Fall vegetative cover and summer precipitation predict abundance of wintering grassland birds across the Chihuahuan desert
Introduction
Deserts are by definition ecosystems where biological processes, including wildlife population dynamics, are ultimately limited by water. Relatively few Nearctic migratory birds overwinter in arid lands, preferring instead less water-limited tropical and subtropical environments. The overwintering of migratory grassland birds in arid lands warrants attention given that populations of North American grassland birds continue to decline at alarming rates (Sauer et al., 2017) and the drivers of these declines remain poorly understood for most species. The ongoing loss of grasslands throughout Canada and the United States since the 19th century (White et al., 2000) suggests that loss and deterioration of breeding habitat is a common factor. However, grassland birds that overwinter in the Chihuahuan Desert grasslands of Mexico and the southwestern USA have declined by twice as much as those wintering elsewhere (North American Bird Conservation Initiative, 2016). As in other guilds of migratory birds, increasing evidence suggests factors in the non-breeding (wintering) season can strongly influence population trends (Calvert et al., 2009). Grassland birds that largely depend on Chihuahuan Desert grasslands are threatened by conversion of wintering habitat to farmland (Macías-Duarte et al., 2009; Pool et al., 2014), overgrazing (Curtin et al., 2002), invasive grasses (Sánchez-Muñoz, 2009) and woody plant encroachment (Van Auken, 2000). The extent to which grassland birds will be able to adapt to the loss and deterioration of wintering habitat depends on the suitability of the remaining habitat and the capability of these birds to track resources at a broad scale, especially in light of profound changes in temperature and precipitation projected in arid lands as a consequence of climate change (Seager et al., 2007).
Local winter abundance of grassland passerines is highly variable throughout the Chihuahuan Desert among years (Macias-Duarte et al., 2011). For instance, winter density of passerine birds can reach a 50-fold difference between two years (0.63 vs. 31.82 birds·ha−1) in desert grasslands of central Chihuahua in a 9-year period (Macías-Duarte et al., 2009), and in some years a species may be completely absent from a region where it normally occurs (Macias-Duarte et al., 2011). This extreme temporal variation in local abundance of wintering grassland passerines in the Chihuahuan Desert may reflect the ability of populations to ‘shift’ their distributions to alternate locations with better conditions. More precisely, we hypothesize that spatio-temporal variation of abundance of wintering grassland birds in the Chihuahuan Desert follows the ideal free distribution model (Fretwell, 1972), i.e., grassland birds are able to track the spatial distribution of resources (ideal) and can move without constraints (free) throughout their winter range so that individual birds can maximize their intake rate of resources. This phenomenon has been documented in Palearctic migratory birds on their wintering grounds of the Sahel, a similarly arid environment, and has been called “itinerancy” (Kristensen et al., 2013) or nomadism (Andersson, 1980).
Quantifying the spatio-temporal covariation of bird abundance and environmental factors throughout the Chihuahuan Desert may reveal factors limiting winter survival. Because most grassland passerines are granivorous in winter, feeding on grass and forb seeds, it is likely that the annual production and availability of these foods affects winter distribution and survival. In addition, grass and forb biomass are important components of habitat quality, providing protection from low temperatures at night (Macías-Duarte et al. in review) and from predation (Macías-Duarte and Panjabi, 2013). Therefore, tracking changes in vegetation biomass in this desert environment may lead to being able to predict abundance, distribution and even survival of grassland birds in winter.
In this context, the objective of this study is to determine if plant biomass in grasslands is a predictor of wintering bird abundance in space and time in Chihuahuan Desert grasslands. Grass biomass production in the Chihuahuan Desert is driven by seasonal precipitation (Khumalo and Holechek, 2005) and herbaceous plant growth is particularly synchronized with monsoonal summer rainfall (Moreno-de-las-Heras et al., 2015). A positive relationship between abundance of grassland birds during the winter and precipitation in the preceding summer has been documented in the Chihuahuan Desert at the local scale (Dunning and Brown, 1982; Gordon, 2000; Macías-Duarte et al., 2009), suggesting a direct link between plant biomass and bird abundance at the local scale. If this relationship between plant biomass in grasslands and bird abundance is also true at the regional scale, the information generated by this study could help inform conservation strategies for grassland birds, especially in light of projected increases in aridity in the Chihuahuan Desert due to climate change (Seager et al., 2007).
Section snippets
Study area
We conducted avian surveys in up to 17 Grassland Priority Conservation Areas (GPCA; CEC and TNC, 2005; Pool and Panjabi et al., 2010) in northern Mexico, southern Arizona, southern New Mexico and western Texas in winter from January 2007 to March 2011 (Levandoski et al., 2009; Macias-Duarte et al., 2011; Panjabi et al., 2010). GPCAs included in this study are Armendaris, Cuatro Ciénegas, Cuchillas de la Zarca, Janos, Lagunas del Este, Llano Las Amapolas, Malpaís, Mapimí, Marfa, New Mexico
Results
We found that NDVI values reflect plant biomass levels at bird transects in Chihuahuan Desert grasslands (Fig. 2). We found evidence that ocular estimates of herbaceous cover are a predictor of NDVI values at the bird transect level through a quadratic regression model (extra-sum-of-squares F test, F = 11.40, d.f. = 1, 1715, P < 0.001). However, we also observed a relatively large unexplained variance (R2 = 0.20), likely associated to the additive effects of observers’ variation in ocular
Discussion
The mechanism for habitat selection in migratory land birds has been hypothesized to be a hierarchical scale-dependent process (Hutto, 1985): at the broadest scale, birds are programmed to head along a certain migratory route to a wintering ground, whereas at the local scale, wintering habitat selection involves exploratory assessment and habitat use primarily determined by ecological factors. This hierarchical decision-making process, in theory, may sometimes limit the ability of birds to
Acknowledgments
Financial support was provided by the U.S. Fish and Wildlife Service through the Neotropical Migratory Bird Conservation Act (NMBCA), the USDA Forest Service International Programs, The National Fish and Wildlife Foundation, The Nature Conservancy (TNC), the Commission for Environmental Cooperation, the USDA Rio Grande Research Center at Sul Ross State University, Texas Parks and Wildlife Department, the Sonoran Joint Venture, and the Bureau of Land Management. Implementation of this project
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