Sometimes sexing birds can be very easy:
Sometimes it is difficult:
While there can be size differences between males and females of the same species, there is often an overlap, which makes it difficult to carry out a reasonably secure sex estimation. If you have a large sample, you can hopefully get two (somewhat overlapping) bell curves, but bird bones are rarely found in such quantities*.
From a European point of view**, normally there are two ways of sexing birds: presence of spurs (only on galliforms, i.e. chicken, pheasant etc) and presence of medullary bone. Medullary bone is formed in the bone shaft cavity on female birds when they’re laying eggs. This acts as a calcium buffer for the egg shell development. The level of medullary bone can vary, from a millimetre lining on the cavity to a fully filled bone shaft, probably depending on where the hen was in the egg laying cycle, and/or how calcium rich the feed is. Medullary bone are most often found in the femur and tibiotarsus (the thigh bone and the drumstick bone). Medullary bone sexing has two major drawbacks: It can only be observed in fragmented bones. Since bird bones normally are rather small, there is little need to portioning them by cutting them in two, as is done with bones from cattle, sheep and pig. It is therefore likely that we miss several egg laying hens in bone assemblages. Secondly, the method only counts egg laying hens, possibly not the preferred animals to slaughter. Medullary bone inside a fowl femurClose-up of medullary bone
Spurs are more straightforward when it comes to sex estimation, even if they only appear in a few species. However, just as with horse canine teeth, occasionally females have them too. The bone part of the spur forms separately and fuses with the bone shaft at six months of age, although this is very variable.Male and female fowl tarsometatarsus
Sometimes spurs have been cut off, either as a way to castrate the fowl (or, possibly to prevent fights within the flock) or to make it easier to tie metal spurs on for cock fighting. Not that that was necessary, cock fighting did and does happen with "natural" spurs too. Sometimes a predominance of male fowl in an archaeological assemblage has been interpreted as occurrence of cock fighting, although, at least for the Roman period, cocks would also have been the preferred sex for sacrifice.Medieval cock fighting, from MS Bodleian 264, fol. 50r.
Conclusion: bird sexing is much easier with live ones! (read more? Dale Serjeantson, 2009, Birds, Cambridge University Press.)
*: Not only are there normally less chicken on the menu than cattle, sheep and pig, but the thin bird bones are easily destroyed in the soil.
**: I don’t have the experience to say whether non-galliform birds on other continents have sexually distinct skeletal elements or not. If some of my readers know, please tell me!