la revisión exhaustiva de nuestro trabajo hecha por McCaffery y McIntyre (2005) y su interés genuino en evitar especulaciones no fundamentadas. Sin embargo, creemos que la profundidad de su crítica no es justificable porque la precisión de los datos de los conteos de migración que presentamos es simplemente insuficiente para permitir la inspección detallada de todos los cambios anuales. Más aún, aunque no podemos estar en desacuerdo con ellos en cuanto a las limitaciones de los datos que presentamos, mantenemos firmemente nuestro argumento de que de hecho existen razones para estar preocupados por el estatus de A. chrysaetos en el oeste de Norte América, particularmente en la eco
Our primary objectives in publishing Hoffman and Smith (2003) were to present for the first time in the peer-reviewed literature a multisite, multispecies review of two decades of raptor migration count data from western North America, to draw attention to notable patterns and trends in the data, and to offer reasonable speculation about the underlying causes of those trends. McCaffery and McIntyre (2005) offer an extensive critique of the “conclusions” we drew from these data concerning the migration ecology and status of western Golden Eagles (Aquila chrysaetos). We do not offer here an equally detailed, point-by-point rebuttal of their criticisms, because we believe that is unwarranted. We do sincerely appreciate their interest in our work, however, and applaud their thorough review and genuine concern for guarding against unwarranted speculation in the pages of peer-reviewed scientific journals.
In fact, our primary goal in seeking to publish this paper was to challenge colleagues to help us carefully consider our data, help formulate reasonable hypotheses concerning the underlying causes of patterns and trends in the data, and ultimately seek ways to test these hypotheses to improve our ability to properly interpret the indicated trends, and thereby stimulate appropriate conservation action when needed. We genuinely value the constructive criticism of our colleagues, and hope it will stimulate further work so that more definitive answers will be forthcoming.
Monitoring continues annually at each of the project sites represented in our paper, as well as at several other sites around the West where datasets have now exceeded, or are quickly approaching, a decade in length. Analyses of these datasets will be updated and expanded in a few years, at which point several of the datasets will exceed two decades in length and provide better coverage across multiple population cycles of primary Golden Eagle prey species. In addition, more sophisticated statistical models will be available to increase the precision of our trend estimates, and significant new information about the species' migratory habits (on-going satellite-tracking studies) and status in the West (a growing body of regional productivity and nest survey information), will be available to help improve both the quality of the data and our ability to interpret them accurately.
Many of McCaffery and McIntyre's (2005) points, such as concern for qualifying the importance of site-specific results based on count volume and the sparse spatial coverage of available migration data, are well taken. Contrary to the flavor of their critique, however, we were not offering any definitive conclusions about the status or habits of eagles, or any other species for that matter. Rather, we merely sought to describe the tendencies in the data and offer what we thought— based on our collective experiences, knowledge of each species, and review of other pertinent literature— was reasonable speculation about the potential underlying causes of the documented patterns and trends. Perhaps we did not pay enough attention to qualifying the speculative conclusions and interpretations we offered as just that, speculation. However, we believe that the depth of McCaffery and McIntyre's (2005) critique was unjustified because the data simply do not warrant such close inspection; that is, the precision of the data is not sufficient to support detailed inspection of every annual change. Instead, we focused only on broad, multiyear patterns, looking for commonalities across sites, and attempting to reconcile the longer-term patterns and trends we discerned in our data with other available, albeit equally limited in most cases, information about the species' status and ecology in western North America.
As discussed at length in the opening sections of the paper, a wide variety of both intrinsic (e.g., observer skill and effort) and extrinsic (e.g., weather and variable migration behavior) factors may influence the accuracy and precision of long-term raptor migration count data. Long-term methodological standardization is critical for reducing the influence of intrinsic confounders, but we have no control over the complex of extrinsic factors that may apply. There is the potential, however, to use sophisticated statistical tools to model the apparent influence of variables such as weather. In fact, HawkWatch International (HWI), the Hawk Migration Association of North America, and Hawk Mountain Sanctuary in Pennsylvania are currently partnering to develop and explore the efficacy of more sophisticated multivariate models for calculating annual indices from migration count data, analyzing long-term trends in site-specific indices, and using a network of sites to assess regional and continental population trends. Development of improved statistical tools will increase the precision of the annual indices derived from migration counts (in terms of reflecting true population trends) and improve our ability to accurately track long-term patterns and trends in those indices.
With the application of better analytical tools in combination with longer and more diverse datasets (by 2006 HWI will have three additional 10-year datasets), we expect that our ability to accurately discern regional patterns and trends in western migration count data will improve markedly. However, because the movement ecology of Golden Eagles is complex and still poorly understood for many regional populations, truly accurate assessments of the species' regional status and population trends will require more research to clarify how Golden Eagle migration and dispersal behavior varies across the continent and from year to year. Some of the needed research is underway in the form of satellite tracking of juvenile birds from Alaska and of birds outfitted at several migration sites in the western U.S.
