Pigeon Week Part 2: Mating, Meiosis and Milk

The next way cool thing about Pigeons is how they raise their young. The next time you hear someone go on about “feathered rats”, consider this: on average, Pigeons are arguably more doting- and certainly more egalitarian- parents than we are. Pigeons are prolific breeders* and both sexes take the business of child-rearing quite seriously.

*Feral pigeons, especially, for reasons we’ll get into in Part 3.

Climate permitting, feral pigeons breed year-round. Here in North America that means they’ll breed year-round, or close to it, up to about latitude 40 degrees North*. In the UK, it’s clear up to 54 degrees North.

*Which is, by wonderful coincidence, the latitude of Salt Lake City. How cool is that? Pretty freaking cool, that’s how. I tell you, the longer I live here, the more I feel like it is the center of the universe.

Pigeon courtship involves a fair amount of cooing, bowing, playful running, and- apparently- foreplay, including something called “billing”, in which, in a weird analog of French-kissing, the female inserts her bill inside the male’s open bill, usually with both their eyes closed. Following mating, the male often makes a brief “display flight”, in which he claps his wings behind his back*. Pigeons mate for life.

*I love this. It’s like he’s strutting about, high-fiving himself.

About 10 days following mating, a female pigeon lays 2 eggs, roughly 40 hours apart. There are 2 really interesting things about these eggs. The first is that the eggs are almost always 1 male and 1 female, which seems neat but odd. In pigeons, like all birds, sex is determined chromosomally, in a manner similar to- and yet completely opposite from- that of mammals.

This Part You Already Know But I Included It Because On The Off-Chance You Don’t Know It You’ll Get Totally Lost

In mammals, like us, sex is determined by an XY chromosomal system. We have 46 chromosomes arranged in 23 pairs. Each pair represents 2 of the same type of chromosome, one of which received from our mother, the other from our father. The exception is our sex chromosomes. Females have 2 of the same type, called “X’, but males have 1 “X” and one “Y”. Only men carry a “Y” chromosome. When a human female produces an egg, it contains just 23 chromosomes*, one of which is always an “X”. When a human male produces a sperm cell, it contains just 23 chromosomes, one of which is either an “X” or a “Y”. So the chances of a human couple conceiving a boy or a girls are, generally speaking**, 50%.

*The obvious exception is the occurrence of trisomy caused by the production of a sex cell (by either parent) that contains 2- and not just 1- chromosome at a given location, resulting in a fertilized zygote containing 3- and not 2- chromosomes at that location. Trisomy can occur on any chromosome, but is only survivable (to birth) at a few locations, including the X/Y chromosomes, and chromosomes #21 (Down’s Syndrome), #13 or #18 (the latter 2 always resulting in severe birth defects.)

**It’s actually a bit more complicated than this. Slightly more human boys than girls are born. But that’s a topic way outside the scope of this post.

This Part You Already Know If You Are A Longtime Reader Of This Blog (Or Just Knowledgeable About Birds)

In birds, the exact opposite is going on. Pigeons have 18 chromosomes arranged in 9 pairs, 1 of which is the sex chromosomes. But it’s males who have the same chromosome- called “Z”- while females bear 1 “Z” and 1”W”. Only female pigeons carry a “W” chromosome. When a female pigeon produces an (unfertilized) egg*, it contains just 9 chromosomes, one of which is either a “W” or a “Z”. When a male pigeon produces a sperm cell, it contains just 9 chromosomes, one of which is always a “Z”. So the chances of a pigeon couple conceiving a male or female squab is, generally speaking, 50%.

*As opposed to a fertilized, make-an-omelette kind of egg. So I guess “egg” is loaded term here, and I probably should say “ovum.”

If you have a 50/50 chance for a boy or a girl, and you have 2 children, then out of the 4 possible combinations- boy-boy, girl-girl, boy-girl, and girl-boy- 50%, or 2, of your two-child pairs will include 1 boy and 1 girl, and the other 2 will be either both boys or both girls*.

*And of course this is the case with human fraternal twins, such as Twin A & Twin B. The reason most twins overall are one sex or the other is because all identical twins are always the same sex. BTW, the single dopiest question you can ask a parent of boy-girl twins is, “Are they identical?” Seriously, you might just as well tattoo “I HAVE NO CLUE” across your forehead.

