Migration efficiency sustains connectivity across agroecological networks supporting sandhill crane migration

Abstract

Preserving avian flyway connectivity has long been challenged by our capacity to meaningfully quantify continental habitat dynamics and bird movements at temporal and spatial scales underlying long-distance migrations. Waterbirds migrating hundreds or thousands of kilometers depend on networks of wetland stopover sites to rest and refuel. Entire populations may rely on discrete wetland habitats, particularly in arid landscapes where the loss of limited stopover options can have disproportionately high impacts on migratory cost. Here, we examine flyway connectivity in water-limited ecosystems of western North America using 108 GPS tagged greater sandhill cranes. Bird movements were used to reconstruct wetland stopover networks across three geographically unique sub-populations spanning 12 U.S.-Mexican states and Canadian provinces. Networks were monitored with remote sensing to identify long-term trends in wetland and agricultural resources supporting migration and evaluated using network theory and centrality metrics as a measure of stopover site importance to flyway connectivity. Sandhill crane space use was analyzed in stopover locations to identify important ownership and landscape factors structuring bird distributions. Migratory efficiency was the primary mechanism underpinning network function. A small number of key stopover sites important to minimizing movement cost between summering and wintering locations were essential to preserving flyway connectivity. Localized efficiencies were apparent in stopover landscapes given prioritization of space use by birds where the proximity of agricultural food resources and flooded wetlands minimized daily movements. Model depictions showing wetland declines from 16% to 18% likely reflect a new normal in landscape drying that could decouple agriculture-waterbird relationships as water scarcity intensifies. Sustaining network resilience will require conservation strategies to balance water allocations preserving agricultural and wetlands on private lands that accounted for 67-96% of habitat use. Study outcomes provide new perspectives of agroecological relationships supporting continental waterbird migration needed to prioritize conservation of landscapes vital to maintaining flyway connectivity.

Functional Wetland Loss Drives Emerging Risks to Waterbird Migration Networks

“…Wetland hydrology and agricultural flooding were monitored using Landsat 5 Thematic Mapper and Landsat 8 Operational Land Imager satellite imagery to depict the timing and duration of wetland surface water. Following an approach outlined by Donnelly et al (2021), surface water conditions were measured monthly (January to December) from 1988 to 2020 as a 5-year running mean beginning in 1984. Normalizing estimates in this way moderated annual climate variability influencing hydrologic conditions (Rajagopalan and Lall, 1998) and improved detectability of long-term trends.…”

“…Although growth has significantly altered most wetland and riparian ecosystems, these systems remain fundamental to biological processes sustaining migratory waterbirds. Waterbirds in some regions have adapted to landscape change by utilizing agricultural food resources and flood irrigation practices to offset historic wetland losses (Elphick and Oring, 2003;Taft and Haig, 2005;Donnelly et al, 2021). Emerging impacts of climate change in these regions raise concerns over the sustainability of continental wetland networks as water scarcity triggers land-use change and ecological effects misaligned with waterbird habitat needs (Haig et al, 2019;Donnelly et al, 2020).…”

Functional wetland loss drives emerging risks to waterbird migration networks

“…Wetland hydrology and agricultural flooding were monitored using Landsat 5 Thematic Mapper and Landsat 8 Operational Land Imager satellite imagery to depict the timing and duration of wetland surface water. Following an approach outlined by Donnelly et al (2021), surface water conditions were measured monthly (January to December) from 1988 to 2020 as a five-year running mean beginning in 1984. Normalizing estimates in this way moderated annual climate variability influencing hydrologic conditions (Rajagopalan and Lall, 1998) and improved detectability of long-term trends.…”

“…Waterbirds in some regions have adapted to landscape change by utilizing agricultural food resources and flood irrigation practices to offset historic wetland losses. (Elphick and Oring, 2003; Taft and Haig, 2005; Donnelly et al, 2021). Emerging impacts of climate change in these regions raise concerns over the sustainability of continental wetland networks as water scarcity triggers land-use change and ecological effects misaligned with waterbird habitat needs (Haig et al, 2019; Donnelly et al, 2020).…”

Importance of flood irrigation for foraging colonial waterbirds

“…Generally, sprinkler-irrigated systems leave much less surface water after the irrigation turns off and this leaves significantly less water to slowly descend through the soil to underlying aquifers. In addition to providing less potential for aquifer recharge, the reduced surface water associated with sprinkler irrigation almost certainly affects habitat suitability for various wildlife species, including wetland birds (Shuford et al 2016, Donnelly et al 2021). Thus, an important component of waterbird conservation is understanding how waterbirds use agricultural fields near wetlands, and how irrigation practices affect forage availability.…”