A forest’s edge zone is the area experiencing climatic and ecological influences from adjacent, open areas (Flagstaff Unified School District). Brown-headed cowbirds thrive at the forest edge because they can invade areas of the forest they would have otherwise been excluded from if there was no disturbance such as a clear cut. Entering from open areas enables the brown-headed cowbird to approach the nests of interior forest birds (Flagstaff Unified School District). The brown-headed cowbirds is called a brood parasite because it lays eggs in the nests of others birds. Typically the host is some type of songbird. After laying its egg, the female cowbird usually will leave to lay eggs in more nests. Cowbird eggs hatch sooner and the young grow faster, so the cowbird chicks get most of the food and have been known to push the host’s chicks completely out of the nest. The majority of the time the adult host birds end up feeding and caring for only the cowbird chick, rather than raising the next generation of their own kind (Texas Parks and Wildlife Department). As needed, the female cowbird will return to remove eggs of the host birds and even destroy their nestlings. Throughout North America, songbird numbers are declining. While there is no single reason for this decline, one major contributing factor is the spread of the cowbirds due to landscape changes. The removal of one female cowbird enhances the survival of 35 songbirds per year (Texas Parks and Wildlife Department). Therefore, the reproductive strategy of the brown-headed cowbird is to leverage a disturbance by entering through the forest edge zone and winning in competition against the other birds in the area. Since the brown-headed cowbird forages in open fields, but roosts in trees, it is a forest edge specialist because it can survive before the disturbance and excels after the disturbance occurred. A lesson to take from this is disturbance brings out competition amongst species, and only the ones who can superiorly adapt will survive. Also, a species with a short amount of time between egg laying and hatching, as well as rapid growth tendency has the upper hand. Further, a species evolved to feed in open fields and roost in trees is equipped to live in a forest edge zone.
Flagstaff Unified School District. “Habitat Fragmentation and the Edge Effect”. Accessed October 4. https://www.fusd1.org/site/handlers/filedownload.ashx?moduleinstanceid=9742&dataid=20535&FileName=3.2%20Habitat%20Fragmentation.pdf
Texas Parks and Wildlife Department. “Brown-headed Cowbirds”. Accessed October 4. https://tpwd.texas.gov/huntwild/wild/nuisance/cowbirds/
Comment by Amanda Ruffini:
I found this to be such an interesting topic to research on! I knew nest parasitism existed, but the brown-headed cowbird is a great example of this, as well as a great example of a forest edge specialists. They really use this specialization to effectively target other species. Human activity is only increasing this ability as it clears out more forest and opens the cowbird up to species of the forest that are not yet adapted to its reproduction strategy. What was even more fascinating to learn was they adapted this strategy due to following bison herds! While I didn’t find a lot of information to support that this particular cowbird is causing a decline in other species, do you think this is possible or saw any information to support it?
Comment by Professor Fenton Kay:
Great post, Mary. Do you think there may be ways to ameliorate the cowbird problem with respect to forest harvesting?
My Comment #1:
Original Post by Michael Muehlberger:
A forest edge is characterized by the transition of different types of vegetation that generally flow from shorter shrub vegetation to taller forest trees. These edge areas are the preferred habitat of the Brown-headed cowbird since it provides the perfect place to satisfy the dietary needs of this species (seeds and insects) as well as an ideal habitat to reproduce. These birds are considered brood parasites which means the females will lay their eggs in the nests of other bird species, and as a result the bird who owns the nest will feed and raise the cowbird chick alongside its own hatchlings. Unfortunately, the brown-headed cowbird chick can outcompete the other hatchlings for essential resources like food, and often the female cowbird will remove other host eggs from the nest. They also develop at an incredibly fast rate and leave the nest within 10-11 days! This poses many problems for the host species since the host species’ chicks will have a limited amount of the resources needed to grow, and the adult birds providing these resources will have to work harder to provide enough food for their entire nest.
Adult brown-headed cowbirds tend to roost in the taller forest trees and forage in open fields making the forest edge a perfect habitat to thrive in. Since these types of habitats are generally formed due to a disturbance event, like a clear cut, the cowbirds are able to easily adapt to the new environment. They can arrive earlier to the site, and over the span of just one nesting season a single female can lay up to 40 eggs. This will greatly impact the success rate of cowbird chicks that mature, and soon enough the cowbird population will thrive compared to other local bird species. The brown-headed cowbird is a great example of how a one-time event (i.e. clear cutting) can dramatically alter a habitat, and it helps strengthen the argument that ecological decisions should be carefully considered before being implemented.
National Audubon Society. n.d. “Brown-headed Cowbird.” Accessed October 5, 2020. https://www.audubon.org/field-guide/bird/brown-headed-cowbird.
United States Department of Agriculture. n.d. “Forest Edge Helps Wildlife Gradually.” Accessed October 5, 2020. https://www.nrcs.usda.gov/wps/portal/nrcs/mt/technical/ecoscience/bio/nrcs144p2_056670/.
I appreciate the way your post illustrates the advantageous interplay between a species being at a place where it can satisfy dietary needs and efficiently reproduce. It is a good reminder that the survival of any species significantly depends on those two factors.
It is interesting to consider the definition of a forest edge with your infographic in mind because you outlined the transition from shorter shrub to taller forest tress in the infographic. So, by matching up this post with infographic photos, I can specifically imagine when the cowbird arrives at the forest edge.
