Multi-Isotopic (δ2H, δ13C, δ15N) Tracing of Molt Origin for Red-Winged Blackbirds Associated with Agro-Ecosystems

Abstract

We analyzed stable-hydrogen (δ2H), carbon (δ13C) and nitrogen (δ 15N) isotope ratios in feathers to better understand the molt origin and food habits of Red-winged Blackbirds (Agelaius phoeniceus) near sunflower production in the Upper Midwest and rice production in the Mid-South of the United States. Outer primary feathers were used from 661 after-second-year (ASY) male blackbirds collected in Minnesota, Montana, North Dakota and South Dakota (spring collection), and Arkansas, Louisiana, Mississippi, Missouri and Texas (winter collection). The best-fit model indicated that the combination of feather δ2H, δ13C and δ15N best predicted the state of sample collections and thus supported the use of this approach for tracing molt origins in Red-winged Blackbirds. When considering only birds collected in spring, 56% of birds were classified to their collection state on the basis of δ2H and δ13C alone. We then developed feather isoscapes for δ13C based upon these data and for δ2H based upon continental patterns of δ2H in precipitation. We used 81 birds collected at the ten independent sites for model validation. The spatially-explicit assignment of these 81 birds to the δ2H isoscape resulted in relatively high rates (~77%) of accurate assignment to collection states. We also modeled the spatial extent of C3 (e.g. rice, sunflower) and C4 (corn, millet, sorghum) agricultural crops grown throughout the Upper Midwest and Mid-South United States to predict the relative use of C3- versus C4-based foodwebs among sampled blackbirds. Estimates of C3 inputs to diet ranged from 50% in Arkansas to 27% in Minnesota. As a novel contribution to blackbird conservation and management, we demonstrate how such feather isoscapes can be used to predict the molt origin and interstate movements of migratory blackbirds for subsequent investigations of breeding biology (e.g. sex-specific philopatry), agricultural depredation, feeding ecology, physiology of migration and sensitivity to environmental change.

Matching geographical assignment by stable isotopes with African non-breeding sites of barn swallows Hirundo rustica tracked by geolocation

“…The method has been successfully applied to many terrestrial species and regions world wide (Europe/Asia: [59], North America: [60], South America: [61] and is very efficient to track individuals or carry-over effects in large sample sizes allowing for robust conclusions (i.e. [62]).…”

Multi-isotopic (δ2H, δ13C, δ15N) tracing of molt origin for European starlings associated with U.S. dairies and feedlots

“…We assumed this would be directly related to the proportion of corn in diets of starlings and largely reflect their dependence on agricultural based products. Following Werner et al [1], we assumed that a starling feeding on a 100% C3 diet would have a feather δ 13 C value of -26‰ and one feeding on a 100% C4 diet would have a feather δ 13 C value of -11‰. This allowed a simple two-endpoint, one isotope mixing model to estimate percent C4 in the diet.…”

“…Dispersal and movements of migratory birds on breeding and non-breeding grounds are fundamental to planning conservation and management efforts, both nationally and internationally [1]. Overabundant species, invasive species and agricultural depredation caused by some wild birds pose considerable management challenges [2].…”

“…The primary objective of this study was to use δ 2 H, δ 13 C and δ 15 N measurements in feathers of European starlings collected at U.S. dairies and feedlots to better understand their molt origin and interstate movements. We expected δ 2 H and δ 13 C measurements to provide greatest resolution for spatial assignments but included δ 15 N analyses because this isotope is also sensitive to land-use practices and other anthropogenic factors that may vary across our study area [1,19,20]. We included each of δ 2 H, δ 13 C and δ 15 N measurements because previous studies have demonstrated best-fit modeling associated with a multi-isotopic approach to the analysis of feather isotopes in pest birds [1].…”

The Future of Blackbird Management Research

“…Although diet and molt pattern in relation to agricultural damage have been evaluated for red-winged and yellow-headed blackbirds, updated data would elucidate changes occurring with changing habitat, climate, and agricultural practices (Linz et al 1983;Twedt et al 1991; Twedt and Linz 2015; Chapter 2, this volume). Further investigations into migration, molt patterns, and food habits can be evaluated using stable isotope markers to understand the full annual cycle of blackbirds at a continental scale (Werner et al 2016). An understanding of species' biology, such as molt, migration, habitat use, diet, dispersal, survival, and reproductive success, could link different periods in the annual cycle and lead to new approaches for managing conflict with blackbirds.…”