Do Mute Swans Migrate in North America?

This Mute Swan was in a city park in Boise,
Idaho.  It is probably not countable.

Most birders in North America are familiar with the Mute Swan, a striking species introduced to our continent in the 1800s, and still widely and frequently planted as live ornamentation in private ponds and city parks around the continent.  The Mute Swan is considered an established species around the Great Lakes and in the parts of the northeast and east U.S. and southeastern Canada, and is "countable" by ABA standards.  Indeed, its population in that area has grown very rapidly in recent decades, roughly doubling every 7-8 years.  However, in much of the rest of North America, Mute Swans are not considered countable because they are not established.  They fall victim to ABA rule interpretation 2.B.iii: "an introduced species may be counted only where and when it meets the ABA Checklist’s definition for being an established population. An introduced species observed well away from the accepted geographic area is not counted if it is more likely to be a local escape or release rather than an individual straying from the distant population."

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.

While it is not dramatic, I think this map at least supports the references that mention "local" winter movements of the species, and I'd even go so far to say there is evidence that some birds seasonally migrate.  Watch how most of the records north of the St. Lawrence River (roughly a line between Toronto and Montreal on this map) disappear in the winter (perhaps due to a lack of open water).  Also, more obviously, watch how the southern edge of the distribution moves from around the Indiana/Kentucky border in summer to around northern Alabama and Georgia in the winter.  There is also an expansion of what looks like either a small migration or a pattern of vagrants in winter extending roughly from southern Illinois to central Texas.  Finally, look for lateral movement of the heart of the distribution: it is not just an even expansion and contraction around the range or post-breeding wandering.  The western border of the distribution is vague but seems roughly stable throughout the year around the Mississippi River, between Illinois and Iowa and between Wisconsin and Minnesota.

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?