American Purple Gallinules are champions of long-distance vagrancy, with records from as far north as Iceland, as far south as South Georgia Island, as far west as the Galapagos Islands, and as far east as Italy and South Africa. This species, and many other rails, are habitat-based dispersalists, adapted to respond to ephemeral habitats and with the machinery to travel long distances. This winter has seen a big influx. What’s driving this?
In late fall 2013 and winter 2014 there have been a surprising number of records of this species far out of range (originally compiled by our friend Louis Bevier and subsequently amended by Teams BirdCast and eBird): 7-11 November 2013, Parque Monsanto near Lisbon, Portugal (see more here; taken in to care 11 Nov and died 13 Nov); 17 November 2013, Seal Island NWR, Maine fide Juanita Roushdy and John Drury; 8 December 2013, Clarenville, Newfoundland; 8 January 2014, Trenton, Maine, thanks to Michael Good; 9 January 2014, Clermont, NJ; 10 and 13 January 2014 from Bermuda; 19 January 2014, Maccallum, Newfoundland fide Bruce Mactavish; 21 January from Bermuda; 29 January 2014, Kettle Cove, Maine (apparently long-dead) fide Richard Jones via George Armistead; 30 January 2014, Iceland; and 2 February 2014, Mullett peninsula, County Mayo, Ireland. That’s 11 far-flung records of birds that were found (American Purple Gallinules are not easy to find!), with three of them crossing the Atlantic!
Two obvious questions come to mind. First, how did they make it across the Atlantic? And what were they doing moving in the first place? We offer an in-depth analysis of the effects of cold, drought, and wind and air parcels on BirdCast and a quick summary below.
The temperate zone winter range for American Purple Gallinule is primarily peninsular Florida, Mexico and Central America south to northern South America, and the Caribbean, with additional populations farther south in South America breeding during this time. The map at left shows this typical distribution from eBird observations of the species from November to February. Of note in this map are occasional winter records away from Florida in the US and frequent records of the species from central Florida south to the Florida Keys. The red balloons represent observations in the last two weeks of January 2014. EXPLORE THE EBIRD MAP HERE.
This range is by no means static, as marsh habitats with floating and emergent vegetation are often ephemeral and subject to drying out. During abnormally dry years birds may be forced to move, and this could also happen in abnormal cold years. During these movements individuals may go far afield in search of suitable habitat, a behavior that is likely echoed in numerous other species of rails. For winter 2013-2014, we contemplate the source region for this recent vagrancy event and explore hypotheses of what might be driving this year’s movement.
It ain’t the cold in North America!
Cold weather in North America appears to have very little to do with this year’s American Purple Gallinule extravaganza. The polar vortex that has received so much hype in North American media has not directly affected the primary wintering grounds for American Purple Gallinule.
Furthermore, there have been virtually no freezes, prolonged or isolated, in the species’s typical winter range on the Florida Peninsula. Unlike areas in the Southeast US north of Florida, only 1 day since 4 November 2013 has seen temperatures dip below 0 degrees Celsius in central Florida, with no days below freezing in the heart of their Florida range.
Drought in the Caribbean
The graphic below shows the standardized precipitation indices for the the last 1-, 3-, 6-, and 12-month periods for southeastern Mexico and Central America, the Caribbean, and north South America. Note the striking red colors that become more intense over the course of the last year in the Greater Antilles (especially the Caribbean islands of Hispaniola and Cuba), where American Purple Gallinule winters (and reside year-round) regularly.
These colors represent precipitation indices 2 standard deviations below the mean for the past 30 years, indicating significant drought conditions. The potential for these conditions to spawn movements of gallinules seems very high. We believe that the vagrant gallinules probably originated here: this is an area with wintering American Purple Gallinules, the conditions are ideal to spawn a large-scale dispersal event, and as we will see, the wind currents can easily connect vagrant records back to this region.
Wind and air parcel analyses explain vagrant records
Wind patterns this winter have been favourable to support movements of gallinules over the ocean to Iceland and Ireland. There is a strong Caribbean connection as well, with high altitude winds linking an air parcel beginning on Hispaniola with the North Atlantic. Similar conditions existed on 22 January to support movements of gallinules over the ocean to Iceland and Ireland. Several other maps of air parcels are included in the full BirdCast feature.
Although this winter’s cold temperatures could yield far-flung American Purple Gallinules, evidence this year is stronger for drought driving dispersal from the Caribbean and south Florida. The same systems that have brought extreme cold to the eastern U.S. are also bringing these strong wind fields as the storms spin up the east coast, and this surely has aided the successful trans-Atlantic flights by these gallinules. South America does not seem a likely source, given the prevailing flow of winds in potential source areas for gallinules on that continent. But the origin, motivation, and mechanism of movements are open questions and discussions worth continuing, as we have barely scratched the surface of these patterns. For example, conditions are generally favorable this winter for Nearctic and Neotropical vagrants to reach the Palearctic, with general flow of winds to the east across the Atlantic in the presence of an Azores High, a pattern referred to as positive North Atlantic Oscillation (NAO). (As an aside, these conditions and the prevailing trade winds also make for favorable conditions to deposit Palearctic migrants in the Neotropics, a topic we will discuss further in an upcoming BirdCast post.) NAO phases are cyclic, albeit irregular in their timing and strength. Previous years with strongly positive NAO may well correlate with other instances of North Atlantic vagrancy in this species: if anyone looks into that we’d love to hear back! That would help to answer the question of whether this year is different, whether something fundamental changed, or if American Purple Gallinules this year just encountered a perfect combination of drought conditions, positive NAO, and a wobbly polar vortex that is sending numerous strong low pressure systems up the Atlantic Coast. Check out the Full BirdCast Feature.