Smith (1999) suggested cowbird control to be justified if the parasitism level exceeds 60% over 2 years, however, small isolated bird populations facing multiple threats may be non self-sustaining at levels as low as 20%. Factors influencing the intensity of cowbird parasitism include the type of microhabitat nests are built in (including the level of nest concealment and the structural diversity of vegetation); forest bird nests in cleared areas may be more vulnerable to cowbird parasitism. A study of the endangered pale-headed brushfinch (Atlapetes pallidiceps) in Ecuador, however, revealed land use to be a major factor determining the impact of cowbird parasitism. In cattle-grazed areas breeding rates of the brushfinch were two times greater than in ungrazed areas due to a decrease in cowbirds numbers (correlated with a decrease in bird diversity and abundance in the grazed areas) (Oppel et al. 2004).
On the other hand, M. bonariensis is associated with dry open habitats (rather than moist forest habitats) and its range expansion (in Chile and the West Indies) may have been facilitated by the conversion of forested areas to early successional habitats (as well as the lack of native brood parasites in the case of some West Indian islands case) (Marín 2000; Post and Wiley, 1977, Cruz et al. 1995).
Physical: Most removal programmes in North America rely on large cage-traps for cowbird control. Selective shooting has also been applied to remove cowbirds, but has yielded mixed results. While site-specific shooting may be an effective complementary tool to support landscape-scale management, shooting alone may not always be sufficient to significantly reduce cowbird parasitism rates (Eckrich et al. 1999., Whitfield, 2000., in Oppel et al. 2004).
Another effective option is to monitor host nests during the breeding season, constantly removing cowbird eggs and chicks. Host eggs must be clearly distinguishable from cowbird eggs. When nest monitoring is required the host nests should not be approached while either parent close to the nest, and damage to the surrounding vegetation should be kept to a minimum to avoid creating gaps to the nest and encouraging predation. While this method is intrusive and requires a considerable level of skill, nest manipulation is efficient and cost-effective, especially in areas where trapping is impractical (Schmidt and Schaefer, 2003; Oppel et al. 2004).
Integrated management: Cowbird control has to be maintained for an infinitely long time, as cowbird populations at a regional level are not affected by most removal programmes. Despite often leading to reduced parasitism rates, cowbird removal has only occasionally triggered an evident increase in the target host population, and it has been suggested that habitat quality or quantity might be more limiting than cowbird parasitism rates alone (Oppel et al. 2004).
Location Specific Management Information
The chalk-browed mockingbird, rufous-bellied thrush and brown-and-yellow marshbird reject pure white cowbird eggs, while the rufous hornero rejects eggs according to size (Mermoz and Reboreda 1999).
A study of brown-and-yellow marshbird nests found that about three-quarters of 74 marshbird nests observed were parasitised by M. bonariensis. Hatchling failure was twice as high in parasitised nests (43.1% compared to 21.1%). The marshbirds’ eggs have reddish brown spots and are similar to spotted M. bonariensis eggs. In 5 out of 6 cases the experimental addition of a white egg in a nest (already infested with M. bonariensis spotted eggs) resulted in the ejection of the white egg from the nest. Although the cowbird chicks were smaller in size than the marshbird chicks they were not outcompeted for food as generally happens when the host species is largerer (for example the chalk-browed mockingbird or the rufous-collared sparrow) (Fraga 1978 1985, in Mermoz and Reboreda 1994). A study concluded that the reproductive success of M. bonariensis was due to some of the life history traits of the marshbird for e.g nest attentiveness during egg-laying, a modal clutch size of 4–5 eggs, and a longer incubation period which caused the parasitic birds eggs to hatch a day or two earlier than the hosts (Mermoz and Reboreda, 2003).
Molothrus bonariensis (shiny cowbird) was first recorded in Cuba in 1982 (Garrido 1984, in Kluza 1998).
Breeding birds which may be at risk of parasitism include the yellow oriole (Icterus nigrogularis), troupial (Icterus icterus), yellow warbler (Dendroica petechia), black-whiskered vireo (Vireo altiloquua and rufous-collared sparrow (Zonotrichia capensis).
Some wild individuals have been caught and put in a cage. A small trap was then set up on top of the cage, and ten more wild individuals were caught in this trap
North Andros Is. (Andros Is.)
Sightings during the winter season suggest this species has established itself as a permenant resident. The potential for range increase of the speices on the island is great since several host species have been identified on the island including the black-cowled oriole, yellow warbler (Dendroica petechis), black-whickered vireo (Vireo altiloquus). Current agricultural development will also probably increase the amount of foraging habitat for cowbirds (Wiley 1985; Baltz, pers. obsv, in Baltz 1995).
Baltz and Burhans (1998) added artificial shiny cowbird eggs to gray kingbird nests in the Bahamas, where shiny cowbirds were first recorded in 1993. Gray kingbirds ejected 85% of artificial eggs within 48 hours of addition. Based upon the short time of contact between the two species, the authors suggest that egg ejection behavior by gray kingbirds in the Bahamas has been retained from ancestral populations.
