Brown-Headed Cowbird (Molothrus Ater) Doug Powless

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Brown-headed Cowbird (Molothrus ater) Doug Powless

Grandville, Kent Co., MI. 5/4/2008 © John Van Orman

(Click to view a comparison of Atlas I to II)

Brown-headed Cowbirds likely flourished Distribution Brown-headed Cowbirds breed in grassland, alongside the Pleistocene megafauna that once prairie, and agricultural habitats across southern roamed North America (Rothstein and Peer Canada to Florida, the Gulf of Mexico, and 2005). In modern times, flocks of cowbirds south into central Mexico (Lowther 1993). The followed the great herds of bison across the center of concentration and highest abundance grasslands of the continent, feeding on insects of the cowbird during summer occurs in the kicked up, and depositing their eggs in other Great Plains and Midwestern prairie states birds’ nests along the way. An obligate brood where herds of wild bison and other ungulates parasite, the Brown-headed Cowbird is once roamed (Lowther 1993, Chace et al. 2005). documented leaving eggs in the nests of hundreds of species (Friedmann and Kiff 1985, Wild bison occurred in southern Michigan and Lowther 1993). The evolution of this breeding across forest openings in the East until about strategy is one of the most fascinating aspects of 1800 before being hunted to near-extinction North American ornithology (Lanyon 1992, across the continent (Baker 1983, Kurta 1995). Winfree 1999, Rothstein et al. 2002), but the Flocks of cowbirds likely also inhabited the cowbird has long drawn disdain. Chapman prairies and woodland openings of southern (1927) called it “. . . a thoroughly contemptible Michigan prior to the 1800s (Walkinshaw creature, lacking in every moral and maternal 1991). Brewer (1991) points out that cowbirds instinct,. . . ” Martin et al. (1961) wrote: were on Sager’s Michigan bird list of 1839. In “Apparently this social parasite is either too lazy contrast, cowbirds were not noted in Ontario or too smart to take care of its own domestic until the 1870s (Falk 2007). duties.” Although ornithologists today are concerned about the effects of cowbirds on Cowbirds likely did not spread farther north in other birds (Robinson et al. 1995, Rosenberg et Michigan until after logging and subsequent al. 1999), current views are not so scornful, and wildfires of the late 1800s (Mayfield 1961a). some consider cowbirds “a fascinating study in The southern LP was logged and farmed by behavior” and “marvelous to watch” (Stokes 1870, but the northern LP and UP were still and Stokes 1983). more than 90% forested then (see map from

Whitney 1994 in Askins 2000, pg. 79). But takes hosts’ eggs and young from the nest soon, logging, agriculture, and large scale (Mayfield 1961a, 1961b, Payne 1965, Lowther wildfires swept across the state and reduced 1993, Elliott 1999). The mating system might forest cover in large areas (Dickmann and vary from monogamy, to polygyny, to Leefers 2003), thus enabling cowbirds to reach promiscuity depending on habitat, sparsity of as far north as the NELP by the 1880s (Mayfield host nests, and sex ratios (Teather and 1961a, 1993). Wood and Frothingham (1905) Robertson 1986). found cowbirds in Oscoda and Crawford Counties during their 1903 quest for birds along Abundance and Population Trends the Au Sable River, but did not comment on (Click to view trends from the BBS) them. Cowbirds were recorded in the NWLP Survey data show Brown-headed Cowbird around Douglas Lake by 1910 (Pettingill 1974). population declines at all landscape scales. Data Wood (1918) also found cowbirds in the early from Berrien County showed declines from the 1900s as far north as northwestern Alger Co. in mid-1970s to 1991 (Booth 1994). Michigan the UP, but he only noted their presence, an BBS routes show a statistically significant indication that he was not surprised to find decline of 1.95% from 1983-2007 (Sauer et al. cowbirds that far north by then. 2008). Regionally, the BBS data also show a slight, but steady and significant annual decline Today, the cowbird is a common summer of 0.55% during the same period. It also resident throughout Michigan. There was no declined across North America 1% per year on apparent change in distribution of the cowbird BBS routes from 1966-1996 (Sauer et al. 2008). since the first atlas. A few areas of Michigan These declines appear corroborated by atlas had no cowbirds during both atlas surveys, data—breeding was confirmed in fewer particularly parts of Lake Superior State Forest townships during MBBA II. However, the in northern Luce County and northwestern number of townships with cowbirds in the SLP Chippewa County, the coast of Gogebic County, was similar between atlases, whereas the parts of Ottawa National Forest in western Iron number of townships with cowbirds decreased County, and in Copper Country and Escanaba within the UP and NLP from MBBA I to River State Forests in northeastern Dickinson MBBA II. This decrease in the UP and NLP County and Marquette County. Most cowbirds seems especially indicative of a reduction in migrate to the south each autumn, but some can cowbird habitat in the north since observer be found in southern Michigan through the effort in those regions was greater during winter. MBBA II. Cowbirds also declined in Ontario between atlas periods (early 1980s to early Breeding Biology 2000s) along Ontario BBS routes (Falk 2007). While some other species in North America, These declines might be due to regrowth of such as the Redhead, sometimes lay eggs in forests in the north. other birds’ nests, the Brown-headed Cowbird is an obligate brood parasite—with no ability to A cowbird was first reported during a Michigan build a nest, the female must lay her eggs in the Christmas Bird Count in 1938, but not again nests of other birds, particularly species that nest until 1953 after which they were seen every low in shrubby, young forest, forest edge, and winter (NAS 2009). The number observed open habitats (Lowther 1993). A female peaked during 1988 CBCs in Michigan, but has cowbird can lay no more than one egg per declined in the past 10 years on Michigan day—usually at dawn—but can lay several counts and across the continent (NAS 2009). dozen over a breeding season, and sometimes Cornell’s Project FeederWatch shows a slightly

