Marissa Wright-LaGreca
I: Its a bird! Its a plane! Its a cow! Its a bird on top of a cow..?
Hello, welcome to the story of a bird’s life; Brown-headed Cowbird edition. On first impression, these strikingly plain birds seem fairly ordinary, however, the Brown-headed Cowbird requires a few dates to really get to know.
Identification
Fortunately, the name of the Brown-headed Cowbird (BHCO) is straightforward; Belonging to the Family Icteridae, adult males can be identified by their black body with a slight green sheen, and chocolate-brown head.
BHCOs have a slightly pointed but stout bill. Molting juvenile males have patchy black and brown coloration. Adult females have a drab brown coloration with fine brown streaks down the belly and a whitish throat.
Brown-headed cowbirds may look completely black in poor lighting, in which case they may be confused with other blackbirds. However, its conical bill (shorter than body), thick neck, large head, and smaller overall size (~40-50 g) can be used to distinguish it from other blackbirds (Cornell…2019).
Another species of cowbird is the Bronzed cowbird. It can be distinguished from the BHCO by the adult male’s red eye, thicker neck, and glossy-blue wing color. The range of Brown-headed Cowbirds and Bronzed Cowbirds has overlapped, resulting in competition for host nests as both species are brood parasites (more on this later). Bronzed Cowbirds tend to parasitize larger birds than the Brown-headed, however, both eggs have been recorded to be in a single host nest (Cornell…2018).
Feeding
Classified as a ground forager, Brown-headed Cowbirds feed largely on seeds and insects. (Cornell…2019). Insect consumption increases while breeding and is an important component of hatchling’s diets.
In the early 1800’s, bison populations in North America were great (estimated 30 million) and were the main “riding hosts” of the cowbird (Bison 2013). As the bison population fell due to over-hunting following European colonization, cowbirds transitioned to horses and cows that grew increasingly abundant with agricultural expansion.
In a sort of cowboy-esque manner, cowbirds sit on horses and cows (hence the name Cowbird). As the cows graze, insects are stirred-up, enabling the cowbird to more easily spot its prey.
Habitat
Brown-headed Cowbirds occupy farms, fields, urban areas and semi-open habitats. They can inhabit a large range of areas, however, tend to avoid dense forests. Unbroken forests may deter Brown-headed Cowbirds as it is difficult to locate and stalk host-nests under dense foliage. Habitat fragmentation has aided in the dispersal of Brown-headed Cowbirds. (Cornell…2019)
Sounds
The male Brown-headed Cowbird sings a uniquely distinct song that is attractive to female BHCOs. The song is filled with gurgling notes and thin whistles. The song sounds as if the cowbird is taking a big swig of water, however, the video looks more as if he is throwing up water. As the bird sings, he puffs out his chest and feathers:
Males and females both make a variety of whistles and chattering. Females make a discernible chattering sound that is attractive to males and may act as a species-recognition “password” for hatchlings (more on this later):
The begging sounds of a hatchling Brown-headed Cowbird tend to be a much louder and persistent compared to its foster siblings. How a young cowbird learns the song and sounds of its own species has long since been a mystery, making brood parasitism a fascinating topic in developmental biology.
Conservation
Although not recently introduced, cowbirds are considered a pest throughout North America due to their parasitic behaviors. According to the International Union for Conservation of Nature Red List, Brown-headed Cowbirds are ranked as a “least concern” as of August 2018. (BirdList 2018)
Many people attempt to remove a cowbird egg themselves if they spot one in a host bird’s nest. However, in many regions it is illegal to remove the egg. Since cowbirds are a native species in North America, they are protected under the Migratory Bird Treaty Act in Canada and the United States. Yes, it seems that our protection of cowbirds, the described “criminals” of nature, is because they are our “criminals”.
Some exceptions to the Migratory Bird Treaty Act include conservation efforts of Kirtland Warblers and Least Bell’s Vireos in the US, in which egg removal and cowbird entrapment is legal with the proper permits. This raises an important ethical question of our place to interfere with processes such as brood parasitism (Audubon 2018).
