I’ve just noticed the carol has pretty much birds and servants through the whole carol. Why is this?
The lyrics are below
On the 12th day of Christmas My true love sent to me 12 drummers drumming Eleven pipers piping Ten lords a-leaping Nine ladies dancing Eight maids a-milking Seven swans a-swimming Six geese a-laying Five golden rings (five golden rings) Four calling birds Three French hens Two turtle-doves And a partridge in a pear tree
The short answer is that no one knows.
What we can say for certain is that the words were published in a late eighteenth century illustrated book known as Mirth without Mischief You can see an online copy at the Internet Archive here.
Beyond that, there are a few French folk songs recorded in the nineteenth century that show a remarkable similarity to the "Twelve Days" lyrics. Here's one example (translated from the original French):
THE GIFTS OF THE YEAR
The twelfth month of the year
What should I give my love?
Twelve good cheeses,
Eleven good hams,
Ten good turkeycocks,
Nine horned oxen,
Eight sheared sheep,
Seven running dogs,
Six hares in the field,
Five rabbits trotting along the ground,
Four ducks flying in the air,
Three wood pigeons,
Two turtle doves,
One young partridge,
Who flies, who flies, who flies,
One young partridge,
Who flies
From the wood to the field.
In addition to the overall structure of the songs being essentially the same, the gifts for one (partridge) and two (turtle doves) are identical.
These similarities show beyond reasonable doubt that there is a connection between the French songs and "The Twelve Days". (They don't determine in which direction the influence flowed: even though the English text is attested earlier, most commentators assume that the song is ultimately French in origin).
Of more interest for our present purposes: only the first four gifts in "The Gifts of the Year" are birds. In fact, when we generally compare all the different versions of the song (including later variants in English), it is the lower numbered gifts that are more likely to be preserved, and the later ones that are more likely to change. This becomes obvious if you look at the exhaustive table of variants on the carol's Wikipedia page.
What we can tentatively suggest is that there is a core text, common to the English and French versions, that includes the first one or two gifts, which are both birds (partridge and turtle doves). As the numbers increase to twelve we get more variation both within and across languages.
References:
French versions: