February 28, 2006

Gallus Domesticus

Ok, here are the answers I've been able to locate so far. I was quite amazed at the vast amount of information on chickens out there on the web. Not living in an agrarian community, I guess it's not something I think about. But it's quite the science. So, without further adieu, I give you my chicken research.

What is important to know concerning chicken anatomy? Chickens (which we'll all remember are female) have one main, um, lower opening called a "vent." The vent is used for the expusion of excrement as well as the laying of eggs. You'd think this would be terribly unsanitary for the eggs, but it isn't because just inside the chicken there are two tracts that lead up to the vent (think of how we have both an esophagus and a trachea in our throats and how, unless we're choking, the contents of each do not blend). See visual here.

What is important to know concerning rooster anatomy? The following quote describes it well: "The rooster's reproductive system is simple when compared to humans or other mammals. The rooster does not have a prostate gland or any of the accessory reproductive glands. Like all other animals, chicken sperm carry the genetic material from the rooster and are produced within the testes. The rooster has two very large testicles within the abdominal cavity on each side of the backbone. After sperm leave the testes, they enter the epididymis, where they gain the ability to swim. Next, the sperm enter the vas deferens, where they are stored until the rooster mates with a hen. Sperm formation takes about 15 days. The rooster's semen contains around 5 billion sperm per cc, about 40 times as much as that of a human. Once a rooster is mature and if he is maintained properly, he will manufacture about 35,000 sperm every second of his life."

Do chickens just lay eggs on their own, or do they need a rooster? Yes and no. Being that chickens are not asexual critters, they do need the rooster's assistance in the area of reproduction. But here's the deal. Chickens lay two different kinds of eggs: fertilized and unfertilized. The average chicken lays one egg every 25 hours. If there is rooster sperm inside her, it will fertilize the egg. If fertilized eggs are cared for, they will hatch into baby chicks. If the rooster hasn't been around for a while, then the eggs will remain unfertilzed. These are what we buy at the grocery store. (Actually, one site said, "Many [italics mine] of the eggs we purchase from the local supermarket are unfertilised eggs." I found this a tad disturbing.)

How can you tell if you accidentally got a fertilized egg? You know the white spot on the yolk? If "the egg has been fertilized this white spot will soon become red with blood as the embryo starts to develop."

What is the reproductive process? Unlike humans, baby chicks do not start out as glimmers in their parents' eyes. They start out as a yolk, which is produced in the chicken's ovary (again, about once a day). Like in humans, it is released down the chicken's oviduct. If fertilization is going to happen, it happens here. Then it continues to spiral its way through the oviduct, gains a membrane, and starts to acquire layers of albumin (which we know as the egg white). There are structural fibers that hook the yolk in place within the egg white. Finally, right before the egg is laid, it gains its calcium carbonate shell. If all is well, the blastoderm (soon-to-be chicken) becomes an embryo, lives off the yolk, deposits waste in a different place within the shell, breathes through the shell, and after 21 days hatches. For a better explanation, try this site.

How does the rooster fertilize a chicken's eggs? As Heidi informed us earlier, roosters mount the chickens by getting on top of them. For pictures (gosh, I'm distributing chicken porn!), see here and here. The process of copulation only lasts a couple seconds.

How long can a chicken be able to produce fertilized eggs after copulation with a rooster? According to Dr. Thomas Caceci, "Near the junction of the vagina and the shell gland, there are deep glands lined with simple columnar epithelium. These are the sperm host glands, so called because they can store sperm for long periods of time (10 days to 2 weeks!). When an egg is laid, some of these sperm can be squeezed out of the glands into the lumen of the tract, whence they will migrate farther up to fertilize another egg. This is one of the really remarkable things about birds; they sperm remain viable at body temperature, something mammals can't do." So basically, the sperm can remain viable up to 2 weeks inside the female. (Probably a good thing this isn't the case with humans, eh?)

Do chickens have an even ratio of male/female offspring? Hmm. Now this was interesting. Theoretically, the answer is yes. However, I found in an article titled "Female Birds Can Bias the Sex of Their Chicks" that there are certain factors that can influence the predominance of female or male chicks among birds. The discussion uses finches as an example, but it appears that this principle applies to chickens as well: "Whether a bird is more likely to lay a male or female egg depends on which sex will have the greatest chance of doing well. Rutstein et al. (2004) adjusted the food intake of female Zebra Finches [see photo of female (left) and male (right) Zebra Finches below right] & found that well-fed females were more likely to produce daughters, while less well nourished birds were more likely to have sons. This is exactly as predicted by the fact that female offspring need to be better nourished than males if they are to survive and grow well. The authors noted that: 'In most animals sex ratio is close to 50:50 and extremely resistant to change. In mammals, including humans, the sex of the baby is determined by whether the sex chromosome in the sperm is male or female. But in birds, it is the female’s egg rather than the male’s sperm that determines what sex the chick will be. Thus the female has the potential to determine the sex of her young by whether she ovulates male or female eggs. In some way, female Zebra Finches seem to be able to exert control over whether to produce a male or female egg depending on which of the two is most likely to be successful.'"