A brief interlude for a Christmas story
T’was a white Christmas in Hopkinsville, Kentucky, in the winter of 2012. A flock of an estimated 5 million blackbirds (including Brown-headed Cowbirds) and European Starlings rolled through the small town, leaving a white landscape of bird excrement (Cornell…2019). It is common for Brown-headed Cowbirds to flock with other blackbirds (such as Red-winged Blackbirds) and European Starlings. However, this particular gift of Christmas may carry the fungus, Histoplasma capsulatum, which is poisonous to humans and dogs. The massive flock’s unexpected arrival is linked to climate change. Due to increasing temperatures, the birds did not have to travel as far South as they would normally (Audubon 2013).
II: “Well that’s (b)rude”: Brood parasitism of the Brown-headed Cowbird
Although, the Brown-headed cowbird has charmed us thus far, its brood parasitism may complicate things.
Continuing with the Christmas theme, female Brown-headed Cowbirds act as a sort of baby-giving Santa Claus. However, this Santa not only leaves its own egg, but also kicks out or swallows one of your own eggs in exchange. So how does Santa decide who is on the nice list this year? Well, Brown-headed cowbirds are host-generalists and have been recorded to parasitize 220 species, however, only successfully in 144 (Cornell…2019).
The clutch size of Brown-headed cowbirds is between 1-7 eggs, laying around 40 eggs in a breeding season! With only a 3% survival rate, the Brown-headed Cowbird is certainly playing a numbers game. Usually, a bird’s clutch size and number of clutches laid in a breeding season is constrained by the available parental care. Since the BHCO does not raise its offspring, their clutches are presumably not limited by parental care but rather by the available host nests and physiological constraints. The Brown-headed Cowbird will only have a 1-2 day break between each clutch. Although lazy in the parental department, cowbirds certainly have their work cut out for them in terms of egg laying. This is where the nickname of the “passerine chicken” has stemmed from. Other passerines experience ovary and oviduct regression in between laying periods, however, the Brown-headed Cowbird’s ovaries and oviducts do not regress. Since the Brown-headed Cowbird is almost constantly fertile, it is able to synchronize its laying with the laying of the host (Scott and Ankney 1983). The BHCO, unlike 97% of birds, are not monogamous and will have multiple mates in a breeding season.
Once the host has laid its eggs, the Brown-headed Cowbird can sneak in while the mother is out, kick out or eat one of the host eggs, and lay her own. But why not just kick out all of the host eggs? Well, that may give away the cowbird’s presence, causing the host bird to abandon the nest or destroy the eggs. The Brown-headed Cowbird’s parasitic “magic act” is reliant on the host-mother never knowing she was there. Watch the Brown-headed Cowbird in action:
Species’ anti-parasitic defenses can be divided into three categories: front-line defenses (aggression towards adult Cowbirds), egg-stage defenses (limit egg hatching), and fledgling defenses (decrease survival of the hatchling) (Abolins-Abols and Hauber 2018). Egg-stage defenses include egg rejection. American robins will reject an egg if it differs in both size and color from their own blue eggs. (Luro et al. 2017). Co-evolution between the appearance of Cuckoo’s eggs and their host-species’ eggs is driven by the Cuckoo’s host-specific brood parasitism. Brown-headed cowbirds are general brood parasites, so they more loosely mimic a wide-range of host-species’ eggs rather than have few and specific hosts as the Cuckoo does.
