Where do Swainson's hawks winter? Satellite images used to identify potential habitat

Abstract

During recent years, predictive modelling techniques have been increasingly used to identify regional patterns of species spatial occurrence, to explore species–habitat relationships and to aid in biodiversity conservation. In the case of birds, predictive modelling has been mainly applied to the study of species with little variable interannual patterns of spatial occurrence (e.g. year‐round resident species or migratory species in their breeding grounds showing territorial behaviour). We used predictive models to analyse the factors that determine broad‐scale patterns of occurrence and abundance of wintering Swainson's hawks (Buteo swainsoni). This species has been the focus of field monitoring in its wintering ground in Argentina due to massive pesticide poisoning of thousands of individuals during the 1990s, but its unpredictable pattern of spatial distribution and the uncertainty about the current wintering area occupied by hawks led to discontinuing such field monitoring. Data on the presence and abundance of hawks were recorded in 30 × 30 km squares (n = 115) surveyed during three austral summers (2001–03). Sixteen land‐use/land‐cover, topography, and Normalized Difference Vegetation Index (NDVI) variables were used as predictors to build generalized additive models (GAMs). Both occurrence and abundance models showed a good predictive ability. Land use, altitude, and NDVI during spring previous to the arrival of hawks to wintering areas were good predictors of the distribution of Swainson's hawks in the Argentine pampas, but only land use and NDVI were entered into the model of abundance of the species in the region. The predictive cartography developed from the models allowed us to identify the current wintering area of Swainson's hawks in the Argentine pampas. The highest occurrence probability and relative abundances for the species were predicted for a broad area of south‐eastern pampas that has been overlooked so far and where neither field research nor conservation efforts aiming to prevent massive mortalities has been established.

Migration Patterns, Use of Stopover Areas, and Austral Summer Movements of Swainson's Hawks

“…The areas occupied with large numbers of Swainson’s Hawks in northern La Pampa Province during our study were virtually devoid of Swainson’s Hawks. Also much of the pasturelands, which were mainly used by the Swainson’s Hawks during our study (Canavelli et al 2003), have been converted to row crops (Sarasola et al 2008b). Transformation of Argentinean agriculture from a system of range-based livestock production to one of intensive agricultural cultivation could affect Swainson’s Hawks negatively (Woodbridge et al 1995).…”

“…How Swainson’s Hawks respond to environmental and land use changes on the austral grounds is largely unknown. Record high precipitation during the summers of 1997 and 2000 through 2002 resulted in a distinct change in the distribution of Swainson’s Hawks on the austral summer grounds (Canavelli 2000, Sarasola et al, 2008b, J. Sarasola pers. com., M. Bechard pers.…”

Vegetation dynamics and avian seasonal migration: clues from remotely sensed vegetation indices and ecological niche modelling

“…Vegetation indices have been used in remote sensing studies to estimate canopy characteristics, such as leaf area index, and chlorophyll and water content (Houborg et al, 2007). In ENM, the normalized difference vegetation index (NDVI; Tucker, 1979) has been used successfully, as vegetation seasonality and type parameter, to address diverse challenges (Osborne et al, 2001;Roura-Pascual et al, 2004;Peterson et al, 2005;Cayuela et al, 2006;Osborne & Suárez-Seoane, 2007;Sarasola et al, 2008;Ferreira de Siqueira et al, 2009;); more recently, leaf area index (LAI; Chen et al, 1997) has been used to estimate vegetation productivity in studies predicting species' distributions in the Neotropics (Buermann et al, 2008;Prates-Clark et al, 2008).…”

APalaearctic migratory raptor species tracks shifting prey availability within its wintering range in theSahel

“…As leaf‐eating grasshoppers depend on green vegetation, we used vegetation greenness (NDVI) as a proxy for grasshopper densities and vegetation cover, and thus food availability for Montagu's harriers, similar to studies in Swainson's Hawk Buteo swainsoni , which consume grasshoppers during non‐breeding periods in South America (Johnson, Nickerson & Bechard ; Sarasola & Negro ; Sarasola et al . ).…”