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Individual study: Effects of supplemental feeding on timing of breeding, clutch-size and polygyny in red-winged blackbirds Agelaius phoeniceus, Winchester Wasteway, Washington, USA

Published source details

Ewald P.W. & Rohwer S. (1982) Effects of supplemental feeding on timing of breeding, clutch-size and polygyny in red-winged blackbirds Agelaius phoeniceus. Journal of Animal Ecology, 51, 429-450

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Provide supplementary food for songbirds to increase reproductive success Bird Conservation

A replicated, controlled experiment on marshlands sites in Washington state, USA, in the springs of 1977-8 (Ewald & Rohwer 1982), found that red-winged blackbirds Agelaius phoeniceus from territories supplied with supplementary food laid eggs significantly earlier than blackbirds from control (unfed) territories (1977: 17 days earlier in eight territories where feeding began on 31st March, 12 days earlier in five territories fed from 5th April; 1978: 26 days earlier). However, in 1978 clutch size was significantly lower in fed territories (3.4 eggs/clutch, 122 fed clutches vs. 3.6 eggs/clutch for 126 controls), the authors suggest this is due to early-laid clutches being smaller – if clutches laid before April 10th were excluded then clutches were not significantly smaller. Fed territories provided with supplementary food also had higher rates of chick mortality than control territories (numbers not provided) and were invaded by non-territorial red-winged and yellow-headed blackbirds Xanthocephalus xanthocephalus at significantly higher rates than controls in 1978 (4.9 intruders/ha in 33 fed territories vs. 1.1/ha for 54 controls), but not in 1977, or when feeder density was reduced in 1978 (0.8 intruders/ha for 17 fed territories vs. 1.3 intruders/ha for 19 controls). Supplementary food consisted of sunflower seeds, cracked corn and puppy food on 30 cm square trays. In 1977 one tray was placed in each of 13 territories on two marshes (controls were on five other marshes); in 1978, trays were placed on a 6 m grid on a single marsh, a tray density approximately ten times that in 1977, reduced to one feeder in each territory several weeks after females arrived.