Using stable isotopes to estimate reliance on agricultural food subsidies and migration timing for a migratory bird

Abstract

Abstract Anthropogenic activities have adversely transformed terrestrial ecosystems consequently limiting many species to more fragmented areas and increasing human–wildlife conflicts. Under some circumstances, this creates a need for management programs to support wildlife populations by subsidizing food resources. Evaluation and improvement of supplementary feeding practices should be implemented to determine dietary importance of supplementary food and identify when to make food resources available, an important consideration for migratory species using seasonal habitats. Large aggregations of greater sandhill cranes ( Antigone canadensis tabida ) wintering in the Middle Rio Grande Valley of central New Mexico have come into conflict with agricultural practices. Resulting crop depredation on private lands has consequently required a mitigation program that subsidizes cranes with cultivated corn to manage their foraging behavior and provide nutritive support. To assess dependency of cranes on corn subsidies and estimate arrival dates of migratory sandhill cranes, we measured stable isotope ratios of liver and muscle tissues of sandhill cranes and their food items during winter. Over 60% of sandhill crane diet in the winter came from corn subsidies. Rates of carbon isotope incorporation in liver and muscle tissues were 0.03 d −1 ± 0.02 (mean ± SE ) and 0.02 d −1 ± 0.01, respectively, and differed predictably by metabolic activity of different tissues. Estimated arrival date on wintering grounds derived from rates of carbon isotope incorporation was November 6 ± 3 d (mean ± SE ) and was within 17 d of the estimated arrival date on the wintering grounds of sandhill cranes equipped with satellite transmitters (November 23 ± 2 d). Our approach demonstrates a field‐based application of intrinsic biomarkers to inform supplementary feeding practices for wildlife populations by identifying dietary response to supplementary food. Additionally, estimating arrival on wintering grounds supports management and conservation decisions by synchronizing availability of supplementary food resources with arrival times.