Background information and definitions
Brood parasites are species that lay their eggs in other species’ nests, with the chicks cared for by the hosts. They can have extremely detrimental impacts on bird species, with parasite chicks frequently evicting all host eggs or chicks. However, some brood parasite species, such as common cuckoos Cuculus canorus are themselves declining or threatened, making control or the reduction of parasitism undesirable.
However, in North America, brown-headed cowbirds Molothrus ater are increasingly abundant and expanding in their range. Brown-headed cowbirds have also been implicated in the near extinction of at least one species, the pale-headed brush finch Atlapetes pallidiceps in South America (Oppel et al. 2004), meaning that their control is seen as a conservation priority. All the studies we captured concern cowbird species.
Oppel, S., Schaefer, H.M., Schmidt, V. & Schroeder, B. (2004) Cowbird parasitism of pale-headed brush-finch Atlapetes pallidiceps: implications for conservation and management. Bird Conservation International, 14, 63–75.
Supporting evidence from individual studies
A before-and-after study in pine forests in Michigan, USA (Kelly & DeCapita 1982) found that the population of Kirtland’s warbler Dendroica kirtlandii did not significantly increase during 1972-81 when a total of 33,536 brown-headed cowbirds Molothrus ater (a brood parasite) were removed, compared to 1931-71, when no control was in place (average population of 207 individuals during 1975-81 vs. 201 for 1931-71). Population size remained similar despite parasitism rates being significantly lower and warbler productivity being significantly higher during the period with cowbird control (3.4% of nests parasitised and 2.8 chicks/nest fledged with cowbird control, number of nests not provided vs. 59% and less than 1 chick/nest fledged before control, 91 nests examined).Study and other actions tested
A replicated trial in 1980 in Puerto Rico (Wiley et al. 1991) as part of the same study as in (López-Ortiz et al. 2002 and López-Ortiz et al. 2006) found that parasitism of yellow-shouldered blackbird Agelaius xanthomus nests was significantly lower in two mangrove forest sites where shiny cowbirds Molothrus bonariensis were removed, compared to sites where cowbirds were not removed (45% of 11 nests parasitized where all cowbirds were removed vs. 30% of ten where female cowbirds were removed; 67% of nine where males were removed and 92% of 12 in control sites). This study also investigated the impact of different nest boxes on parasitism, discussed in ‘General responses to small/declining populations - Provide artificial nesting sites’.Study and other actions tested
A before-and-after study in young jack pine Pinus banksiana forests in Michigan, USA (Kepler et al. 1996) found that, following the trapping and removal of a total of 84,937 brown-headed cowbirds Molothrus ater between 1972 and 1992, parasitism rates of Kirtland\'s warbler Dendroica kirtlandii nests by cowbirds fell significantly and productivity increased, compared to 1944-57 or 1966-71, before cowbird control was started (1944-57: 55% of 137 nests parasitised vs. 1966-71: 69% of nests parasitised, 0.8 fledglings/nest, sample size not provided; 1972-1977: 6% of 230 nests parasitised, 2.7 fledglings/nest; 1989-91: 2% of 48 nests parasitized). However, these changes did not result in increased numbers of singing males, with seasonal numbers fluctuating from 167-243 during 1971-89.Study and other actions tested
A before-and-after study at four coastal scrub sites in California, USA (Braden et al. 1997), found that levels of parasitism of California gnatcatcher Polioptila californica nests (initiated after 5th May) by brown-headed cowbirds Molothrus ater was significantly lower in 1994-5, following the removal of a total of 507 cowbirds over the two years, compared to in 1992-3 (10% of 132 nests parasitized in 1994-5 vs. 46% of 107 nests in 1992-3). However, nest success over the whole breeding season was no higher in years with cowbird control (21.7% of nests successful in 1994-5 vs. 11.2% in 1992-3).Study and other actions tested
A replicated, controlled before-and-after study between 1984 and 1995 at three 50-100 ha (2 experimental and 1 control) hardwood forest sites in Pennsylvania, USA (Stutchbury 1997) found that the proportion of hooded warbler Wilsonia citrina nests parasitised by brown-headed cowbirds Molothrus ater was significantly lower during years when cowbirds were removed (11% of 241 nests parasitised in 1991-5 vs. 64% of 28 nests parasitised in 1984-90). When comparing sites, parasitism rates were lower on sites with cowbird removal (0-11% of 280 nests parasitised at removal sites vs. 38-58% of 32 nests at controls). However, there were no changes in warbler nesting success between sites with low (?5%) and high (>30%) levels of parasitism (average of 1.7 fledglings/nest in sites with low parasitism vs. 1.6 fledglings/nest in sites with high parasitism). The authors suggest that nesting success may be more determined by high rates (22-52%) of predation, than by parasitism. An average of 11-20 female and 7.5-17 male cowbirds were removed each year.Study and other actions tested
A study in 1996-9 in coastal forests on Puerto Rico (López-Ortiz et al. 2002) found that only a single yellow-shouldered blackbird Agelaius xanthomus nest was parasitized by shiny cowbirds Molothrus bonairensis in the study period. The authors argue that this was due to a widespread cowbird eradication programme initiated in 1984. This study is discussed in more detail in ‘Provide artificial nesting sites’.Study and other actions tested
