BROWN-HEADED COWBIRD (Molothrus ater) BHCO Sample size: 1,746 |
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Perhaps best known as a brood parasite, the Brown-headed Cowbird is a short-distance migrant ranging throughout North America. Cowbird parasitism has caused much concern in terms of declining songbird populations since hosts of this parasite have limited success in raising their own young [Terborgh 1989]. Historically, cowbirds bred in the short-grass plains in association with buffalo [Lowther 1993 (BNA)]. Since European settlement of North America and the consequent clearing of forests, cowbirds have spread throughout the U.S. and southern Canada, breeding in a multitude of habitats [Mayfield 1965]. The species requires some open grassland, thus preferring forest edges, second growth, pastures, and residential areas; unfragmented forests are usually avoided [Lowther 1993 (BNA)]. Cowbirds winter from New England in the east, through the southern states to central Mexico [Lowther 1993 (BNA)]. In the west, their northern wintering range is restricted to the coast and southern Arizona and New Mexico [Lowther 1993 (BNA)]. In Massachusetts, towards the northern extent of the wintering range, cowbirds are most abundant in the Connecticut River valley in winter months [Veit and Petersen 1993].
Large winter flocks move north early in the spring. Cowbirds often occur in mixed-species flocks of blackbirds both in winter and during migration [Bent 1958, Lowther 1993]. Goddard [1971] reported a maximum of 100,000 cowbirds in a mixed species roost in November in Oklahoma. Migrants arrive in Massachusetts in March and April [Veit and Petersen 1993] and reach their northernmost breeding areas by early to mid-April [Bent 1958]. The first individuals to arrive are males, followed by females [Bent 1958]. Migrants precede residents and older birds arrive before younger ones [Bent 1958]. During their diurnal migration, cowbirds forage in fields during the day and often roost in wetlands at night [Bent 1958, Lowther 1993].
In the fall, cowbirds form large, wandering flocks well in advance of migration [Bent 1958]. When egg laying is complete (late June in Massachusetts [Veit and Petersen 1993]), the adults gather in flocks in fields and pastures [Bent 1958]. Young birds join the flocks once they are able to fly [Bent 1958]. Migration does not begin until September [Bent 1958], with peak counts of migrants occurring in October in Massachusetts [Veit and Petersen 1993].
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Observations
This rather common open-country species clearly and significantly preferred A sites along the main Connecticut River. Very few were recorded in any state in the more closed canopy B and C sites. In the CT data there was a peak (possibly earlier migrant males) in periods 1 and 2, and a second peak (possibly later-arriving females) noted in period 4.
The southerly distribution of cowbirds was also clearly documented; many in CT, some in MA and very few in NH or VT. The greatest transect total was 40 recorded in period 1 in an A site in CT, 1996. Most circle counts were single birds, with a maximum flock size of 10 recorded in two circles in A sites in CT, 1997. |
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BHCO Map
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