Only with pigeons, that’s not what happens. The vast majority- far more than 50%- of pigeon squab nest-mates are male-female. How can this be? The mechanism is unknown.

It gets even weirder. 70% of the time, the first egg laid is male, and the second, ~40 hours later, is female. How can this be? Again, the mechanism is unknown. The male, laid earlier, generally hatches before the female, begins eating, and gains weight rapidly. If food is sparse, and only 1 squab makes it, it’s usually the slightly bigger, first-born male who survives, and the female who dies. The benefit of laying the male egg first seems to be a greater survival likelihood for the male, though the advantage of this to the parent is unclear.

Conjectural Tangent*: Whatever’s happening, it’s happening in the female. Remember, the “W” chromosome can only come from her, so it’s her ova that are determining the sex. Presumably there’s some benefit to pigeons, some slightly better statistical success-payoff in the long run, to bearing young in pairs of opposite sex. And presumably, if only one of your 2 squabs is going to make it, there’s some slightly better statistical success-payoff in the long run if that survivor is male ~2/3 of the time.

*Warning: All of my conjectural tangents are just that- totally conjectural, and almost certainly wrong. But they’re fun to think about.

I say “presumably” because of course I have no idea. But there are millions and millions of pigeons, with an average generation-time of just a couple of years. And across thousands/millions of years, and billions of pigeons, these weird statistics have worked out and hold steady worldwide. There has to be a long-term statistical driver behind it.

So somehow, female pigeons have evolved to either a) release 2 ova roughly concurrently, but slightly staggered, or b) release 2 ova concurrently, but, following fertilization, stagger development of the embryos by roughly 40 hours. And more remarkably, she’s evolved to produce these ova, in most cases, alternating between W and Z ova, which in turn implies that whatever mechanism is at work here is at the meiosis level. That’s incredible! How is it controlled? What is going on?

Meiosis- or the division of diploid cells into haploid gametes- is the process by which all sex cells are created, and occurs in 4 distinct phases, encompassing 2 stages of cell division. I won’t go through all the details, but in human females, the first phase- called Prophase 1- happens before birth. In Prophase 1 the chromosomes in the initial germ cell split, which, in a female bird, means that one half ends up with a “W”, and the other with a “Z.” The proto-eggs then remain “on ice” until puberty, when they complete the meiosis process. But the initial splitting of the germ cell’s chromosomes happens in Prophase 1.

Does the timing work this way in birds? I don’t know*. But if it does, it would mean that the sex determination of gender- which ultimately occurs in Prophase 1, specifically in the female- would happen before birth. If this is the case, it means that somehow the female pigeon is positioning or packaging eggs, by gender, for eventual sequenced release, before birth.

*Specifically, does Prophase 1 occur before birth? I spent- no kidding- an hour online trying to find out before I gave up.

NOTE 9/22/10: Anonymous commenter answered this footnote-question. In birds Prophase 1 occurs 24-48 hours before ovulation (very different from the timing in mammals). See comments for details, source..

This sounds crazy, but the other possibility sounds even crazier: the female pigeon, as an adult, actively and real-time recognizes and sorts eggs by gender internally. How could this be? What “signature” would an egg bear based on whether it contained a W or a Z chromosome?

Whatever is going on inside of female pigeons, it is some seriously complicated shit.

The couple shares incubation duties, the male incubating by day, the female by night. After about 18 days the 2 eggs hatch, usually about 24 hours apart. After the squabs hatch, the parents feed them, which is pretty normal for all sorts of birds. But what they feed them is not; they feed them milk.

It isn’t actually “milk”, in the strict, mammalian sense of a white emulsion of fatty globules in a water-based fluid. But it is a remarkable analog, and like so many avian-analogs we’ve looked at- color vision, foveal vision, thermoregulation, gender-determination, intelligence/self-awareness- it’s a fundamentally different way of solving the same problem.

Pigeon milk is produced by fluid-filled cells lining the crop, which is a specialized organ present in many birds, located at the bottom of the esophagus, and generally used for food storage. Crops are thought to have come about as a mechanism for a bird to quickly gather and accumulate food from a location more rapidly than it might consume it, thereby minimizing on-location exposure to potential predators. Pigeons and game birds tend to have larger crops. The evolution of development of “milk”-production in the crop is an apparent example of an exaptation, or the leveraging by natural selection of a component(s) originally evolved for a different function.

