Weather affects diet composition of rusty blackbirds wintering in suburban landscapes

Abstract

The rusty blackbird (Euphagus carolinus) is a species of conservation concern throughout its range and the cause of the species' population decline is unknown. We studied diet composition of rusty blackbirds with stable isotope mixing models in suburban landscapes in the southeastern United States. We captured blackbirds in Georgia and South Carolina from 2009 to 2012, and estimated proportions of earthworm, other animals, pecan, and acorn incorporated into individual diets. On the Piedmont Plateau, terrestrial and aquatic earthworms constituted the largest proportion incorporated into the diet (39% AE 2.9; mean AE SD by site and year) and animals other than earthworms (mostly larval invertebrates Odonata and Diptera) constituted 27% AE 12.9. In contrast, on the Coastal Plain, which featured milder winters than the Piedmont, earthworms constituted a lower proportion (19% AE 1.2) of incorporated food items and animals other than earthworms comprised 62% AE 3.3% of the diet. Increased incorporation of earthworms in the diet was related to increased upcoming precipitation and daily maximum temperature. Rusty blackbirds incorporated more tree mast into their diet on the Piedmont Plateau than the Coastal Plain. Increased incorporation of tree mast was related to advancing cold temperature. Mast, including crushed pecans (Carya illinoenensis) and pre-opened small-seeded red oak (Quercus spp.) acorns, is a high-lipid dietary component of blackbirds wintering in colder climates, and is incorporated prior to extreme cold weather. Therefore, planting mast trees, especially lipid-rich pecan, could be used to augment resources in known rusty blackbird wintering hotspots. Maintaining shallowly flooded wetlands with a fluctuating water regime and residential lawns with abundant red oak (Quercus spp.) leaf litter would promote acorn and invertebrate resources including earthworms. Ó 2015 The Wildlife Society.

Rusty blackbird patch use during winter in suburban landscapes

“…). Short‐term weather variables were related to diet composition in the same study sites in a concurrent study (Newell Wohner et al ) and can affect the availability of wetland invertebrates (Neckles et al , Wehrle et al , Magee et al , Batema et al ).…”

“…We developed a priori models based on our predictions that rusty blackbirds would increase their use of wetlands after precipitation events when invertebrates are concentrated, and during warm weather when invertebrate larvae are most available (Fredrickson and Batema , Batema et al ). We predicted increased use of residential lawns with increased precipitation because rusty blackbird diets were comprised of more earthworms with precipitation (Newell Wohner et al ), and terrestrial earthworms are the predominant resource in residential lawns (Fig. ; Neckles et al , Corcoran et al ).…”

“…These patch aggregations include fragments of shallowly flooded forested wetlands and creeks, residential areas with ornamental oak and pecan ( Carya illinoinensis ) trees, and abandoned or partially harvested pecan groves (Luscier et al , DeLeon , Borchert , Mettke‐Hofmann et al , Newell Wohner et al ). The species has adjusted to suburban landscapes (Meanley , Luscier et al , DeLeon , Newell Wohner et al ). For example, some of the largest and most persistent flocks in the eastern population occur sporadically in suburban areas on the Piedmont Plateau (Hamel and Ozdenerol ).…”

“…However, rusty blackbird flocks have a patchy distribution, are rare, and do not occur in just any suburban landscape. Their irregular distribution across the wintering range may indicate blackbirds are at least somewhat specialized on specific aggregations of foraging patches and corresponding food resources (DeLeon , Borchert , Newell Wohner et al ).…”

“…In suburban landscapes, the availability and use of predominant food resources in discrete foraging patches (i.e., wetlands, residential lawns, and pecan or oak groves) may vary with fluctuations in short‐term air temperature and precipitation (Newell Wohner et al ). High availability of invertebrate prey such as Odonata and Diptera larvae in wetlands is governed by short‐term water fluctuations from precipitation events, flooding, and subsequent water receding (White , Fredrickson and Batema ).…”