Importance of Flood Irrigation for Foraging Colonial Waterbirds

For a colonial-nesting bird, like the white-faced ibis ( Plegadis chihi ), the landscape surrounding the breeding colony can be important. White-faced ibis must rely on areas outside their breeding colony for foraging, but this part of their life history has received little attention, and the management of this landscape even less so. To address this knowledge gap, we conducted road-based driving surveys and a randomly selected, spatially balanced sample survey of agricultural fields within a 22-km radius of the 2 largest white- faced ibis breeding colonies in Idaho, USA: Market Lake Wildlife Management Area and Mud Lake Wildlife Management Area. Our study took place in 2012 and the primary objective was to quantify patterns of foraging habitat use of this marsh-nesting species, particularly associations with specific irrigation practices and crop types. We documented the majority of foraging birds in flood- irrigated and wheel-line sprinkler-irrigated agricultural fields (76%) and natural wetlands (13%), which were limited in our study area (3% of land cover). Even though 70% of the agricultural landscape included center pivot sprinkler irrigation, only 11% of foraging observations came from this irrigation type. Most agricultural fields (>85%) used by foraging ibis were flood-irrigated and all had standing water or recent moisture at the time of use. Though ibis used many crop types when foraging in flooded agricultural fields, ibis use of alfalfa (58%) was greater than availability (38%). We also observed distinct distribution patterns around the 2 breeding colonies. Whereas birds foraged in all directions around Mud Lake (~80%) within a 12-km radius from the colony, we observed over half of birds around Market Lake foraging within 12–22 km, and almost exclusively to the south and southeast of the colony, reflecting the distribution of flood-irrigated agriculture in the area. The most common foraging distance (12–22 km) around Market Lake is greater than found in existing literature, suggesting that the foraging habitat is limited within 12 km of the colony and that the birds may need to travel farther to find adequate foraging habitat. Flood-irrigated agriculture and natural wetlands provide foraging habitats for white-faced ibis in eastern Idaho and should be considered in future management and conservation of wetland birds.