Breeding & Genetics
Can birds have twins? Indeed they can, and Romanoff states in charmingly stilted language that "Prenatal duplicity in birds is not less common than in other animals". Rates of up to 2.8% have been observed in domestic birds, although most observers have reported much lower rates (Betuel et al). Producing twins from a hard-shelled egg is more complicated than giving birth from a mother's body, so successful hatches are rare. Most observations of avian twins have been in poultry, but scientific papers have documented twinning in at least 14 species of wild birds, including a successful hatch of twin bluebirds (Bailey & Clark). It happens with pet birds too - there are informal reports from breeders of twins in cockatiels (Talk Cockatiels, Watts).
With fraternal twins, each twin came from a different ovum that was fertilized by a different sperm, and the twins do not have the same DNA. With identical twins, a single ovum fertilized by a single sperm splits into two individuals with the same DNA. An egg yolk is the hen's ovum, and a double-yolked egg is an obvious case of fraternal twins since there are two ova. It's also possible for fraternal twins to share a single yolk (Romanoff, Riddle), although I haven't been able to find an explanation for how this occurs. Riddle and Susanne Russo mention extra-large yolks in these cases, and a doubtful-looking post on Yahoo talks about double egg yolks merging together. If this can actually happen, it would explain the situation. Identical twins came from the same ovum, so they obviously have to share a single yolk. (Romanoff, Bailey & Clark)
Double-yolked eggs are usually the result of multiple ovulations at the same time, but it's also possible for two yolks to be encased in the same shell if one moves down the reproductive tract so slowly that a second yolk catches up with it. Double yolks are particularly common in young hens who are just beginning to lay, which is attributed to the immaturity of their reproductive systems. The fertility of double-yolk eggs is significantly lower than that of single-yolk eggs, and the death rate for embryos in double-yolk eggs is significantly higher than for single yolks (Jeffrey et al, Salamon & Kent).
The eggshell is produced in an area of the hen's reproductive tract called the shell gland. Enough eggshell is laid down to cover the amount of material that's present, whether that amount is normal or not. As a result, a double-yolked egg is much bigger than a normal egg because there was more material to cover. A yolkless egg (called by such names as fart egg, cock egg, or fairy egg) is much smaller than normal because there is less material to cover (PoultryHelp). Yolkless eggs are more common in young hens than in mature hens.
There are major obstacles to survival for avian twins. An egg yolk is designed to sustain one chick, so when twins share a single yolk it's unlikely that there will be enough for both of them. This should be less of an issue with a double-yolked egg, although it's not clear whether there will be enough albumen (egg white) for both. Having enough space for two embryos is obviously a bigger problem in a normal-sized single-yolk egg than in a larger double-yolk egg. But a double-yolk egg is less than twice the size of a normal egg, so there are still some space constraints. An eggshell is designed to let in enough oxygen for one chick, and even in a larger egg there may not be enough oxygen for two.
There are two peaks of mortality in twin chicken embryos, one about one-third of the way through incubation and the other near hatching. The reason for the first peak is unclear. The deaths at hatch time occur because the hatching process is extra-hazardous for twins. A chick needs to be in a specific position in the egg at hatch time; it needs to have its head in the air cell end of the egg; it needs to be able to turn in the egg to break the shell in a circular pattern; and it needs to NOT rupture the egg yolk or puncture the blood vessels lining the shell. With twins, it's likely that one or both of them will not be in the right position, and pecking the eggshell in the wrong spot can be disastrous. Even in the right position, they will get in each other's way as they try to break the shell. It looks like most of the hatchtime mortality for twins is caused by rupturing the yolk sac or suffocating because the chick's head was not in the air cell (Romanoff, Pattee et al). For twins that make it to hatch time, human assistance is usually needed for a successful hatch. But in spite of all the obstacles, there have been cases where twins hatched successfully with no assistance at all.
Fraternal twins are much more common than identical twins in mammals, and this is also true of birds. Identical twins in birds are a lot less likely to be normal than mammalian twins. Romanoff and Riddle report that most identical avian twins are conjoined (incompletely separated aka Siamese twins), and they are more likely to be joined at the head than at the lower body. Levin et al discuss the reasons for this in very technical language. Being conjoined obviously reduces the odds of survival even further. But in spite of the heavy odds against identical avian twins, there is a confirmed case of normal identical emu twins that hatched with human assistance (Bassett et al).
It's reported that exposing already-laid eggs to cold temperatures increases the incidence of twins (Batt et al). These would have to be identical twins - a splitting of the original single embryo. Subjecting the hen to hypothermia prior to egg laying also has this effect (Lokemoen & Sharp).
There's an online story about conjoined barn swallow twins that hatched and survived to fledging age in the wild (The Telegraph, ScienceBlogs, Chicago Tribune). But it looks to me like this is either a misunderstanding or a hoax. The remains were allegedly sent to the Smithsonian for further study, but there are no reports on what the Smithsonian found and no scientific papers on these birds. I don't see how twins that were joined at the hip like this could fit in the egg or hatch successfully. And the picture is most unconvincing - it just looks like a couple of barn swallows sitting side by side in someone's hand. It wouldn't be hard to have someone brush the feathers aside enough to show the conjoined flesh, or hold the birds in a position that showed the conjoined area better. There's an odd thing between them that looks like a vestigial leg with just one toe, but this is the Photoshop age and we don't know if it's real, or if a bird with an abnormal extra leg was photographed next to a normal bird.
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