Info

Figure 25.3 Percentage reduction in the breeding success of various North American songbirds caused by the brood-parasitic Brown-headed Cowbird Molothrus ater. Effect calculated as % nests parasitised x % difference in success between parasitised and unparasitised nests. *Long-distance Neotropical migrants. Based on data assembled by Payne (1997), in which references to the original studies may be found.

1983, Temple & Cary 1988, Donovan et al. 1995, Robinson et al. 1995). Many of these new hosts, including Neotropical migrants, have no innate defences, and will not desert or eject strange eggs, but raise the resulting young. Because cow-birds use many host species, moreover, they are not vulnerable to decline in any one, and could in theory parasitise favoured species to extinction, while maintained at high density by other common hosts (May & Robinson 1985). The same features that make Neotropical migrants nesting near the edges of woods more vulnerable to nest predators than residents also make them more vulnerable to parasitism, namely their use of accessible nest-sites and their predominant single-broodedness.

1900 1920 1940 1960 1980 Year

Figure 25.4 Increase in the numbers of Brown-headed Cowbirds Molothrus ater in the southeastern USA. Points show the proportion of Christmas Bird Counts on which the species was recorded during 1900-1980. Redrawn from Brittingham & Temple (1983).

1900 1920 1940 1960 1980 Year

Figure 25.4 Increase in the numbers of Brown-headed Cowbirds Molothrus ater in the southeastern USA. Points show the proportion of Christmas Bird Counts on which the species was recorded during 1900-1980. Redrawn from Brittingham & Temple (1983).

During the 1960s, cowbirds laid their eggs in up to 70% of nests of the rare Kirtland's Warbler Dendroica kirtlandii, thereby reducing the production of young warblers to less than one per nest (Mayfield 1983, Walkinshaw 1983). This was fewer than needed to maintain the warbler population level, which during 19611971 declined by 60%. Then, with the start in 1972 of a programme of cowbird removal, parasitism dropped to 6%, warblers fledged about three young per nest, and their breeding numbers stabilised (Kelly & DeCapita 1982, DeCapita 2000). Later, they increased more than three-fold when the habitat expanded as a result of a forest fire (Rothstein & Robinson 1994) but, to facilitate this increase, and to maintain Kirtland's Warbler in the long term, cowbird control was continued. The historic suppression of wild fires had clearly not helped this species, which prefers young stands.

The effects of cowbird removal have also been examined in various other host species in different areas (Table 25.2). Overall, of six experimental removals of cowbirds, five were followed by an increase in the breeding success of the host species, and at least three by an increase in host breeding density. Only in one study, on Hooded Warblers Wilsonia citrina, did cowbird removal lead to no obvious increase in either nesting success or breeding numbers, but in this study parasitism rates were low (Stutchbury 1997). Other passerine populations may have declined following greater contact with this generalised brood-parasite. Examples include the Golden-cheeked Warbler Dendroica chrysoparia in parts of Texas, the Black-capped Vireo Vireo atricapillus in parts of Oklahoma (Grzybowski et al. 1986), the Eastern Warbling Vireo Vireo gilvus in parts of California and British Columbia (Verner & Ritter 1983, Ward & Smith 2000), the White-crowned Sparrow Zonotrichia leucophrys in parts of California (Trail & Baptista 1993), and Bell's Vireo Vireo bellii in Missouri (Budnik et al. 2000). In addition, some other species seem

Table 25.2 Effects of the experimental removal of parasitic Brown-headed Cowbirds Molothrus ater on their songbird host species

Host species

Area

Increased breeding output

Increased breeding numbers

Source

Kirtland's Warbler Dendroica kirtlandiia

Michigan

+

+

DeCapita (2000)

(Least) Bell's Vireo Vireo belliia

S. California

+

+

Griffith & Griffith (2000)

Black-capped Vireo Vireo atricapillusa

Central Texas

+

+

Hayden et al. (2000)

Song Sparrow Melospiza melodia

Mandarte Island, BC

+

Smith et al. (2000)

Southwestern Willow Flycatcher Empidonax trailliia

California

+

Was this article helpful?

0 0

Post a comment