Also, your post effectively illustrates the advantage cowbirds have in terms of being able to get an early start. Since cowbirds can adapt to a recently disturbed site, they can arrive early and understand the lay of the land before other species. The early arrival combined with the high visibility they experience from tall tree tops helps them thrive. Further, they have a head start in terms of undergoing additional adaptation before others arrive. Therefore, they are equipped with superior traits when competition begins. It is amazing the cowbird can lay up to 40 eggs at a time. Also, thanks for providing the specific amount of days it takes for the young cowbird to leave the nest. That is rad! I see the cowbird’s rapid development as a major reason why the species is able to be so healthy at a forest edge. Lastly, phenomenal application showing how the information can be applied to ecological decision-making.
My Comment #2:
Original Post by Amanda Ruffini:
The forest edge serves as a transition zone between different types of vegetation zones, such as a tall forest and a field. The USDA even states that a forest edge can be a key component to “good wildlife habitat” on a farm (USDA n.d.). Brown-headed cowbirds, a forest edge specialist, prefers open habitats, such as the forest edge, but also pastures, meadows, and fields. This is because the brown-headed cowbird does not create its own nest when laying eggs. Instead, the bird is considered a “brood parasite” in which the females will lay eggs in the nest of other species. According to Cornell University, this bird seeks out females of other bird species who are actively laying eggs and once she has found a host, will “sneak” into the hot’s nest, damage or remove the eggs, and replace the eggs with her own (Cornell University n.d.). Because the bird has a shorter incubation time and grows quickly, it can outcompete its host family essentially.
Historically, the brown-headed cowbird followed bison herds and have adapted their reproductive strategy to fit the movement of these herds which move frequently. Clear cutting and removal of forest cover has only served to benefit this species of bird because it provides the brown-headed cowbird with more habitat and gives them access to hosts that have yet to adapt to their reproductive strategy of nest parasitism (Cornell University n.d.). Of course, as mentioned before, the brown-headed cowbird impacts the reproductive strategies of the forest species because they destroy the hosts egg and lay their own to be raised by the host. These chicks than hatch faster and grow at a faster rate, allowing them to outcompete the host family for food. Not only this, but the brown-headed cowbird does not only affect one species, it has invaded the nests of over 220 other species (Cornell University n.d.).
An interesting lesson from this is the adaptability of host species to the brown-headed cowbird. If a host were to reject the egg, the female would destroy the nest in retaliation. Therefore, while some species recognize it is not their egg, they do not reject the egg to prevent the destruction of their nest (Cornell University n.d.). However, a big takeaway seems to be the affect our actions have on the ecosystem. Clear cutting is increasing their habitats, which not only result in habitat fragmentation, further impacting wildlife, but allows for the brown-headed cowbird access to species that are not adapted to their reproductive strategy. Even without just clear cutting, human activity is resulting in climate change, which are affecting the biomes. On the Audubon website, a guide to north American birds, it shows how climate change will result in their habitat only increasing. The image below represents an increase in habitat during the summer season at a 3-degree temperature increase. While a worst-case scenario, it represents the effect climate change will have on a species such as the brown-headed cowbird, which in turn, will impact other species.
Audubon. N.d. “Brown-headed Cowbird”. Accessed October 6, 2020. https://www.audubon.org/field-guide/bird/brown-headed-cowbird
Cornell University. N.d. “Brown-headed Cowbirds”. NestWatch. Accessed October 6, 2020. https://nestwatch.org/learn/general-bird-nest-info/brown-headed-cowbirds/?__hstc=75100365.13b254c56c26bca328efe27f24e87ee8.1597348341084.1597348341084.1601974433438.2&__hssc=75100365.4.1601974433438&__hsfp=31047428#_ga=2.48449774.912533559.1601974432-1522454141.1597348336
United States Department of Agriculture (USDA). N.d. “Forest Edge Helps Wildlife Gradually”. Natural Resources Conservation Service”. Accessed October 6, 2020. https://www.nrcs.usda.gov/wps/portal/nrcs/mt/technical/ecoscience/bio/nrcs144p2_056670/
It is interesting the brown-headed cowbirds adapt well to a variety of areas humans distinguish including pastures, meadows, farms and fields. It is interesting to link its ability to do well in the various contexts with the fact it does not create its own nest while laying eggs. That being said, the cowbird’s mobility as a reproductive characteristic impacts where it can live.
I wonder why, originally, brown-headed cowbirds were drawn to following the bison herds. Is there some feature about the bison herds in which the cowbird is attracted to?
I also wonder, how may another host bird adapt to the cowbird’s reproductive strategy? It is interesting the hosts have learned to not reject the cowbird’s egg.
Thanks for showing the cowbird’s scope of impact in terms of biodiversity. 220 species is astounding!
Response by Amanda Ruffini:
So in trying to do a little more research, I found on a Stanford University page some information that may partially answer your questions. Links to an external site.They noted that 97% of cowbird eggs and nestlings never reach adulthood, given that a female cowbird lays up to 40 eggs a year, and 1/3 of all nests parasitized have more than 1 egg. This may be due to adaption by the host species, who destroy the egg, bury it to prevent it from hatching, or abandoning the nest all together. While the evidence is also circumstantial, the website also discusses the possibility that eggs are being laid in particular vulnerable host species, the ones not yet adapted to the cowbird and in which the expansion of their habitat is giving them new access too. Its all pretty interesting!
However, I still cannot find anything that really explains why the cowbirds originally followed the bison. I did find a CBS articleLinks to an external site. that briefly explained that cowbirds evolved with the bison herds, eating the insects disturbed by the bison grazing. It is possible then, that because bison also resided in open fields, like the cowbird, that it came down to the bison providing a food source for the birds. This, though, would require a little more digging to find out exactly why.