© 2011 Kalamazoo Nature Center Brown-headed Cowbird (Molothrus ater) Doug Powless declining trend in winter in the Great Lakes warbler population growth was limited solely by region over the past 20 years (PFW 2009). amount of suitable habitat (Rothstein and Peer 2005); but most contend that a combination of Conservation Needs intensive habitat management along with Brown-headed Cowbirds have been found to lay cowbird control saved the Kirtland’s Warbler eggs in nests of over 220 species (Lowther (Kelly and De Capita 1982, Walkinshaw 1983, 1993) including some of the rarest warblers in Mayfield 1993, USFWS 2008). As the warbler our region, such as Prairie (Nolan 1978), reaches recovery goals, baseline data on warbler Kirtland’s (Mayfield 1961a, 1992), Golden- productivity and brood parasitism with and winged (Coker and Confer 1990), Hooded without cowbird control will likely be (Ogden and Stutchbury 1994), and, to a lesser important. In the meantime, cowbird control is degree, Cerulean (Rogers 2006). However, a still recommended (Mayfield 1992, Olson 2002, recent review of brood parasitism by the Wells 2007). Prior to cowbird control, National Audubon Society reports that cowbirds Kirtland’s Warbler hosted over 90% of cowbird do not appear responsible for rangewide eggs laid in the area (Walkinshaw 1983). declines of various songbirds, with the Interestingly, De Groot and Smith (2001) have exception of some rare species (Muehter, no shown that cowbirds have no community-wide date given). Since MBBA I, ornithologists have impacts on other songbirds in the jack pine published extensively on the effects of forests of Michigan. landscape factors on songbird productivity and brood parasitism (Askins 2000). Several studies Literature Cited indicate that the songbirds most vulnerable to cowbirds build open cup nests in open and edge Askins, R.A. 2000. Restoring North America’s habitats, but forest-nesting species, such as Birds: lessons from landscape ecology. 2nd Wood Thrush, can also be very vulnerable in Edition. Yale University Press, New Haven agricultural landscapes where forests are & London. 332 pgs. fragmented (Robinson et al. 1995, Trine 1998, Baker, R.H. 1983. Michigan Mammals. Fauth 2000, Ford et al. 2001; but see Fauth Michigan State University Press, East 2001, Knutson et al. 2004) and brood parasitism Lansing, Mich. can increase near patches of agriculture even in Booth, W. 1994. Numerical changes in the forested landscapes (Rodewald and Yahner nesting birds in Berrien County, mid-1970s 2001). Rodewald (2009) also shows that to early 1990s. Michigan Birds and Natural habitat-altering invasive shrubs associated with History 1(1): 10-15. urbanization can promote brood parasitism on a Brewer, R. 1991. Original avifauna and forest bird such as the Acadian Flycatcher. postsettlement changes. In Brewer, R., G.A. McPeek, and R.J. Adams, Jr. (eds.). 1991. In Michigan, the cowbird is infamous as one of The Atlas of Breeding Birds of Michigan. the main threats to the Kirtland’s Warbler Michigan State University Press, East (Mayfield 1960, 1963, 1977, 1992, Radtke and Lansing, Mich. Byelich 1963, Ryel 1981, Walkinshaw 1983, Chace, J.F., C. Farmer, R. Winfree, D.R. 1991, USFWS 2008). The severe effect of Curson, W.E. Jensen, C.B. Goguen, and brood parasitism on Kirtland’s Warbler S.K. Robinson. 2005. Cowbird (Molothrus necessitated the nation’s first cowbird control spp.) ecology: a review of factors program in 1972, and tens of thousands of influencing distribution and abundance of cowbirds have been trapped since (Kelly and De cowbirds across spatial scales. In Ortega, Capita 1982, Mayfield 1993). Some argue that C.P., J.F. Chace, and B.D. Peer (eds.).

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