Pigeon milk, like mammalian milk, is a highly nutritious, easy-to-consume, digest and metabolize food source rich in fats and proteins. Only more so: the protein, fat and overall dry matter content of pigeon milk is higher than human or cow milk. “Milk” is also a misnomer in that the substance secreted is more like a yellowy cottage cheese in appearance and texture, consisting of 35% dry matter, compared with 8% dry matter for human, and 4% for cow milk.

For the first several days, the squabs are fed the milk by their parents, who regurgitate it up from their crops, and into their mouths. After a few days, the parents add regurgitated seeds- mixed in with the milk- to the squabs’ diet, and gradually wean them off the milk at around a week or so.

Tangent: To a human, regurgitating food into someone else’s mouth sounds like, well, the grossest thing imaginable. But it’s remarkable how common it is in the animal world. Birds do it all the time, as do all kinds insects, and wolves regurgitate partially-digested meat to their pups. Lots of critters do it, and it seems to work just fine. And besides, I suspect some of the things we do would seem pretty disgusting to other creatures. I sometimes think that to an egg-laying creature like a bird, a live birth must be about the grossest thing imaginable.

The evolution of “milk” in pigeons is cleverer than it seems. Pigeons are mainly vegetarians; they don’t generally eat bugs. One of the challenges for non insect-eating birds is getting enough protein to their chicks that they attain critical mass prior to fledging*. Some species alter their diets to include bugs when in need of additional protein, but milk-production represents another solution to the problem.

*And it was this aspect of House Finch diet that turned out to be the undoing of the Brown-Headed Cowbirds who parasitized their nests, as we saw in last year’s Bird Feeder Week. (Man, was that an great week or what?)

Side Note: Still another cool thing about milk-production in birds is that it has evolved multiple times independently. Flamingoes (pic right, not mine) feed “milk” to their young, which is produced, not in a crop, but rather in glands lining the upper digestive tract. In a creepy vampire-like twist, the milk contains large numbers of both red and white blood cells. Emperor Penguins (pic below, left, not mine) also feed their chicks “milk”, in this case produced directly in the esophagus.

Interestingly, though all 3- pigeons, flamingoes, and penguins- produce milk, the evolutionary drivers for doing so appear to have been different. In pigeons it was driven by the need to ingest maximum protein prior to fledging. In flamingoes, they’re only able to ingest liquid food while their specialized feeding apparatus develops. In Emperor Penguins, it serves as a critical supplement in a food-barren environment. (Penguins eat fish, but incubate/hatch eggs fairly far inland.)

If you didn’t notice already, I’ve been glossing over something in my description of pigeon milk. I’ve consistently said “parent” in place of “mother”, and the reason for this is that both sexes of pigeon produce milk, starting to do so a few days before eggs hatch. With the exception of a couple of species of bat, no male mammal is known to routinely produce milk.

I’ve already mentioned the admirable parenting habits of pigeon-fathers, and such behavior isn’t unusual in the world of birds. Although certainly many bird species are “bad dads”, the males of many, many species are exceptional caregivers, and in many species even assume the lion’s share of incubation and chick-rearing duties. In mammals, with a few notable exceptions, such as humans and wolves, the vast, vast majority of species- including bears, cats, deer, rats, and former presidential candidates from North Carolina- are completely uninvolved- if not outright absentee- fathers. 90% of bird species are primarily monogamous*; only 10% of mammal species are.

*Though virtually all will cheat, given circumstance and opportunity.

One reason why this might be so is eggs. A female mammal carries her young until birth, and there’s not all that much many male mammals can or need do to help things along. By the time the offspring is delivered, he’s long gone. But since so much of a bird’s prenatal development takes place outside the mother, it opens up all kinds of incubation and egg-protection scenarios, many, many of which involve ongoing paternal participation, in turn keeping the male around till hatching. Some number- a big number- of these high-paternal-involvement scenarios led to more chicks surviving to hatch, fledge, mature and mate, which in turn led to… well, you get it already.

