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NOVA ScienceNOW

Gangster Birds: Expert Q&A

  • Posted 08.31.09
  • NOVA scienceNOW

On August 31, 2009, Scott Robinson answered questions about cowbirds and other brood parasites in the animal kingdom.

Scott Robinson

Scott Robinson

Scott Robinson is the Katharine Ordway Professor of Ecosystem Conservation at the Florida Museum of Natural History. Full Bio

Photo credit: Courtesy Scott Robinson

Scott Robinson

Scott Robinson is the Katharine Ordway Professor of Ecosystem Conservation at the Florida Museum of Natural History, part of the University of Florida in Gainesville. Robinson has research interests in the areas of community and population ecology, conservation biology, and avian brood parasitism. He's particularly interested in cases of coevolution—when host and parasite evolve in specific ways because of their relationship. Robinson earned his B.A. at Dartmouth College and his Ph.D. at Princeton University, both in biology. Before moving to Florida, he served as head of the Department of Animal Biology at the University of Illinois at Urbana-Champaign.

Q: In the show, it was presented as if the brown-headed cowbird was seeking retribution on the birds that evicted the brown-headed cowbird eggs from their nest. However, isn't it more likely that the reason for damaging the potential host's eggs is so that the host will establish a new nest, which the brown-headed cowbird could then parasitize? I'm presuming that increasing the species is a greater driving factor than mere retribution. Lisa Weeks, Overland Park, Kansas

Scott Robinson: Dear Lisa,
This is exactly the scenario for the "farming" hypothesis. Sometimes cowbirds evict host eggs from nests that never had cowbird eggs, presumably to force them to re-nest, which would then allow the new nests to be parasitized by cowbirds. The retribution scenario is based on evidence that cowbirds also penalize hosts that eject their eggs, which prevents the evolution of host defenses against cowbirds. In either case, individual cowbirds benefit by making more nests available and by slowing the evolution of host defenses. Animals behave in ways that increase their individual reproductive success, not necessarily that of the species.

Q: Could it be that the reason some cowbirds destroy the warblers' eggs after their eggs have been removed from the nest is to prevent the host bird from passing on a technique that disrupts the cowbird's method of continuing its species? Sort of like the pirate leader killing off the gang members that know where the buried treasure is. Larry Lieb, Lake Orion, Michigan

Robinson: Dear Larry,
This is an excellent analogy. Host defenses will not evolve unless they actually work. By penalizing the hosts that have learned or evolved this behavior, they prevent hosts from doing this again. The selection is intense on the hosts, but even more intense on the cowbirds, which must increase their own individual reproductive success.

Q: Do the birds who are tricked/forced into raising the cowbird young recognize that the chicks aren't their offspring? Have any tests been done to see if the parent birds can truly distinguish their offspring from "invaders"? Robby Armaline, Lakewood, Ohio

Robinson: Dear Robby,
This is an excellent question and one that has puzzled scientists for a long time. For some reason, no hosts are known to recognize cowbird nestlings even though they are very different-looking from their own young. Perhaps, because birds imprint on their own young as they grow in the nest, it is too dangerous to evolve rejection of unfamiliar-looking nestlings. Because cowbirds eggs usually hatch earlier than those of their hosts, the hosts would actually be more likely to imprint on the cowbird nestling and then reject their own young!

Q: Is revenge a reasonable motivation for a cowbird to exhibit? It seems just as likely that the cowbird destroys the warbler eggs to encourage the mother warbler to lay new eggs. If the cowbird simply laid a new egg to replace the missing one the warblers would hatch earlier than the cowbird chick putting it at a developmental disadvantage. It seems to me that instinctual maternal motivations such as this might better explain the cowbird's behavior. Can you explain why this cowbird behavior is seen as punishment or vengefulness? Lee Hesselroth, Minnesota

Robinson: Dear Lee,
The motivation is not exactly revenge; the advantage is that hosts do not gain any advantage from rejecting an egg and, therefore, the behavior does not evolve genetically or is not learned culturally from watching other hosts reject foreign eggs. Cowbirds do destroy the entire clutch, which makes the scenario you describe an unlikely explanation for this behavior because the host builds an entirely new nest. You make an excellent point—it is important for the cowbird egg to hatch first; otherwise, they have no particular advantage. Cowbirds do "farm" some hosts to stimulate re-nesting, but the retribution behavior occurs at a much higher frequency.

Q: Maybe the cowbird is taking revenge on the humans for taking out her egg. What if you removed the eggs in such a way that is impossible for the cowbird to see or smell human involvement? Jeff Allen, Exeter, New Hampshire

Robinson: Dear Jeff,
This is a good point and illustrates the importance of doing controls in experimentation. To make sure that this is not happening, scientists visit the nests and manipulate the eggs even from nests from which no cowbird eggs have been removed. Host eggs are only removed from the nests from which cowbird eggs have been removed, which "controls" for human visitation effects.

Q: I have heard that birds will rebuild if nests are destroyed early in the breeding season. Do not cowbirds need to lay an egg while the host is laying? If a cowbird egg is destroyed, then is not timing wrong for the cow bird to lay another in that same nest? Does destroying the nest encourage host birds to try a new nest which may give the cowbird a new opportunity to reproduce? Delphia Jones, Newport, Virginia

Robinson: Dear Delphia,
This is an excellent point. Rejection usually occurs after a clutch is nearly complete and just laying a new egg would not work because it would be late and the host would just remove it again anyway. Therefore, it makes more sense to destroy the entire clutch because it forces the hosts to re-nest and it also imposes the maximum penalty for any rejection.

