North American Birds

VOLUME 69 No1 2016

A Quarterly Journal of Ornithological Record Published by the American Birding Association. The mission of the journal is to provide a complete overview of the changing panorama of our continent’s birdlife.

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Page 39 of 179

N O R T H A M E R I C A N B I R D S T H E C H A N G I N G S E A S O N S : O U T P O S T S (e.g., by David DeSante involving vagrant war- blers in California). A solid, current, accessible summary of vagrancy patterns is presented in Rare Birds of North America (Howell et al. 2014). The four most important vagrancy mecha- nisms in North America are misorientation (reverse migration and the perhaps related mir - ror-image migration), drift (weather-induced vagrancy), overshooting, and irregular disper - sal. Overshooting is most common in spring, when, for example, a number of "southern" species overshoot to the northern United States and Canada. Irregular dispersal is often the result of food shortage or (temporary) habitat change due to weather and often involves, for example, a number of well-known winter ir - ruptive species, as well as the irregular late-fall and winter dispersal of several eastern Mexico species into southern Texas. Grosbeaks. These birds, as pointed out by Brian Sullivan (2004), are not "late lingering" birds that have been just suddenly noticed by birders (though we still call them "lingering") but are rather returning northward late in the season, either reverse-migrants or borne northeastward by strong storms (or both), probably entrained when trying to cross large bodies of water. Reverse (opposite direction) and mirror-im - age (mostly confusing east and west) misorienta- tion are especially apparent in autumn, as frst- year birds are the most likely age-class prone to this form of hard-wired vagrancy. Although some argue that such mechanisms are invoked excessively, most authorities embrace these specifc types of misorientation because much vagrancy appears to show certain patterns of oc - currence that are more predictable than would those resulting purely from chance. Thus, a re - verse migrant in fall that "should" have departed eastern Russia in a southwestward direction for Southeast Asia wintering grounds may end up going northeastward instead and is found on one of the Bering Sea islands. And if such birds encounter an eastward-moving storm-system coming off the Asian continent at the same time, drift may enhance this level of vagrancy. Or a mirror-image misoriented "eastern" warbler, hatched in the western Great Lakes or central Prairie Provinces regions, which "should" have oriented to the southeast toward the Caribbean, instead heads to the southwest and perhaps ends up in the Great Basin, Southwest, and/or along the West Coast. The surges in a number of "southwestern" species to eastern North Amer - ica, such as Cave Swallows and Ash-throated Flycatchers, occur late in autumn. Unlike the cuckoos and buntings noted above, these spe - cies do no make major water crossings during migration, and so these infuxes are likely also primarily of reverse migrants, their appearances tied to specifc weather conditions. Reverse migrants often represent the most extreme cases of vagrancy. The distances these birds may occur outside their normal range sometimes boggles the mind. Long-distance mi - grants may easily end up traveling at least the same distance in the wrong direction than they "should" have gone in the correct direction. And those long distances may also be augmented by the bird being stuck out over the ocean and forced to fy appreciably farther to fnd land, or having local weather conditions effectively re - sult in adding overshooting to the reverse course. During active migration, adverse weather or contrary winds are a constant danger to birds, particularly when they are crossing large expanses of water and are unable to land. Some birds may simply fy downwind, whichever way the wind is blowing, even if the weather is not particularly severe, but the addition of in - clement weather (precipitation, fog, unstable air masses) will likely cause them even more diffculty. "Drift" is the term frequently used to describe birds that are moving with a parcel of air in a direction contrary to their typical (or intended) direction. Drift is often cited as the principle mechanism for bringing vagrants from Europe, Iceland, and Greenland to Atlantic Canada and the northeastern United States in spring, and fallouts of Asian species to the Aleu - tians and Pribilofs in western Alaska in spring (mostly) and fall—with some especially strong and fast-moving fall weather systems likely re - sponsible for facilitating the full Bering Sea and North Pacifc crossing of several Asian species to mainland western North America, from Alaska to California, Baja, and beyond. The same holds true for strong and fast low-pressure systems crossing the North Atlantic in fall, which may entrain a suite of North American species rid - ing the westerly and southwesterly winds in the "warm sector" of the low all the way east to the Azores, United Kingdom, Ireland, and France. In contrast, overshooting probably plays a major role in spring bird vagrancy at St. Lawrence Is - land in the northern Bering Sea and to northern mainland Alaska (e.g., Barrow). In eastern North America, drift migration also likely plays a major role in the fall occurrence of unseasonably late migrants in the northern United States and Can - ada, such as numbers of late October and No- vember Yellow-billed Cuckoos, Chimney Swifts, swallows, warblers, Indigo Buntings, and Blue Figure 5. This Willow Warbler was the second of two at Gambell, Alaska in autumn 2014 (here 2 September). This bird brought the fall season total to 17 individual Willow Warblers noted at Gambell since 2002; most records are from late August. Photograph by Neil Hayward. Figure 6. On St. Lawrence Island, Alaska, two Tree Pipits hung around the middens at Gambell in September 2014. This frst bird, present 2-5 (here 3) September, marked the second fall record for Gambell and Alaska's fourth record of the species, which winters in India and Africa. Photograph by Neil Hayward. 38

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