![Page [unnumbered] of Differential Ejection of Cowbird Eggs and Non-Mimetic Eggs by Grassland Passerines](https://lh3.googleusercontent.com/cub-UZ8zUFuZLy0tUg0xIt5wK4psCaeGqQfA62NxoruXuqJFMvHIOSz7JwXyxymhwwOfBgkenoEYNrH3S3MWyduCf5W4GLLk30L7jeiOLMymx3-kCa34HufZ-leR5q5VFlKFDyxa)
Egg rejection seems to be lost in several species of birds. The study, Differential ejection of cowbird eggs and non-mimetic eggs by grassland passerines, placed both cowbird eggs and non-mimetic blue eggs in the nests of six grassland species. All six species accepted nearly all cowbird eggs that were placed in their nest, demonstrating almost no egg rejection towards cowbird eggs. However, when the non-mimetic blue eggs were placed into the nests, only one of the species (Western Meadowlark) rejected the non-mimetic blue eggs most of time (92%), while the other five species only demonstrated rejection 9-20% of the time (Klippenstine and Sealy 2008). This suggests that the five species that did not eject the non-mimetic blue eggs, showed an absence of egg rejection behavior despite obvious egg morphology discrepancies. The loss of egg rejection behavior may be a result of an evolutionary equilibrium in which the trade-offs of potentially rejecting one’s own egg is too great of a risk. Therefore, rather than risk the accidental ejection of one’s own eggs, it is more favored to accept all eggs, reducing the selection for “picky” mother birds.
Another proposed reason for the loss of anti-parasitic defenses in host-species is mafia behavior. Documented in species of parasitic Cuckoos (Soler et al. 1995) and later in Brown-headed cowbirds, mafia behavior may encourage brood parasitism. Mafia behavior is a strategy utilized by the brood parasite mother to encourage egg-acceptance by the host. The brood parasite will remain near the nest after laying to watch if the host will accept her egg. The theory of mafia behavior arose due observations of increased predation of rejector nests compared to acceptor nests (Hoover and Robinson 2007). If the host rejects the egg, the parasitic mother will attack the nest in retaliation. This behavior enforces the acceptance of the egg or the “mafia” will make you pay.
Often, cowbirds parasitize birds of much smaller size, increasing the competitive-edge of the cowbird hatchling in the host nest. However, this almost comical size difference can greatly influence the fitness of the host. The development of the cowbird is rapid compared to many other birds both before and after hatching. Relative to egg-volume, Brown-headed Cowbirds often have rapid incubation rates (10-12 days). Yellow Warbler eggs (whom BHCOs parasitize) are much smaller yet have almost the same incubation duration (10-13 days). In fact, the mere presence of a parasitic egg has been shown to disrupt the incubation of the host’s eggs, lengthening incubation duration by reducing the amount of heat available to the host’s eggs (McMaster and Sealy 1998).
Hatching before any of its foster siblings gives the cowbird a developmental head-start. As well, after hatching the cowbird growth is quite speedy. The rapid development is reliant on a large amount of insects, putting a constraint on cowbirds to parasitize insectivores. The young cowbird will beg much more persistently and louder than its foster siblings (Dearborn 1998). If the bird hosts the cowbird egg, it may allocate much of its energy to support the massive cowbird young (but I mean, how could you resist such a beautiful face?).
A study conducted in 2014 tracked Brown-headed cowbird fledglings in an attempt to answer the mystery as to how young cowbirds learn their identity. Louder et al. hypothesized that the mother cowbird may linger around the host nest to facilitate their offspring’s dispersal and development (2015). The study found no evidence that the mother facilitates the young cowbird’s learning of their identity, but rather, they found that the young cowbird, at around 20-25 days old, begins to live a secret life…
Much like an angsty teenager, young cowbirds begin to take excursions in the middle of the night. The desire to leave the safety of the nest can be compared to zugunruhe or migratory restlessness. These late-night excursions may serve to expose the young cowbirds to con-specific songs and behavior, gathering in areas were adult cowbirds roost. There was no evidence of parental care by the adult cowbirds in the roosting colonies, rather, the young cowbirds just observed the behavior of the adults. These late-night adventures creates separation from the young cowbird and the host nest.
