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Thoughts about origin and flight pattern for North American vagrant birds arriving to the Azores.

Somewhere in the Azores soon? A Palm Warbler Dendroica palmarum photographed in Florida. Photo: Staffan Rodebrand.

When preparing data for lectures about autumn migratory birds arriving in the Azores, I was surprised about how well spread over time the arriving dates was for birds from North America. Furthermore the correlation was very poor with heavy storms moving eastwards over the Atlantic. The “general idea” about American birds reaching Western Palearctic has since long been that birds are “caught” in storms, and just been following the hard winds eastward over the ocean. Scanning the literature gives a lot of “evidence” where the occurrence of North American birds are claimed to be the result of name given tropical storms or hurricanes. Some of the birds have clearly arrived together with the storm, while others have not. Even birds arriving long after or even before (!) are claimed to be the result of name given storms.
   A more thorough check of bird appearance correlated with storms reveal some interesting general facts:
   First of all it is very clear that westerly winds are very important, but more in a sense of a steady flow rather than passage of heavy low pressure systems. With easterly winds almost no American birds will make it to the Azores. And, of course, wind directions which not are absolutely favorable for a flying bird, will result in a wind drift that the bird may not be able to compensate for.
   Secondly the major effect of severe storms is that larger birds which also are good flyers are affected. A typical situation is an influx of waders, and maybe also gulls, ducks and swifts. These are also birds that often migrates in flocks, and to a certain extent also birds that might try to outrun or fly around bad weather.
   Thirdly the form and the route of severe storms are very important when discussing their effect on migrating birds. The storms will only affect birds already in the air, and mostly so birds migrating over sea. Either far out over the Atlantic or at least near the shoreline. Taken into account the fact that winds circulate in low pressures, and not are steady in one direction, birds have all possibilities to leave the system when wind direction is favorable.
   The distance from the Azores (western islands Flores and Corvo) to North America differs from around 2000 km (Newfoundland) to 3900 km (Cape Hatteras, North Carolina). This can be compared with the distance from Newfoundland to Ireland and the British islands which are at a distance of 3000 to 3500 km. With an estimated flight speed of 60-100 km/hour for fast flyers (ducks, waders, swifts) to 30-50 km/hour for slow flyers (herons and most small land birds) it is a simple mathematical problem to calculate the time it will take for birds to travel these distances. And how much “fuel” they will need. However the winds are circulating in storms even if storms in general move west. There is also a clear tendency that winds are stronger and more westerly further north. This all makes the calculation more difficult. How much is wind speed affecting, and how straight does a bird really fly when affected by bad weather?
   Using different ways of calculating do however point to a couple of facts that is very difficult to set aside. First of all the flight distances involved are too long for birds in “normal” or even bad condition. They will just not make it! And if they have to fight their way against unfavorable winds it is even worse. So what are we looking at here?
   Presume instead that most of these birds are in perfect condition, having their fat resources built up to a maximum, and are ready to set out for a long flight. And that they are by free will aiming to migrate to a sector from northeast to east, setting off in favorable westerly winds. Then almost everything fit! Then we are of course talking about “reverse migration”. This is a phenomenon that European birdwatchers since long have been aware of. Even if “reverse migration” in this sense still are controversial and need much more scientific confirmation, we have in fact a known reality where a lot of autumn migrants from far east seem to take a more or less opposite flight direction than the one presumed. A lot of eastern birds move westwards over Siberia and Europe following the earth great circle and in the end even moves out over the Atlantic. The same can of course be expected in North American migrants but in an opposite direction. There are in fact more and more evidence being put forward about this phenomenon in eastern North America.
   So theories about tired migrants in bad shape, getting caught in westward storms and keep on flying west even when there are opportunities to get out of the weather systems and turn to the aimed direction, does not at all make sense and do not match with known observations. On the other hand if birds are by will heading this “reverse” way they will go when they are fully loaded with energy, when the wind is favorable, and they will keep their direction also after a first stop somewhere. Thus also explaining why some birds reaching Western Europe keep on moving even further east instead of turning south.
   In general one would expect that a large number of at least October transatlantic North American vagrants come from more southerly places along the American east coast, and not so many from high north. This due to the simple fact that many species migrate rather early and the number of individuals is not so high further north later in autumn. Except for the flight distances involved, this also explains some of the differences in which species are found in the Azores compared to northwestern Europe. Good flyers with a far northern distribution like some wood thrushes, Red-eyed Vireo and Blackpoll Warbler would show a higher percent of the American land birds reaching Northwest Europe. On the other hand species with shorter, earlier and more inland migrating routes would be more common in the Azores. Among these species like White-eyed Vireo, several wood warblers and Indigo Bunting.
   Making prognoses using the “reverse migration” theory and comparing with the relatively good knowledge of migrants turning up on the Bermuda gives an interesting list of species that would be expected to turn up in the Azores within a short future. The first group of potentials should be: Ruby-crowned Kinglet, American Robin, Golden-winged Warbler, Blackburnian Warbler, Yellow-throated Warbler, Prarie Warbler, Mourning Warbler, and White-throated Sparrow. In the next group there would be species like: Eastern Kingbird, Warbling Vireo, Nashville Warbler, Cape May Warbler, Bay-breasted Warbler, Palm Warbler and Kentucky Warbler.
   Really some nice birds to look out for!

Staffan Rodebrand

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