Q: Hey Scott, how've you been? Looks like Florida's been good to you! I'm teaching jr. high Gifted now - my class visited Jeff in the Shawnee in May. We're visiting Andy and his ants next month. Here's a question: How are bronzed cowbirds in the southwest effecting songbirds there? How has development affected that? Thanks! Take care! Scott Saffer, Forrest, Illinois

Robinson: Dear Scott,
Good to hear again from one of my all-time best assistants. I also have a couple of kids taking gifted science classes in Middle School, so I know how extremely important these years are in the development of scientific thinking and interests. The bronzed cowbird is having a very negative effect, especially on orioles in the desert Southwest. They are not as common as the brown-headed cowbird, but they are also more specialized, which increases the negative effects on their hosts. Unfortunately, this species is spreading into tropical mountains as a result of global warming and drying, which improves cowbird habitat at the expense of birds that need cloud forest habitats.

Q: Dear Scott,
I have been a birder for aprox. 30 years or so. I have only seen this once and it was shocking and cool at the same time. Starlings battling to the death on a sidewalk on a busy street, completely oblivious to the fact that they were rolling around at two women's feet, traffic, noise. They were really going at it. I read up on it. It seems to be a territorial thing/nesting. Can you give your take? Theresa, Toronto, Ontario

Robinson: Dear Theresa,
Good to hear from a fellow birder. I have several times seen birds fight to the death, although in all cases it was something else (an alligator or a car) that actually killed one of the birds fighting. Most likely, this was a fight over nest sites, which are limited in birds such as starlings that nest in cavities (tree holes drilled by woodpeckers, eaves in buildings), but cannot make their own. When they find a good, safe cavity, it is so valuable that it is actually worth fighting over and taking incredible risks. Males will also fight to the death over especially desirable mates. Usually, fights over something like food would not be so extreme because the consequences of losing a few food items are not so great that it is worth taking such extreme risks.

Q: Dear Scott,
How have cowbird eradication campaigns affected the population of these birds? If their numbers are drastically reduced, would it create a negative environmental impact, or are the campaigns justified? Thanks. Aaron Dawson, New Hampshire

Robinson: Dear Aaron,
This is a very good question: cowbird removal programs are controversial, even among scientists. In my opinion, they are essential in cases where there are endangered species that are being so heavily parasitized that they cannot breed successfully. Cowbird removals have been very successful in southern California and in parts of Texas where endangered species have recovered after cowbird trapping and intensive habitat management have greatly improved nesting success.

I do not favor massive cowbird killing because most hosts have large populations, some of which nest in extensive forests where they are not exposed to cowbird parasitism. Cowbirds need human-altered habitats where they can forage. In the absence of these feeding sites, there are no cowbirds. Populations in large tracts free of cowbird feeding habitat seem secure and may even provide a surplus of young to repopulate areas where cowbirds are abundant. As long as we maintain relatively large, unbroken habitat patches, we can probably control cowbirds without killing them and also benefit many other species that need larger habitat patches.

Q: What would cause the cowbird to evolve traits that make it parasitic? It seems like laying eggs in other birds' nests and watching over them is more trouble than just laying them in a nest of their own. Marisa Marraccini, Charlottesville, Virginia

Robinson: Dear Marisa,
This is an excellent question and is currently the topic of intensive research and mathematical modeling. The advantage of brood parasitism is that it frees birds from parental care. In stead of taking care of young, parasites can spend all of their energy making new eggs, which could greatly increase their won nesting success. Some cowbirds in South America are estimated to lay more than 150 eggs a year! However, as hosts evolve defenses (egg rejection), it becomes much harder to find an appropriate home for so many eggs because the parasites have to become more specialized in which species they parasitize (egg mimicry). Therefore, some parasites such as cuckoos may not lay many more eggs than other non-parasitic species. In this case, the main advantage may be in spreading the risk among many nests—sometimes, it is better not to put all of their eggs in one basket.

In either case, the behavior probably first evolved from the habit of some birds to lay "extra" eggs in the nests of other members of their own species. Sometimes, birds lose their nests during the egg-laying period, which means that they just find a place to lay the rest of the clutch. The best place, of course, would be the nest of another member of their own species, but if none are available, the nest best place would be a nest of another species. There are even some females of species such as the starling that may lay clutches even when they do not have a nest site of their own. These females may lay their eggs in other starling nests. This is truly a step towards the evolution of parasitism.

Q: Why are cowbirds always referred to as "parasites"? It seems that a name like that would just help justify killing more and more of them off. Emily G., St. Louis, Missouri

Robinson: Dear Emily,
This has become a loaded term because of its human implications, but you have to remember that this term was first applied to behavior of non-human animals and then adopted to describe seemingly similar human behavior. Unfortunately, we humans cannot seem to resist being judgmental about such behaviors. In the birds we study, parasitic behavior is just a way that individuals maximize their own breeding success. I agree that their behavior makes them extremely unattractive to humans and makes it easier for many people to kill them.

Q: Are there any other birds out there that have similar habits as the cowbird, or is it totally unique in its behavior? Dan Rony, Montgomery County, Maryland

Robinson: Dear Dan,
Cowbirds are the most "generalized" of brood parasites in that two species parasitize more than 200 host species, but there are roughly 100 other species that are also "obligate" brood parasites. There are ducks, cuckoos, honeyguides (African birds, some of which guide humans to honey sources), and finch-like birds called indigobirds, among others. This behavior has evolved independently many times. Most are more specialized than the cowbird.

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