However, these late-night excursions still do not answer how cowbirds are able to identify adult cowbirds as their own species. Previously it was thought that species recognition in brood parasites was exclusively genetically determined, however, now it is thought to be a blend of genetics and environment. The “blend” theory is supported by observations of Brown-headed Cowbirds raised without the presence of adult cowbirds (in captivity), in whom adopted characteristics of their host and attempted to breed with members of the host species (Audubon 2016). Louder et al. purposes that there may be a “password” (vocalizations) that initiates the juvenile’s learning behavior toward the password giver (2019). In the study, cowbird hatchlings showed evidence of recognition towards the chatter call (audio 1). Perhaps, think of the chatter call as a kind of “open sesame!” in bird language.
How shall I deter Cowbirds?
Not a fan? Well, if you wish to deter Brown-headed Cowbirds from your backyard, it is recommended to use bird feeders that are too small for cowbirds to access or use feed that cowbirds will not eat such as whole peanuts, suet, safflower seeds, or thistle (A Bird’s Delight 2018). Further, increased research on Brown-headed Cowbirds may assist in population control by broadening our understanding of the harsh but fascinating topic of brood parasitism.
Thank you for reading, I hope you enjoyed!
References
Abolins-Abols, M., & Hauber, M. E. 2018. Host defences against avian brood parasitism: an endocrine perspective. Proceedings. Biological sciences. 285(1886). doi: 10.1098/rspb.2018.0980
All about birds [internet]. 2019. The Cornell lab of ornithology: [cited 2019 Nov 10]. Available from: https://www.allaboutbirds.org/guide/Brown-headed_Cowbird/overview
American Redstart feeding a cowbird [internet]. 2013. Project FeederWatch: [cited 2019 Nov 20]. Available from: https://feederwatch.org/birdspotter-2013/american-redstart-feeding-a-cowbird/
Bison [internet]. 2013. The Canadian Encyclopedia: [cited 2019 Nov 10]. Available from: https://www.thecanadianencyclopedia.ca/en/article/bison
Blackbirds Swarm Kentucky Town [internet]. 2013. Audubon : [cited 2019 Nov 10]. Available from: https://www.audubon.org/news/blackbirds-swarm-kentucky-town-it
Brown-headed Cowbird [internet]. 2014. San Diego bird spot: [cited 2019 Nov 10]. Available from: http://sandiegobirdspot.com/brown-headed-cowbird/
Christmas Events, Parades & Holiday Theatre in Calgary 2019 [internet]. 2019. Things To Do Canada: [cited 2019 Nov 10]. Available from: https://www.todocanada.ca/christmas-plays-calgary/
Cow bird song and password [internet]. 2019. BirdNote: [cited 2019 Nov 10]. Available from: https://www.birdnote.org/show/cowbird-song-and-password
Ehrlich, P. R., Dobkin, D.S., and Wheye, D. Cowbirds. 1988. Stanford, CA. [cited 2019 Nov 10]. Available from: https://web.stanford.edu/group/stanfordbirds/text/essays/Cowbirds.html
Finch feeds cowbird. 2008. flickr: [cited 2019 Nov 10]. Available from: https://www.flickr.com/photos/27089900@N00/2572586176
Hoover, J. P., & Robinson, S. K. 2007. Retaliatory mafia behavior by a parasitic cowbird favors host acceptance of parasitic eggs. Proceedings of the National Academy of Sciences. 104(11): 4479-4483. Available from: https://doi.org/10.1073/pnas.0609710104
How does a cowbird learn to be a cowbird? [internet]. 2016. Audubon: [cited 2019 Nov 10]. Available from: https://www.audubon.org/news/how-does-cowbird-learn-be-cowbird
How to get rid of cowbirds [internet]. 2018. A bird’s delight: [cited 2019 Nov 15]. Available from: https://abirdsdelight.com/how-to-get-rid-of-cowbirds
Is it okay to remove cowbird egg’s from host nest? [internet]. 2018. Audubon: [cited 2019 Nov 10]. Available from: https://www.audubon.org/news/is-it-okay-remove-cowbird-eggs-host-nests#
