Monitoring raptor migration is perhaps one of the most effective tools for tracking changes in the natural world. Ecologically, raptors are located atop many food chains and their populations are sensitive to changes occurring one or several trophic levels below, making them ideal sentinels of ecosystem function (Bildstein 2006).
Diurnal raptors (vultures, hawks, and falcons), however, are naturally rare and their low population densities makes them difficult to detect and appropriately survey. Two continental-scale surveys, the Christmas Bird Count and the Breeding Bird Survey, are the keystones of the landbird monitoring system and fail to provide statistically robust data for most raptor species (Farmer et al. 2007).
The migration season offers the best opportunity to monitor their populations. Raptors migrate during daytime and are influenced by geographic features that constrain their migratory movements to predictable routes. Shorelines, for example, divert and concentrate the flights of migrants reluctant to cross over large bodies of water. Conversely, mountain ridges that possess conditions for energy-saving flights create pathways that lead migrants to follow them. Migration counts have been validated as an appropriate method to detect changes in migratory bird populations.
In terms of efficiency, a single migration monitoring site can track a substantially large proportion of the regional or continental population of a given species. Migration monitoring sites in the southern United States routinely tally between 1-30% of the entire North American population of species such as Broad-winged Hawk or Turkey Vulture.
Last, raptors are large and easy to detect and identify relative to other species of birds, and have attracted the attention of the bird watching public for decades. This recreational activity has been turned into a conservation opportunity in many locations where volunteer citizen scientists collect standardized records of their migration.
The Florida Keys Hawkwatch (FKH) is the southernmost migration monitoring site in the continental United States. It is strategically located in a region with a very low density of monitoring sites and it is the last location along the Atlantic seaboard where migrants can be tracked before crossing the Gulf into the West Indies. It is also a continental record holder for the migration of Peregrine Falcons, the longest continually operating site in Florida and one of the longest-running sites in the southern half of the country.
Synthesis and Significance
Few sites in the North American migration monitoring network have the conservation and ecological significance of FKH. The magnitude of its migration is not only one of the largest in the country (including migrants of 18 species, Lott 2006, Ruelas 2008), but the most important/among the top-three most important for falcons of three species, and one of the few locations in the country where Mississippi Kites and Short-tailed Hawks can be tracked on migration.
The significance of continuing long-term coverage at FKH grows as the dataset increases in length. The monitoring site at Curry Hammock State Park, founded in 1999 (Lott 2006), has now achieved more than a decade of continuous coverage, a “reasonable” threshold for using datasets to estimate change in populations (Farmer et al. 2007). Support from the following organizations will make the 2011 migration season possible, Tropical Audubon Society, Hawk Migration Association of North America, Florida Keys Audubon Society, Helen G. and Allan D. Cruickshank Research Award – Florida Ornithological Society, Space Coast Audubon Society, Florida Keys Birding & Wildlife Festival and Leica Sport Optics.
Raptor population monitoring at FKH contributes strategically to the coverage of the North American network and its continuity is of a highly significant importance. The appropriate documentation of protocols and seasonal metadata, a recommendation applicable to any migration monitoring site, will help FKH to allow replicability of current practices in the future and serve as a training tool for volunteers, citizen scientists, and staff operating the site.
The adopted population monitoring activities follow the standard protocols for data collection that are in use in more than 200 sites in North America (HMANA 2011). These migration counts consist of hourly records of a series of variables of weather and observation conditions, and its corresponding hourly tally of migrants recorded. These general standards are refined to fit the conditions of the flight at each site; e.g., FKH requires two observers during the field season, its optimal count period occurs between 15 September-15 November (in order to include ≥95% of the migration window for selected species), and specific instructions to document site routines are critical at this site, which experiences unique migration features such as the “return” migration of birds that do not cross open water and need to be adjusted from the seasonal counts, posing a challenge for analyzing data.
For the 2011 migration season, FKH will be monitored by two full-time field biologists that will be the primary staff onsite and will also recruit, train, and organize a small group of volunteers to participate in these counts throughout the season, to cover main observers on their days off and to achieve the target of two staff onsite continuously throughout the field season.
FKH is currently preparing written protocols for data collection in accordance to HMANA recommendations and in the tradition of methodologies published in research reports, e.g. Lott 2006, Smith et al. 2008. These documents will include instructions and the specific operation routines for the site, including start and end times/dates of observations, target species, sex and age class data, collection of records of weather and flight conditions, seasonal metadata, etc., that need to be documented to assist data interpretation.
Migration count data from most monitoring sites in the continent is stored in the online repository HawkCount.org. This electronic archive, managed by HMANA, is federated with other large observational data repositories such as Cornell University’s Avian Knowledge Network and the Global Biodiversity Information Facility, and provides safe and permanent archival with some analytical tools available for the general public, and data accessibility for research and conservation applications. Data from FKH is only partially archived in HawkCount.org, and plan to complete the transfer of all paper and electronic records collected since 1999 to HMANA’s online archives. Educational activities and recreational opportunities will be made available at the monitoring site, aiming to increase public awareness of raptor ecology.
Excerpt from Raptor Population Monitoring in the Florida Keys, Rafael A. Gálvez and Ernesto Ruelas Inzunza, 2011.