Hybrids are normally sterile. For simplicity I'd just agree hybrids are sterile. But there are cases of mules getting pregnant. Without looking it up now I don't know what the percentage of live births are or how long the young normally live. I know that many hybrids are not very thrifty, they either die before birth/hatch or shortly after. Some will make it a couple of years. Some species do fine and have a normal lifespan, it really depends on species and how closely related those two species are. I do find hybrids interesting, but for the most part I don't believe many of them should be purposely bred. In domestic animals it isn't such a big problem but you see a lot of intentional hybrids in exotics like parrots and snakes. Some of these species have no new (wild) blood being introduced into the captive population. Some snakes are hard to tell the hybrids from pure blood and those hybrids can breed and produce fertile offspring. It seems popular to hybridize macaws currently and laws prevent (for good reason) the capture of wild birds.
But now I'm going off topic so I'll stop there. It is a very in depth and interesting topic.
It is interesting. I recall when I was a kid someone near our farm had a mule that gave birth to a live foal.
I think there are now 8 species of macaws thought to be extinct in the wild.
I worked at a breeding facility in Central America raising both Scarlet and Great Greens with the goal of returning a viable population to the wild. There were sometimes as many as 400 there. At each prospective release site, environmental impact studies had to be performed that could take as long as 3 years. Even though they were being released into their former native range, it still had to be studied if there would be any negative impact to other birds and things like sloths, howler, squirrel, spider monkeys, capuchins, etc.. And studies were also done on the potential danger from poaching. They would usually wait for young adults to pair up in the flight pens and choose those for release.
Wouldn't this interspecies hybridization (in the case of the Macaws) be the same as chicken crossbreeding?
Not the same. All Macaws are the same genus (ara) but are comprised of different species. For example, Scarlets are
ara macau and Great Greens are
ara ambiguous.
Chickens on the other hand, even though the various breeds look so different, are all the same species -
gallus gallus domesticus. Therefor, any breed of chicken can mate with any other and produce viable eggs/chicks.
Galliformes is the order made up of 5 families. The family phasianidae includes chickens, peafowl, pheasants, turkeys and old world quail.
Two other families include one just for new world quail and one for guinea fowl.
Carrying the topic just a bit further, old world quail and new world quail are only distantly related and probably can't interbreed.
Regarding chickens, there are 4 gallus species. Gallus gallus is the ancestor of all domestic chickens, common name is red jungle fowl. The other 3 species are green jungle fowl (gallus varius), grey jungle fowl (gallus sonneratii) and Sri Lankan jungle fowl (gallus lafayettii). There were reports of cross matings between red and either green or grey but almost never result in viable chicks hatching.
Some speculate that one of those matings are the source of dark brown eggs in some chicken breeds but I've never seen any real scientific or documented evidence of that.
I'm probably belaboring the point here but in modern biological understanding of taxonomy there are 7 main levels; kingdom, division, class, order, family, genus, species. Then in domestic animals, you have breeds (shape) and varieties (colors) of those breeds.