Klippenstine, D. R., & Sealy, S. G. (2008). Differential ejection of cowbird eggs and non-mimetic eggs by grassland passerines. The Wilson Journal of Ornithology. 120(4): 667-674. Available from: https://doi.org/10.1676/07-074.1
Louder, M. I., Balakrishnan, C. N., Louder, A. A., Driver, R. J., London, S. E., & Hauber, M. E. 2019. An Acoustic Password Enhances Auditory Learning in Juvenile Brood Parasitic Cowbirds. Current Biology. Available from: https://doi.org/10.1016/j.cub.2019.09.046
Louder, M. I., Ward, M. P., Schelsky, W. M., Hauber, M. E., & Hoover, J. P. 2015. Out on their own: a test of adult-assisted dispersal in fledgling brood parasites reveals solitary departures from hosts. Animal behavior. 110. 29-37. Available from: https://doi.org/10.1016/j.anbehav.2015.09.009
Luro, A. B., Igic, B., Croston, R., López, A. V., Shawkey, M. D., & Hauber, M. E. (2018). Which egg features predict egg rejection responses in American robins? Replicating Rothstein’s (1982) study. Ecology and evolution. 8(3): 1673-1679. doi: 10.1002/ece3.3759
McMaster, D. G., & Sealy, S. G. 1998. Short incubation periods of Brown-headed Cowbirds: How do cowbird eggs hatch before Yellow Warbler eggs? The Condor. 100(1): 102-111. doi: 10.2307/1369901
NestWatch [internet]. 2019. The cornell lab of ornithology: [cited 2019 Nov 10]. Available from: https://nestwatch.org/learn/general-bird-nest-info/brown-headed-cowbirds/
Scott, D. M., & Ankney, C. D. 1983. The laying cycle of Brown-headed Cowbirds: Passerine chickens? The Auk. 100(3): 583-592. Available from: https://www.jstor.org/stable/4086459
Soler, M., Soler, J. J., Martinez, J. G., & Moller, A. P. 1995. Magpie host manipulation by great spotted cuckoos: evidence for an avian mafia? Evolution. 49(4): 770-775. Available from: https://doi.org/10.1111/j.1558-5646.1995.tb02312.x
The IUCN Red List of Threatened Species [internet]. 2018. BirdLife International:[cited 2019 Nov 10]. Available from: https://www.iucnredlist.org/species/22724354/132175819
Hi Marissa,
Thank you for such a super engaging and funny blog! You kept me hooked right until the end with the puns, jokes, and media! I love the comparison of the Brown-headed Cowbird to an angsty teenager!
I liked your Christmas story interlude. That is a really interesting occurrence! Did it only happen in Hopkinsville, or was it reported anywhere else? Has this ever happened again since 2012? So neat (maybe not for humans and dogs though)!
You mentioned that the song-learning method of the Brown-headed Cowbird is still a mystery. That is super interesting considering what a hot topic song development is! Regarding the parasitism side of the Brown-headed Cowbird’s life history, you mentioned that they have a clutch size of 1-7 eggs. Are the cowbirds depositing each egg in a different host nest? Or do they sometimes double+ up on the eggs in one host nest? I guess I’m asking because when I think of a clutch, I think of a unit that sticks together in the next.
Thanks again for such a great blog!
Samuelle
Hi Sam, glad you enjoyed! I believe out-of-the-ordinary and large flocks like this have occurred elsewhere, this was just a well-documented occasion because of the shockingly large numbers. And yes I agree song development is very interesting, particularly in the weird case of the cowbird! The BHCO often lays their clutches in several different nests, splitting up the clutch family :'( . However, they have been recorded to “double-up” (ie. lay up to 2 eggs in a single nest) in rarer cases. Although, this would again cause competition between their offspring , may cause the host to become suspicious, and will increase the “energy-load” of the host (she must take care of 2 massive cowbirds instead of one). Like a parasite, the cowbird doesn’t want the host to not survive, rather, depends on exploiting a living host.
