|This Mute Swan was in a city park in Boise,|
Idaho. It is probably not countable.
Sometimes, in the western U.S., I've seen Mute Swans in situations where I wondered if they might be natural vagrants from the established population centers to the east. There was one Mute Swan that turned up on a local warm spring in northern Utah two winters in a row, but was not seen in the area during the summer. Another mid-winter observation, in southern Utah, was miles from the nearest homes on a relatively remote reservoir. The source of these birds is difficult to determine: local movements from feral releases? Or are at least some of them natural vagrants from established eastern populations? Until a banded Mute Swan is documented moving from an established population, the best we can do is guess. Most, if not all, of the Mute Swans in the west are just as they are thought to be: non-countable artificial transplants. But if some of our Mute Swans were natural vagrants from the east, we'd expect that to be a result of migration within the east. Some references say that North American Mute Swans don't migrate. Others, more accurately, say that there is post-breeding movement as a result of weather or local seasonal movements. But few if any seem to indicate that there is a pattern of north-south movement in North America's Mute Swans that qualifies as a migration.
To assess whether there is any indication of seasonal migration in Mute Swans in North America, I looked at eBird data for each month of the year, then animated those 12 pictures into a looping gif:
|Mute Swan distribution through the year according to eBird data accessed 3 Mar 2014.|
I think it is reasonable that some of these movements are resulting in natural vagrants to the west. Of course, it is very difficult to identify them, and they will be greatly outnumbered by locally released birds. But in the meantime, I encourage birders in the west to continue recording Mute Swans in eBird, even if they're not "countable," and to especially watch for any banded birds. There might be more going on with North American Mute Swans than we realize.
|This Mute Swan was photographed on a warm spring in northern Utah in December 2007 and January 2008, was not seen in visits to the location in March, April, and July, and was then recorded again in December 2008. Where did it go? Did it migrate?|