Abstract
Migratory species are rapidly declining but we rarely know which periods of the annual cycle are limiting for most species. This knowledge is needed to effectively allocate conservation resources to the periods of the annual cycle that best promote species recovery. We examined demographic trends and response to human footprint for Canada warblers (Cardellina canadensis), a threatened Neotropical migrant, using range-wide data (1993–2016) from the Monitoring Avian Productivity and Survivorship (MAPS) program on the breeding grounds. Declines in abundance were steepest in the eastern breeding region, followed by the western region. Breeding productivity did not decline in any region. In contrast, we observed declining recruitment in all regions, low apparent survival in the east and west, and a decline in apparent survival in the east. Abundance declined with increasing disturbance around MAPS stations. Between 1993 and 2009, the human footprint index on the breeding range increased by 0.11% in contrast to a 14% increase on the wintering range. Landscape-scale disturbance on the breeding grounds may influence abundance in some regions; however, the observed trends in demography and footprint suggests limitation during the non-breeding period as the likely driver of overall declines, particularly for eastern populations.
Highlights
Worldwide, many populations of migratory animals are undergoing rapid declines[1,2]
We used a Bayesian approach to analyze temporal trends in demography and response to anthropogenic footprint by Canada warblers across the species breeding range
Using 24 years of data (1993–2016) collected from the Monitoring Avian Productivity and Survivorship Program[25] (MAPS), we first asked how adult breeding density, breeding productivity, adult apparent survival and recruitment varied over time in west, central and eastern portions of the breeding range (Fig. 1)
Summary
Many populations of migratory animals are undergoing rapid declines[1,2]. These declines prompt concern for the general loss of biodiversity per se and for how such losses might influence ecological function[3]. Most studies have been conducted over small spatial scales and typically do not reveal why populations are declining across broad regions; addressing this question requires studies at spatial scales covering the distribution of a species Research at such broad scales is challenging, but the combination of long-term and spatially extensive datasets (e.g. Monitoring Avian Productivity and Survivorship Program[20]; North American Breeding Bird Survey11) with knowledge of the change in factors such as land use or climate can allow us to identify the mechanisms that influence annual variability or long-term trends of populations across broad spatial scales[6,7,21]. We estimated the change in human footprint in the Magdalena/Cauca basins and the eastern slopes of the Andes in Colombia since Canada warblers over-wintering in these regions primarily bred in west/central and eastern parts of the breeding range, respectively[23]
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