Thanks again!
Hi Marissa,
Great blog, definitely had me chuckling. Especially the comment of them being OUR criminals so we need to respect them whether we like them or not 😉
I found the mafia behaviour extremely interesting. I was wondering if anywhere in your research you came across a reason for that behaviour other than for revenge. We can assume the bird is not consciously exacting revenge on the mother however attacking the mother who rejected the egg must be energetically taxing and could result in the cowbirds injury or death. My question I guess then is why? and what evolutionary advantage would this provide?
Thanks!
Emma
Hi Emma, thanks for the comment! And yes it does seem a bit strange that brood parasites like the cowbird would use so much energy to enforce the acceptance of her eggs. Although, since the cowbird is not raising young, they do have some more “energy in the bank” to spend. If we think of this behavior with an evolutionary context, enforcing egg acceptance would benefit the cowbird greatly, increasing the number of hosts the cowbird can parasitize. Although it is energetically costly to continually “bully” the host into accepting the egg, it clearly offers an advantage by reducing the energy that would be expended during parental care. Instead of raising young, the cowbird can keep popping-out babies, and hopefully (with the help of mafia behavior), some of them get accepted by the host. Thanks again Emma!
Hi Marissa!
I found your blog super pleasant to read and super interesting!
Since the cowbirds are a known ‘pest’, are there any laws against shooting them if you lived on a farm far away from civilization to judge you?
Hope you don’t take my question too seriously!
Cheers,
Dan
Thanks Dan! And no judgement here! I actually found a slightly comedic article of a man wishing to shoot cowbirds (to protect the Cardinal nests on his lawn) with air rifles. However, he talks more about getting permission from his wife rather than the law. I also found an entire manual dedicated to cowbird control which has a section on shooting. Apparently in Texas, there are shooting events in remote areas, attracting cowbirds using the female chattering call. So it seems that the legality of shooting cowbirds is very location dependent, but I believe for the most part it is illegal. If it is in fact legal in your location, make sure to double check with your wife first 😉
Thanks again!
links,
manual- https://www.usbr.gov/tsc/techreferences/env/Siegle_Cowbirdmanual.pdf
interesting article- https://www.statesman.com/article/20120924/NEWS/309248343
Hey Marissa,
Really great job on your blog!! I actually never thought that these birds rode on bison and cows and that’s where they got their name! I just assumed it was just another unique name!
I know you mentioned that there wasn’t really any conservation concerns for these birds but since they are frequently inhabiting farms and riding along on cattle, I was wondering whether you came across anything involving Brown-headed Cowbirds being affected by pesticides used at these farms in the past or present?
Thanks,
Danielle
Wow that’s a super interesting question Danielle! Didn’t cross my mind while writing the blog. Since pesticides impact the food chain I would presume that cowbirds would be affected by chronic exposure. While looking into it I found some pretty interesting papers (some of them a bit disturbing :o). The first was a study used to quantify the lethal mobilization of DDT on cowbirds. The study measured the mortality and the effects/level of DDT in cowbirds after they were restricted from food (pretty dark but it was the 70s). Also, I found an article that talks about pesticides being used to control blackbird pests such as the cowbird. With certain permits it seems to be legal to use pesticides such as Methyl Anthranilate, Polybutene, and Avitrol, on bird-pests. Although, I couldn’t find many recent studies considering the effects of pesticides (that aren’t primarily aimed to kill/deter bird pests) on Brown-headed cowbirds in farmlands, it is certainly an interesting topic. Thanks for the great question, hopefully my semi-convoluted answer suffices!
links to the articles 🙂
– DDT article: https://www.jstor.org/stable/3799424
– Pesticides against bird pests: https://www.aphis.usda.gov/wildlife_damage/nwrc/symposia/blackbirds_symposium/jacobs.pdf
Hi Marissa!