The Phenomenal Unfairness of Being Born Female

Tangent: Thinking about birds and mammals and gender always leads me back to thinking about humans and gender, which leads in turn to what for me has always been one of the big, unfathomable mysteries of life: the unfairness of being female.

Any rational way you slice it for a human being, being born female seems like a bad deal. They’re physically weaker, and their entire bodies are, well, structurally compromised for child-bearing. To pass a human head during delivery, our females’ hips have become so wide that women basically walk wrong. Routine childbirth for humans is a far more traumatic, painful, and life-threatening event than it is for pretty much any other large mammal. Every month women are plagued with inconvenience and discomfort, and although all humans fall victim to cancer, breast cancer strikes down under-40 women in their prime far, far more often than prostate cancer afflicts similarly-aged men.

Women routinely live with threat (or reality) of sexual or spousal violence, and the common, unwanted attentions of people bigger and stronger than them. In love, the penalties of misjudgment, indiscretion, or plain bad luck (unplanned pregnancy) are always greater for women, in terms of health, time, missed opportunities and cultural stigma. In the majority of human societies throughout history, and arguably today, women have enjoyed far fewer legal and property rights. Even in modern America, women earn less, and are remarkably under-represented politically.

Obscenely-priced cosmetics and hair products, burkhas/ veils/ headscarves, pantyhose that endure only a handful of wearings, shoes that are both exorbitant and borderline un-wearable. How do they stand it?

Nested Tangent: There’s another mystery to me embedded in this one, and that is how theists- people of faith- regard gender. Theologians seem to spend endless cycles considering or debating specifics of the afterlife, the path to salvation, or the duality of Christ, and yet, with a few exceptions (immaculate conception, virgin birth, original sin), don’t seem to think much about the greatest of God’s mysteries: How’d he pick who gets/has to be male/female? Specifically, do souls have gender? Or are souls neuter, above/beyond gender and base sexuality, and we’re simply assigned a gender in this life. If gender is assigned, what did women do to deserve their female-ness*? And if they’re not assigned, how does God decide which souls to make female?

*Yes, I’ve read Genesis, and I say, “Give me a break.” Assigning punishment to the great (x-times) great granddaughter of a “criminal” is even more morally repugnant than the psychotic-feudal family labor camps of North Korea.

If souls don’t have gender, and you believe in an afterlife, are you and your spouse really going to want to hang together for eternity? Doesn’t anybody think about this stuff*?

*Yes, I know some people do. Mormons are a good example. In Mormon theology, soul is definitely linked to gender, and that’s important, because married couples remain together in the afterlife. For me though, that opens up a whole other can of worms. I love AW dearly, but I wonder if after 500, 10,000, or a million years or so, what would we talk about? Seriously, at some point, after a few millennia or so, you must wake up every day and be like, “What- you again?” Anyway, I don’t think there is an afterlife, but if there is I’m thinking maybe I’d like to play the field a bit.

But that- none of that- is the unfathomable part to me. No, the unfathomable part is that the vast majority of women with whom I’ve had this conversation don’t agree* with me. They’re glad they were born female! How can this be? Don’t they see what an awful, terrible hand life has dealt them? It seems sometimes that no matter how long you live around women, so much about them is forever a mystery.

*Of course, at some level I generally find it unfathomable that anybody ever disagrees with me about anything, but then I guess that’s part of the unfathomable mystery of being male.

After about 4 days, the parents begin to augment the squabs’ diet with regurgitated seeds mixed in with the milk. By day 8-10 they’re almost fully weaned, being fed just seeds. At around day 30-35 they leave the nest.

But usually around day 20 the female lays the next pair of eggs, initiating a somewhat crowded period in the nest called clutch overlap. Clutch overlap is a busy time, with both eggs to incubate and mouths to feed. Guess who picks up the slack? That’s right- Dad. During overlap the male assumes a majority of feeding duties for the squabs.

Clutch overlap, and the rapid, year-round reproductive cycle of pigeons leads to perhaps the most interesting questions about them: Where are they from, and how come there are so darn many of them?

Next Up*: Coloration, Defense, and the Remarkable Natural History of Feral Pigeons.

*Will bleed into next week. These theme “weeks” are tough to pull off on schedule. But aren’t pigeons cool? Jason? Hello? I am talking to you.