For the same reason that the European Cuckoo also knows when the host bird, notably Wrens, detect an egg of another species and removes it. How do they know? They watch nests from a distance, according to wildlife biologists. The Cuckoos also select some species to parasitize more than others, too. Warblers are less likely to eject eggs, Wrens are far more likely. IE Ries Scott Baron <brnpelican@xxxxxxxxx> wrote: Birders get upset about Cowbirds here in the East, where cowbirds are relatively recent colonists from farther West. The story goes that when forests were cleared, these westerners expanded their range to the Atlantic. Our eastern forest birds didn't evolve with Cowbirds, so are less likely to remove the Cowbird egg or abandon the nest. I've never heard any anthropomorphizing about these birds. Rather, humans have caused a range expansion in one species (Cowbird) that has negatively affected several others. John's question about why Cowbirds know that the deposited egg has been removed is a good one. Anyone have an answer out there? Scott Baron Fairfax, Va. --- John Broman wrote: > This raises the question of how a cowbird knows its > deposited egg has been removed. I thought Cowbirds > developed this parasitic behavior due to never > staying in one place on enough to nest (following > the cattle). Perhaps if the urban cowbirds don't > have cattle to follow around, they have time to see > if their egg deposits are still safe. > And while we're discussing Cowbirds...I'm still > puzzled by the anger some birders have for them. It > seems to me this behaviour is what nature developed. > Any similarity to dead-beat dads should not be held > against the birds. Are we anthropomorphising too > much here? We don't consider a lion cruel for > taking down a gazelle for lunch, do we? > > John Broman > Lovettsville, VA > > > On 13/Jun/2007 23:44 Sandy Hevener wrote .. > > And the other side of the cowbird egg story: > > (From March 10, 2007 Science News) > > * > > Mafia Cowbirds: do they muscle birds that don't > play ball? * By S. Milius > > Cowbirds in Illinois that sneak their eggs into > other birds' nests > > retaliate violently if their scam gets foiled, > researchers say. > > > > The brown-headed cowbirds of North America > outsource nest building and > > chick raising. female cowbirds dart into other > birds' nests, quickly lay > > eggs, and rush away. The nest owners are left to > care for big demanding > > cowbird chicks. > > > > Why don't the dupes throw out the odd eggs? When > scientists removed > > cowbird eggs from warbler nests, more warbler eggs > later got smashed or > > carried off than did eggs in nests with cowbird > eggs in place. It was > > cowbird retaliation, conclude Jeffrey P. Hoover of > the Illinois Natural > > History Survey in Champaign and Scott K. Robinson > of the Florida Museum > > of Natural History in Gainesville. > > > > That's the first evidence of gangsterlike behavior > in cowbirds, say Hoover. > > > > A decade of monitoring prothonotary warblers in > nest boxes in > > souther-Illinois swamps gave Hoover the idea for > the new experiment. The > > nest boxes sit on poles coated with axle grease to > thwart raccoons, > > snakes, and most other raiders. Egg-laying > cowbirds still strike, and > > Hoover had for years left the cowbird eggs alone. > In 2002, he and other > > researchers removed cowbird eggs. Nest vandalism > suddenly increased. > > > > No one saw the vandals, but Hoover and Robinson > turned to an idea put > > forward in 1979 by Israeli biologist Amotz Zahavi. > He'd suggested that > > by tending the weird-looking eggs and chicks, the > foster parents protect > > their own progeny. In a rare test of the idea, > cuckoos retaliated > > against magpies in Spain that rejected cuckoo > eggs, scientists reported > > in 1995. > > > > In the new experiment, the researchers recorded > egg damage in only 6 > > percent of the warbler nests where cowbird eggs > remained unmolested. In > > contrast, 56 percent of nests were vandalized > after the researchers > > removed the cowbird eggs. When the scientists > removed the cowbird eggs > > but added new fronts to the nestt boxes with holes > too small for > > cowbirds, there was no damage > > So, the nest thrashes are cowbirds, Hoover and > Robinson conclude in a > > paper now online for an upcoming Proceedings of > the National Academy of > > Sciences. > > > > When the cowbird eggs stayed in the next, some > warbler chicks starved > > because the pushy cowbird nestlings took so much > of the food. Yet with > > the retaliation attacks, the nests where cowbird > eggs had been removed > > produced, on average, only 40 percent as many > warblers as the > > cowbird-fostering nest did, say Hoover. > > > > "This is a surprising result," says Stephen > Rothstein of the University > > of California, Santa Barbara. > > > > Rothstein hasn't tested whether cowbirds > retaliate, but he says, "My > > bet, before this paper, would have been defnitely > no." He's now > > reconsidering but says, "I'd like to see more > direct evidence to date, > > " > > such as video. > > > > So would Naomi Langmore of the Australian National > University in > > Canberra. Still, she describes the evidence as > compelling." > > > > > > > > > > "Best evidence of date, " Says Rebecca Kilner of > the University of > > Cambridge in England. > > > > Comment: Hummmmm. Wonder if we should throw > cowbird eggs from the nest? > > > > > > Posted by: > > > > Sandy Hevener > > General Delivery > > Blue Grass, VA 24413 > > > > > > > > > > > > > > > > > > > > > > You are subscribed to VA-BIRD. To post to this > mailing list, simply send > > email to va-bird@xxxxxxxxxxxxxx To unsubscribe, > send email to > > va-bird-request@xxxxxxxxxxxxx with 'unsubscribe' > in the Subject field. > > > You are subscribed to VA-BIRD. 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