A replicated, controlled study in 1995-9 at three riparian sites in British Columbia, Canada (Smith et al. 2003), found that the success rates of song sparrow Melospiza melodia nests were higher at sites where female brown-headed cowbirds Molothrus ater were removed (51 in 1996, 163 in 1997 at one site; 24 in 1998 at another) than in a control site with no cowbirds removed, both across all years (44% success for 296 nests in removal sites vs. 32% success for 615 in control sites) and just in years when cowbirds were removed (44% success for 296 nests in removal sites vs. 34% success for 399 in control sites). Nest survival rates were higher for song sparrow eggs in removal sites (96.5% daily survival vs. 94.7%), but there was no significant difference in nestling survival (97.4% daily survival vs. 96.6%). Significantly fewer nests were abandoned after being parasitised by cowbirds in removal sites than control sites (9% abandoned vs. 16.5%).Study and other actions tested
A controlled before-and-after study in 2000, 2001 and 2003 in mangrove forests in Puerto Rico (López-Ortiz et al. 2006) found that a significantly lower proportion of yellow-shouldered blackbird Agelaius xanthomus and yellow warbler Dendroica petechia nests were parasitized by shiny cowbirds Molothrus bonariensis following the removal of adult cowbirds (from 1982 onwards), cowbird eggs and chicks from artificial nests used by blackbirds (from 1991 onwards) (blackbirds: 91-95% of 202 nests studied in 1975-83 vs. 3% of 927 nests in 2000, 2001 and 2003; warblers: 63% of nests in 1975-83 vs. 37% of 165 nests in 2000, 2001 and 2003). Decreases in areas without cowbird control were either smaller or non-existent (44% of 32 blackbird nests and 85% of 13 warbler nests in reference areas parasitised). The effect of egg and chick removal is discussed in ‘Remove brood parasite eggs from target species’ nests’.Study and other actions tested
A controlled before-and-after cross-over study between 2003 and 2005 at five tall-grass prairie sites (24-83 ha) in Kansas, USA (9) found that brood parasitism by brown-headed cowbirds Molothrus ater on Bell’s vireos Vireo bellii was significantly lower at sites where a total of 980 cowbirds (171 adult females, 724 adult males and 85 juveniles) were removed in 2004-5, compared to before removals (47 and 58% parasitism in 130 nests in removal plots in 2004 and 2005 respectively vs. 64-81% parasitism in 130 nests in 2003). There was no corresponding decrease in areas when cowbirds were not removed (77-85% parasitism in 278 nests in non-removal plots in 2004-5). Vireo productivity was higher at removal plots (2.6 compared to 1.2 vireo fledglings/pair) and nest success was higher for non-parasitised nests (64% of nests producing at least one chick vs. 51% of parasitised nests). However, cowbird productivity was also higher for removal plots than control plots (0.3 compared to 0.1 cowbird chicks/vireo pair).Study and other actions tested
A controlled cross-over experiment at four 24-36 ha tall-grass prairie sites in Kansas, USA (Sandercock et al. 2008) found that, in 2004, parasitism of dickcissel Spiza americana nests by brown-headed cowbirds Molothrus bonariensis was significantly lower in two plots where 346 cowbirds were removed (76 adult females, 231 adult males and 39 juveniles) than in two control plots (51% of 53 treatment nests and 85% of 27 control nests respectively). However, in 2005 when treatments were reversed and 634 cowbirds (95 adult females, 493 adult males and 46 juveniles) removed from the remaining two plots, there was no such difference (78% of 45 nests and 82% of 44 nests respectively). In neither year were there differences in dickcissel productivity between experimental and control plots (2004: 0.32 and 0.29 chicks/nest respectively; 2005: 0.06 and 0.04 chicks/nest respectively). The authors suggest that nest survival was very low (34% in 2004, 7% in 2005) due to predation and other causes, not because of parasitism.Study and other actions tested
A before-and-after study in Puerto Rico (Vincenty et al. 2009) using some of the data from (López-Ortiz et al. 2006) found that the proportion of yellow warbler Dendroica petechia nests parasitized by shiny cowbirds Molothrus bonariensis fell following cowbird control (see López-Ortiz 2006). Nesting productivity of warblers also increased following cowbird control (0.7 warbler chicks/nest in 62 nests in 2001-2 vs. 0.35 warbler chicks/nest in 1977-80). However, the proportion of nests fledging chicks appeared to be lower following control (31% of nests in 2001-2 vs. 45% of nests in 1977-80). The majority of nests lost were predated (80% of 41 failed nests).Study and other actions tested
A before-and-after trial in arid scrubland in the Yunguilla Valley, Ecuador (Krabbe et al. 2011), found a large increase in the number of territorial male pale-headed brush finches Atlapetes pallidiceps following the control of brown-headed cowbirds Molothrus ater, starting in 2003 (12-29 birds recorded in 1999-2002 vs. 27-50 in 2003-7). In addition, parasitism rates appeared lower, with only a single nest being parasitised in 2003 and none in 2005. Between 18 and 69 cowbirds were shot each year at the site.Study and other actions tested