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

Abstract

Citation: Boggie, M. A., S. A. Carleton, D. P. Collins, J. Vradenburg, and C. J. Sroka. 2018. Using stable isotopes to estimate reliance on agricultural food subsidies and migration timing for a migratory bird. Ecosphere 9(2):e02083.10. 1002/ecs2.2083 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 À1 AE 0.02 (mean AE SE) and 0.02 d À1 AE 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 AE 3 d (mean AE SE) and was within 17 d of the estimated arrival date on the wintering grounds of sandhill cranes equipped with satellite transmitters (November 23 AE 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.

Land Use, anthropogenic disturbance, and riverine features drive patterns of habitat selection by a wintering waterbird in a semi-arid environment

“…Corn, for example, contributes proportionally the most to sandhill crane diets [ 38 ] and provides a highly metabolizable carbohydrate [ 86 ] used to maintain and store energy reserves [ 54 ]. Managed moist-soil wetlands are flooded seasonally and also contain nutritionally valuable plant-based resources (e.g., alkali bulrush [ Bolboschoenus maritimus ], yellow nutsedge [ Cyperus esculentus ], [ 37 , 38 ]), as well as provide a source of protein via invertebrate communities [ 87 ]. Together these land-use types supply sandhill cranes with forage resources and areas with minimal disturbance to carry out diurnal activities while conserving energy.…”

“…The proportion of time spent foraging increased throughout winter, leading up to spring migration in mid-February ( Fig 7C ). Ramping up foraging (i.e., hyperphagia) prior to an energetically expensive event, such as migration, is a common strategy in migratory birds [ 90 , 91 ], and reiterates the importance of public lands and land managers synchronizing food resources with timing of highest population needs [ 38 ], and providing sandhill cranes with a consistent resource base until spring departure.…”

“…Our modeling results identified a high proportional abundance (88%) of perennial crops (i.e., alfalfa and hay pasture) such that their spatially static nature of occurrence discarded the need to reproduce annual crop distribution estimates. To supplement this assumption, we surveyed the study area each season to identify changes in more dynamic annual crop fields known to be food resources valued by wintering sandhill cranes (i.e., corn [ 38 ]). The surveys involved visiting general areas where grain crops were suspected (e.g., dairies), or known (received information from public land managers), to be grown on private lands to confirm their presence, and examining clustering of diurnal GPS locations of sandhill cranes tagged with satellite transmitters to identify and visit agricultural areas that were frequently used and determine crop types of these areas.…”

“…Drought, flood control, water diversion or damming, and human water consumption have drastically altered the natural hydrology of the Rio Grande [ 15 ], influencing habitat availability for sandhill cranes. Transition away from grain-based crops, an important dietary component for wintering sandhill cranes [ 38 ], have likely influenced the availability and quality of food resources [ 39 , 40 ]. Several national wildlife refuges and state managed properties that have grown into important public lands serve a dual purpose within the MRGV; they support migratory birds throughout winter and mitigate human-wildlife conflicts by reducing crop depredation by sandhill cranes on private lands.…”

The Importance of Isotopic Turnover for Understanding Key Aspects of Animal Ecology and Nutrition

“…A final example of the utility of isotopic clocks is a study conducted by Boggie et al [114], in which the authors both estimated the timing of arrival by migratory sandhill cranes (Antigone canadensis) on wintering grounds in the Middle Rio Grande valley of New Mexico and their use of anthropogenic subsidies there. Using carbon isotopic clocks for liver and muscle, the authors estimated that, on average, this population of cranes arrived on the wintering grounds in early November and subsequently relied on corn grown on state and federal lands for approximately 60% of their diet, resources that would otherwise likely be acquired by foraging on agricultural land.…”