In general, the long-term monitoring data currently available are far from adequate to accurately assess the current regional status and trends of Golden Eagles in western North America. In particular, we heartily agree with McCaffery and McIntyre (2005) that the spatial extent of available migration-monitoring data in western North America is still too limited. Unfortunately, financial support for significantly expanding the network is hard to come by. Nevertheless, HWI recently began a new long-term, monitoring project in southwest Wyoming where autumn Golden Eagle counts are as high or higher than at any other site in the western U.S. outside of Montana (HWI, technical reports available at www.hawkwatch.org), and the Rocky Mountain Eagle Research Foundation continues to explore western Canada for potential new eagle migration-monitoring sites. In addition, in the past five years HWI has established a new large-scale nesting study in the northern Great Basin that currently includes ∼130 Golden Eagle nesting territories (HWI, tech. reps.); new Golden Eagle nesting surveys are underway in New Mexico; and the U.S. Fish and Wildlife Service recently contracted for a large-scale aerial survey of Golden Eagles in the western U.S. (Good et al. 2004). Thus, both monitoring and movement-ecology databases for this species are growing and, with time, improved understanding of Golden Eagle ecology will enhance our collective ability to accurately interpret migration count trends and assess this species' status and trends in western North America.
In the meantime, two 25–30 year nesting studies representing substantial numbers of breeding territories in Idaho (Steenhof et al. 1997) and Utah (K. R. Keller, unpubl. data) clearly point to long-term declines in nesting activity that are strongly correlated with widespread loss and degradation of native sagebrush-steppe habitat and attendant declines in the abundance of black-tailed jackrabbits (Lepus californicus) throughout the Intermountain region. Moreover, data derived in conjunction with these studies clearly document three distinct 10–11-year cycles in the abundance of jackrabbits in the Intermountain region. Our Goshute Mountains migration data show a clear correspondence between cyclic crashes in jackrabbit abundance (and periods of minimal eagle nesting activity) in 1985 and 1994, and high migration counts of adult eagles in 1985–1986 and 1994–1996 (Hoffman and Smith 2003).
Thus, although we accept McCaffery and McIntyre's (2005) criticism that the same cyclic pattern is not as apparent in data from other migration sites, and that we may therefore have over-generalized our speculative conclusions, we firmly stand by our hypothesis that cyclical periods of low eagle nesting success due to jackrabbit population crashes result in upswings in adult migration counts at the Goshutes because such conditions stimulate greater movement of adult eagles within the Intermountain region. Similarly, despite their thorough discounting of our hypothesis, we also stand firmly behind our contention that, at least in the Goshutes in the heart of the Intermountain region, multiyear declines in counts of younger birds precede periods of high adult activity. One problem here is that, due to historic limitations concerning in-flight identification of different age classes (recently overcome through improved knowledge of plumage differentiation [Clark 2001]) our categorization of younger birds combines four age classes, rather than allowing for a strict comparison between the abundance of first-year birds and adults. This undoubtedly “muddies” the comparison and may preclude a clear demonstration of the hypothesized relationship based on our migration data. Nevertheless, another sharp decline in counts of immature/subadult birds has occurred in the Goshutes between 2000 and 2004 corresponding to the expected onset of another cyclical jackrabbit low (HWI, tech. rep.); should another spike in adult counts follow in the next 2–3 years, we will be even more confident in the validity of our hypothesis.
In summary, regardless of whether or not the migration data we presented in our paper, or other long-term monitoring datasets like the Breeding Bird Survey and Christmas Bird Count (both of which have marginal utility in assessing regional population trends of Golden Eagles due to poor coverage in prime eagle habitat and generally small sample sizes), provide strong corroborating data, there is little doubt that Golden Eagle populations in the Intermountain West are suffering from long-term habitat degradation. A long-term nesting study in southern California also points to significant declines in the abundance of nesting eagles (J. D. Bittner and J. Oakley, unpubl. data). Moreover, HWI migration data, now collected through 2004, continue to point to significant long-term declines in Golden Eagle migration counts in the Bridger Mountains, Montana, and in the Grand Canyon, Arizona, and the count in the Wellsville Mountains, Utah, dropped to a record low in 2004 (HWI, tech. reps.). Accordingly, while we cannot disagree with McCaffery and McIntyre (2005) concerning the limits of the migration data we presented in our paper and definitely agree that more spatially diverse data and a better overall understanding of the species' movement ecology would be very helpful, we stand firmly behind our contention that there are indeed reasons to be concerned about the status of Golden Eagles in western North America, particularly within the sagebrush-steppe ecoregion (Knick and Dyer 1997, Knick et al. 2003), and that we should be paying close attention to monitoring their regional status and trends.