Yucatan birds may not be as vulnerable to brood parasitism as West Indian birds because they have experienced previous contact with the bronzed cowbird (M. aeneus), which is a resident throughout the peninsula. On the other hand, experience with the bronzed cowbird may be limited among Yucatan birds as the bronzed cowbird it is more specific than the generalist shiny cowbird, mostly parasitising icterids and finches. Yucatan endemics at risk include the Yucatan flycatcher Myiarchus yucatanensis, Yucatan wren Campylorhynchus yucatanicus, orange oriole Icterus auratus, black catbird Melanoptila glabrirostris, cozumel vireo Vireo bairdi, and cozumel wren Troglodytes beani. Regional endemics at risk include the Yucatan vireo V. magister and white-browed wren Thryothorus albinucha. Careful monitoring of Shiny Cowbirds on the Peninsula, and the effects on species already resident there, will be important (Oberholser 1974, Friedmannn 1929, Friedmannn et al. 1977, Friedmannn and Kiff 1985, in Kluza, 1998).
Yunguilla Reserve was established in 1999 by the Fundación Jocotocoto protect a small population of the recently rediscovered pale-headed brushfinch (Atlapetes pallidiceps). This 27 hectare area of land is located in the Yunguilla Valley (in Azuay, a province in southern Ecuador) and contains the only known remaining suitable habitat for the pale-headed brushfinch. In 2003 a local hunter was employed to remove cowbirds from the reserve during part of the breeding season of the brushfinch (from February to April); very few cowbirds had been observed from May to July 2003). A total of 69 cowbirds were shot, 22 were females (each with 15 to 39 developing eggs). A few cowbird nestlings were found when cowbird removal started; they were removed and all affected pairs initiated another clutch 10 to 17 days later. Cowbird removal resulted in a strong increase in the reproductive output of the brushfinch (with 16 brushfinch fledglings in 2003, compared to only 5 in 2002).
As a means to reduce cowbird parasitism, shooting was preferred in this case to minimise the impact of nest searching on the vegetation surrounding the brushfinch nest (which might open paths for predators to the nests). In addition, egg removal is a time-costly operation. Finally, cowbird egg removal was discouraged after preliminary trials showed it was difficult to reliably discriminate between the polymorphic brushfinch egg and the polymorphic cowbird egg. Despite this recommendation, future research into more reliable determination of the cowbird and brushfinch eggs is certainly of high value, and may have positive applications in cowbird management.
Cowbird control does not eliminate the actual causes for declining populations (Hall and Rothstein 1999, Hayden et al. 2000, Rothstein and Cook 2000). Oppel et al. (2004) found that the mean reproductive output of a pair of brushfinches in the grazed area was 1.33 young, as opposed to 0.36 young during the 2002 breeding season. The same study also investigated differences in microhabitat type and breeding success (as cowbirds are thought to be less likely to parasitise concealed nests). However in this case this was not found to be significant. The main factor was land use, and the postulated reason to explain the low breeding success of the brushfinch in ungrazed land was thought to be due to the high diversity and abundance of potential hosts in ungrazed land; cowbirds have been demonstrated to prefer breeding area with a high diversity and abundance of hosts (Barber and Martin, 1997, Evans and Gates, 1997, Robinson et al. 1999, Tewksbury et al. 1999, Young and Hutto, 1999). Habitat management in the reserve began in November 2002, and consisted of strategies that aimed to prevent the succession of grasslands to forest (such as the selective thinning of dense thickets). The Foundation planned to continue cowbird removal in 2004. Members of the Fundación Jocotocoto have made repreated visits to local agencies, land owners and schools to raise local awareness.
2. Lopez-Ortiz, R., Ventosa-Febles, E.A., Reitsma, L.R., Hengstenberg, D., and Deluca, W. 2002. Increasing nest success in the yellow-shouldered blackbird Agelaius xanthomus in southwest Puerto Rico. Biological Conservation. 108 (2): 259-263.
Summary: Information about the artificial nest programme and parasitism rates for the yellow-shouldered blackbird in Puerto Rico.
3. North American Bird Information Web Site. Undated. Observations of Shiny Cowbird in the United States of America.
Summary: Information on distribution in North America.
4. Oppel, S., Schaefer, H.M., Schmidt, V. and Scroder, B. 2004. Cowbird Parasitism of Pale-headed Brush-finch Atlapetes pallidiceps: Implications for Conservation and Management, Bird Conservation International 14: 63 - 75.
Summary: An overview of management strategies and habitat restoration in Yunguilla Reserve in Ecuador.
5. Schmidt, V. and Schaefer, H.M. 2003. Pale-headed Brushfinch Recovery Project in Southwestern Ecuador 2002-2003 (Final Report).
Summary: An overview of the management strategies employed in Yuguilla Reserve in Ecuador.
6. Woodworth, B.L. 1999. Modeling Population Dynamics of a Songbird Exposed to Parasitism and Predation and Evaluating Management Options. Conservation Biology. 13 (1): 67-76.
Summary: A study that documents parasitism including the shiny cowbird.
Results Page: 1