Seven Swans a Swimming
We don't have a lot to work with here as far as identifying the swans, since all we know about them is that they're swimming, which all swans do. Fortunately, there are basically only 6 types of swans: Black, Black-necked, Mute, Trumpeter, Whooper, and Tundra. Swans can live most places, but they aren't generally equatorial birds, meaning we can eliminate the most of the Mediterranean, Middle East, and Central America. Furthermore, you are unlikely to be able to complete the Xmas birding challenge if you are in Africa. We've made some progress, but that's still a lot of ground to cover.
Six Geese a Laying
These are probably domestic Geese, since they are given the designation of 'laying'. Unfortunately, this means they could be almost anywhere. Fortunately, this means it should be pretty easy to find 6 of them. While I will generally try to only count birds in their native habitat, even this degree of nit-picking will not help up, since Geese occur naturally almost everywhere. The consensus best goose for laying is the China Goose, so maybe we're talking about Asia here. Let's keep going.
Four Cally Birds
Yes, I said 'cally birds'. I am assured by multiple internet sources that these aren't 'calling' birds as I have always assumed. And we might finally have our big break in the case, since a Cally Bird is unambiguously an anachronistic name for a member of the Thrush family called (ironically) the Common Blackbird. Coal-y bird. Get it? Anyway, they're native to most of Europe and have a sizable population in Southeast Australia and New Zealand. Now we're getting somewhere!
Three French Hens
Sigh, another domestic bird.. You can find domestic female chickens that originated in France anywhere in the world, as evidenced by this handy list of French Chicken Breeds. But let me make a supposition here. Would you really call it a French Hen if you were in France? Like, in that case, it would just be a Hen, right? Not the strongest evidence, but it could be a tie-breaker.
Two Turtle Doves
|pictured above: Not a Turtle Dove (a Collared Dove)|
Oh god, there are 56 types of partridge. They live on grasslands throughout Eurasia, and I've been able to verify that you're going to find partridges just about anywhere we've already got on our shortlist. Between the Grey and Red-legged varieties, you'll be able to find a partridge anywhere in Europe.
The Pear Tree
So could you bird all the birds in the 12 Days of Xmas? It seems like you definitely could, as long as you live in Central Europe (or even parts of Turkey). However, given that the origin of this Xmas carol is 1780's England and France, we can confirm that you could do this birding challenge in the song's country of origin. If you really want to maximize your odds, we can combine our pear data with our bird data, and come to the conclusion that Belgium is your best bet. Big shout-outs to the IUCN Red List website for its unparalleled animal range data. Merry Xmas everyone, and may all your presents be bird-related again.