Can this happen genetically? According to the same site, yes. "Avian sex determination (Ellegren 2001)--The molecular determinants behind sexual development in birds remain a mystery. The process is known to be different from that in mammals, with no homolog to the gene that confers maleness in mammals found in birds. The failure to identify such a gene in birds is probably a reflection of the fact that, despite the occurrence of two sexes being nearly universal throughout the animal kingdom, the genes involved seem virtually unrelated among metazoan phyla. These differences raise obstacles for comparative or candidate gene approaches in studies of sexual development. In birds, females are the heterogametic sex, with one copy each of the Z and W sex chromosomes. Males are homogametic (ZZ). However, it is not clear whether it is the presence of the female-specific W chromosome that triggers female development, or the dose of Z chromosome that confers maleness. An intriguing additional possibility is that both Z and W matter! In marsupials, for example, Y acts as a dominant testis determining chromosome, while the X chromosome determines the choice between pouch and scrotum. Maybe a system where the two sex chromosomes mediate different aspects of sex differentiation is also used in birds."

So, can farmers determine whether their chickens lay male or female eggs? It appears that they can, to some extent, by altering the living conditions and diet of the chickens. If they want females (for breeding purposes), they will keep the chickens well-nourished and comfortable. If they want males (either for breeding or for eating), they will not keep the chickens as well-nourished or in as comfortable surroundings. And of course, if farmers just want eggs to take to market, they just keep a bunch of chickens on hand without any roosters so they don't have to worry about fertilized eggs getting into the mix.

Can you tell the difference between male and female chicks? You can, but it's extremely difficult. The more accurate method is called vent sexing, but few are skilled enough to do it successfully: "Vent sexing of chicks at hatching has complications that make it more difficult than sex determination of most other animals. The reason is that the sexual organs of birds are located within the body and are not easily distinguishable. The copulatory organ of chickens can be identified as male or female by shape, but there are over fifteen different different shapes to consider." The easier, but less accurate method is feather sexing, which is basically where people observe the physical qualities of the chicks. A lot of this varies with breed, though. Pretty much, if you want really accurate results, wait until the chicks mature.

"How do you know what color shell the eggs will have? The earlobes of the chicken dictate the color of the shell (white ear lobes = white eggs; red earlobes = brown eggs)."

What's the quote of the day? "The biggest problem with being a bird is that everyone is trying to eat you," given to us by Dr. Caceci.


Nicole said...
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Nicole said...

I found this post to be very interesting and yet extremely disturbing. For the record, I did not utilize the links of the "chicken porn" or anything I thought might be remotely disgusting.

I do have one question, though. My uncle (the one you love to hear stories about), raises chickens. He has a chicken that lays green-shelled eggs. If chickens with white earlobes lay white eggs and those with red earlobes lay brown eggs, what color of earlobes does a chicken have that lays green-shelled eggs? (I don't know if these eggs are the "Green Eggs and Ham" we've heard tales about or not.)

Oh, one more question. Is there any way to tell before you take an egg into crack for breakfast if it is a fertilized egg? If you are trying to raise more chickens, you wouldn't want to eat all the fertilized eggs. Yet, you certainly wouldn't want to leave unfertilized eggs out there too long. Chickens are very confusing creatures.

Becky Davis said...

Wow! That is a lot of information.

You need to get a life! :) I have a feeling that I would find Hebrew much more interesting than chickens, but then again, I am not a science person.

Jana Swartwood said...

Oh my gosh, the chicken pictures are no worse than anything you'd see on the Discovery Channel. Seriously. But Nicole, I do have answers to your questions. I'll try to post them tonight.

And Becky, it's not that I don't find Hebrew interesting. It's fascinating. I enjoy it a lot. It's just hard to be motivated to care about a class you're easily aceing, and it's also hard to discipline yourself to study the mundane parts (vocab, charts on infinitive structure in different verbal families) when you could be doing something more creative and inventive (even if that something has to do with chickens).

Jana Swartwood said...

I have more recently heard that the chicken earlobes/egg color connection may be false. So you might want to do some more checking on that before you quote me.

hunny8801 said...

My daughter Annabelle had been learning about farm animals at school. Her teacher decided it would be a treat to hatch some eggs for the class. At the end each student could purchase a chick for a dollar with their parent's permission of course. I bought one for my daughter(6 years old) and was planning on donating it to a farm when it got a little bigger. 3-5 months later Annabelle is very attached to it and it loves to follow her around! I don't know what kind of chicken or rooster I have and I don't even know what it is, a chicken or a rooster. If anyone has some advice about this I sure would appreciate it. I now plan on keeping this animal in a large pen outside and am looking into purchasing "friends" so it's not alone. but before I do that I'd like to know what I have first. My email address is HUNNY8801@HOTMAIL.COM. We keep it in a pen above the ground and I was wondering what to put in the pen? hay, wood shavings?

Anonymous said...

For HUNNY8801, hi im Alexandria and i am a owner of a rooster and a hen. I also was also able to do the same thing as your daughter, and have the opportunity to hand raise my own chicken in the 9th grade. Judging on the gender of your chicken, you may want to get it a hen if its a rooster and i rooster if its a hen. Your chicken is old enough to decipher the gender by using several techniques; such as the height of the comb, the size of the feet, if the chicken is starting to grow "spirr's" or if your chicken is laying eggs are some ways to tell the gender. Chickens are easy animals to care for. If you find your chicken is a hen you may want to get a coop with hay, they usually lay their eggs in dark places. If you want to email me pictures of your chicken so i can tell you its gender, feel free to email me at,
thank you, alexandria

Blondie said...

Thank-you! You answered all my questions. Now off to my coop to candle some eggs!

(it amazes me how some crazy *itch would take the time to criticize your great blog...thinking that Hebrew would be more interesting...WOW)

Keep up the great work!!