Fantastic blog!
You mentioned that female cowbird’s either kick out or eat a host species egg and then replace it with their own. I was wondering if this is the case 100% of the time. I just thought they would have layed their egg among the hosts’ and then just take off. If the host is a species that can count their eggs, I can understand the advantage of removing one before laying . But, not all hosts can count, so why go through the extra effort of removing an host egg? Also, if the added benefit of removing a host egg by eating it is nutrition, then why the option of just kicking one out? Sorry for all the questions at once!
Thanks!
Melissa
Hi melissa thanks for the comment 🙂 and all of those are great questions. As far as I know and have been able to find, cowbirds will always destroy an egg (or at least attempt to). There may be times that the cowbird fails to destroy the egg, not pushing it far enough so that it falls out of the nest, resulting in a higher number of eggs. As you said, the cowbird is motivated to remove an egg because it decreases the chance of the mother rejecting the egg if she is of a species that can count eggs. Since cowbirds lay in so many nests and has no way of knowing which of the nests are from species that can count eggs, it may be a sort of a “safe guard” to act as if all bird species can count.This “safe guard” may have retained the egg destroying behavior in cowbirds. Another advantage of destroying an egg is that it decreases the number of hatchlings the cowbird will have to compete against. Also, I didn’t have a chance to include it in my blog but I found some research into why cowbirds eat the eggs, other then just for basic nutrition; eggs (specifically the shells) are a great source of calcium! Cowbirds are making tons and tons of eggs so they need a high source of calcium. The study I was looking at also recorded females eating mollusc shells as a source of calcium. But like you asked, why not eat the eggs all of the time? Unfortunately, I couldn’t find too many answers on that. It may just be as simple that she does not need the egg nutrients at that time or perhaps eggs are more difficult to digest than their normal seed/insect diet so eating a lot of eggs is not optimal. I looked into an article that studied the egg removing and eating behavior, however, they didn’t come up with any conclusive findings. It can be difficult to record how much eggs are being eaten vs thrown out, making it hard to study. The study also raised the question if egg-eating in cowbirds is a learned behaviour or if naive cowbirds recognize eggs as food from the get-go. Definitely some really interesting questions!
Thanks Melissa 🙂
links to the studies
– egg-eating: https://www-jstor-org.ezproxy.viu.ca/stable/pdf/1369242.pdf
– egg nutrition/calcium source: https://www.jstor.org/stable/pdf/4085740.pdf?casa_token=2R_aejIscpIAAAAA:STLTjqBaxnGv85KExY001o2K2l70-KrcmhHvPgkDO5HLgeq5aH4hNdgGdhFJ-ZmMwDNruJfYJxtmTGE68nXwjf6yCVaN8MOY-wHCI9h6n31jPZpEUg
Hey Marissa,
Loved the casual and humorous but informative style! Very engaging!
The interlude was super interesting if not a little bit eerie :0 … fingers crossed Santa doesn’t bring any fungus this year!
My question is about the nightlife of these young birds! I find it fascinating that they sneak out at night. You mention that they sneak out between 20-25 days after hatching, is this an every night ordeal that sees them returning to the nest for the next day? I had never considered how offspring of brood parasites have an obvious hurdle to overcome in terms of learned behaviours!
I am so conflicted on these birds because they are super interesting, but they hurt other cool birds too 🙁 Nonetheless thank you for posting a thought-provoking blog that highlights the amazing inter-specific interactions of the BHCB
Joel
Hello Anonymous Joel,
Thank you for the comment 🙂
I believe that the late night excursions increase in duration and frequency as the bird gets older. So in the beginning, the cowbird may leave the nest once every couple of days, but as it becomes more separated from the host and is more physically capable of being on its own, it starts to leave every night. And yes I have a similar dilemma with the brown-headed cowbird where I think they’re so cool but also pretty mean! Thanks again,
Marissa