Implications of Prebasic and a Previously Undescribed Prealternate Molt For Aging Rusty Blackbirds

Abstract

Abstract. Aging birds often relies on differences in plumage between immatures and adults, and understanding these patterns can improve our ability to discern demographic patterns within populations. We investigated patterns of prebasic molt of the Rusty Blackbird (Euphagus carolinus) in fall at Whitehorse, Yukon Territory, and developed a new technique for aging based on characteristics of the head plumage acquired during prebasic molt. Furthermore, we investigated the possibility of a prealternate molt among wintering Rusty Blackbirds on the basis of captures in Mississippi and museum specimens from across the species' winter range. Finally, we examined how a prealternate molt might affect the aging of winter birds by plumage. Rusty Blackbirds completed their prebasic molt by the end of September, and immature birds had a more prominent eye ring and a paler chin than adults, allowing a reliable age determination. Previously, the Rusty Blackbird was thought to attain its breeding plumage through feather wear exclusively, but we discovered a partial prealternate molt in our examinations of live captures (76% molting) and museum specimens (59% molting). The prealternate molt was observed in all age and sex classes, was concentrated along the feather tracts of the head, and peaked in occurrence from mid-February to mid-March, when nearly 90% of birds were molting. Between mid-December and mid-February, the prealternate molt did not appear to interfere with aging birds in the hand by the pattern of the eye ring and chin in basic plumage. Age determination later in the spring, however, remains to be investigated.Key words: age determination, Euphagus carolinus, Icteridae, molt limit, prealternate molt, prebasic molt, Rusty Blackbird. Implicancias de la Muda Prebasica y de la Previamente No Descripta Muda Prealterna para Determinar la Edad de Euphagus carolinusResumen. La determination de la edad de las aves a menudo se realiza a partir de diferencias de plumaje entre individuos inmaduros y adultos. Entender estos patrones puede mejorar nuestra capacidad de distinguir patrones demograficos dentro de las poblaciones. Investigamos patrones de muda prebasica de Euphagus carolinus durante el otono en Whitehorse, Territorio Yukon, y desarrollamos una nueva tecnica para determinar la edad basada en las caracteristicas de las plumas de la cabeza adquiridas durante la muda prebasica. Ademas, investigamos la posibilidad de una muda prealterna entre individuos de E. carolinus invernantes basandonos en capturas en Mississippi y en especimenes de museo del rango de distribution de invierno de la especie. Finalmente, examinamos como una muda prealterna podria afectar la determination de la edad de las aves invernales a partir del plumaje. E. carolinus completa su muda prebasica para fines de septiembre, y las aves inmaduras tienen un anillo ocular mas prominente y una barbilla mas palida que los adultos, permitiendo una determination confiable de la edad. Anteriormente, se pensaba que E. carolinus alcanzaba su plumaje nuptial exc...

Habitat Occupancy by Rusty Blackbirds Wintering in the Lower Mississippi Alluvial Valley

“…we found that wintering Rusty Blackbirds (both individuals and flocks) often occupied swamps and bottomland forests with varying amounts of surface water and that in 2008 site occupancy by individuals was higher in wet bottomland forests than in the other habitat types. this pattern reflects observations across the winter range, where Rusty Blackbirds have been found foraging in small groups on the ground, in shallow water, or low in vegetation (Beal 1900, Bent 1958, dickson and nobel 1978, avery 1995 in swamps, wet woodlands, bottomland forests, thickets bordering wetlands, or the edges of ponds and creeks in arkansas (Meanley 1972), the lower colorado River (Rosenberg et al 1991), georgia (Burleigh 1958), louisiana (dickson and nobel 1978), Mississippi (turcotte and watts 1999, Mettke-hofmann et al 2010), ohio (trautman 1940, South carolina (Sprunt and chamberlain 1970), and Virginia (Meanley 1995). wintering Rusty Blackbirds' occupancy of a variety of types of bottomland forest may be a function of the species' winter diet, which includes similar amounts of invertebrates and seeds and berries (Beal 1900, Martin et al 1951.…”

Rusty Blackbird: Mysteries of a Species in Decline

“…data). Mettke-Hofmann et al (2010) examined patterns of molt, finding that Rusty Blackbirds in fresh basic plumage in their first fall can be distinguished in the field from older birds on the basis of a distinct pale eye ring and chin. However, a previously undocumented pre-alternate molt during winter is concentrated on the head feathers and obscures most of these age-related differences by late winter.…”

Rusty blackbird patch use during winter in suburban landscapes

“…Each captured blackbird received a unique United States Geological Survey (USGS) aluminum band and color‐band combination for identification. We recorded sex, and aged blackbirds as hatching year by the presence of a pale eye‐ring and a lighter chin, or after hatching (Mettke‐Hofmann et al ).…”

“…Each captured blackbird received a unique United States Geological Survey (USGS) aluminum band and color-band combination for identification. We recorded sex, and aged blackbirds as hatching year by the presence of a pale eye-ring and a lighter chin, or after hatching (Mettke-Hofmann et al 2010). We attached transmitters (Holohil, Ontario, Canada; BD2, 1.2 g, duration 45 days) to 29 rusty blackbirds with a synsacrum harness (Rappole and Tipton 1991) using 1-2-mm plastic jelly cord.…”