[Maine-birds] recent vagrancy in Purple Gallinule

Last week I posted a list of vagrant Purple Gallinules, including recent birds from Maine and others as far as Iceland and Ireland. Correspondence with BirdCast and eBird team members generated some interesting hypotheses on the origin and transport of these birds. Why would they fly in the first place? What would transport them once airborne? The recent BirdCast makes these fascinating connections (and shows why eBird linked with climate and weather data are powerful tools):  http://birdcast.info/forecast/purple-gallinule/

For birds living under changing or ephemeral conditions, dispersal is a necessary adaptation, and I think there is an interesting link between our Snowy Owl invasion and the vagrancy of Purple Gallinules.

For rails, dispersal is tied to habitat. When the lily pads wither and the marsh dries up, they need to be able to find a new home which is not usually right next door. They occupy patchy and ephemeral habitats. Some populations have developed regular migrations to seasonally flooded marshes; other populations engage in facultative dispersal, which may the birds recently found as vagrants.

For Snowy Owls the habitat remains the same, and it is the resource that fluctuates. Hence they are resource-based dispersalists, roaming to find places with burgeoning lemming populations. They are similar to crossbills in this regard, settling to breed where resources are burgeoning. How far can Snowy Owls go? One chick from a group of nestlings banded on Victoria Island, Canada, in July 1960 was later found on the opposite side of the world at Sakhalin, Russia, a year and a half later. If conditions are right and resources good, large broods are the result and something triggers these birds to disperse.

The similarity between the gallinule and the owl is that they both appear to have evolved the machinery necessary for flying long-distances at some point in their lives. With long-distance migrants, we more or less expect this. We have learned recently the amazing distances shorebirds can travel, altering their physiology to make long flights by allowing the digestive tract to atrophy (paring down weight) and burning fat for metabolic water. (I wrote about shorebird migrants for Mid-Coast Audubon here http://tinyurl.com/kxat5hr). Lighter birds are more efficient fliers, and many of the Snowy Owls that first arrive are thought to be emaciated. Instead, they may be trimmed for long-distance dispersal, recovering when they find a suitable location to settle. The shy and unobtrusive Purple Gallinule apparently is able to make the same changes and fly far and wide, not always with the good results. More often than not, however, this must work for them.

Louis Bevier

Fairfield

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