The orientation system and migration pattern of long‐distance migrants: conflict between model predictions and observed patterns

Abstract

J. 2001. The orientation system and migration pattern of long-distance migrants: conflict between model predictions and observed patterns. -J. Avian Biol. 32: 111-119.The requirements of the orientation system of naïve long-distance night migrants were analysed by comparing data on Barred Warbler Syl6ia nisoria, Marsh Warbler Acrocephalus palustris and Spotted Flycatcher Muscicapa striata with data from a computer model of a clock-and-compass system. These species show, respectively, a rather restricted winter distribution in East Africa, migration through a very narrow corridor in East Africa, and rather widely distributed recoveries in the Mediterranean with more concentrated recoveries south of the Sahara. For all three species, to obtain the observed concentrations either a very high directional migratory concentration was needed in computer simulations to bring the birds successfully to their wintering areas or misorientating individuals would be subjected to a very high mortality. Neither the very high directional concentration nor the high mortality amongst misorientating individuals fit the empirical data sets. On the basis of the present study, the observed patterns seem difficult to explain by a simple clock-andcompass system only, and to account for the exceptionally precise migratory routes shown in this study it is proposed that first-time migrants might be able to use landscape topography on a regional scale in combination with corrections of directional mistakes/wind displacements.

Information from the geomagnetic field triggers a reduced adrenocortical response in a migratory bird

“…First year thrush nightingales (Luscinia luscinia) exposed to a magnetic treatment simulating a migratory flight from southeast Sweden to northern Egypt increase food intake and attain a higher fuel load compared with control birds experiencing the ambient magnetic field in southeast Sweden (Fransson et al, 2001;Kullberg et al, 2003;Henshaw et al, 2008). These results are in line with the suggestion that migratory birds use external cues to get more precise information than the endogenous time programme alone would allow in deducing their latitudinal position (Gwinner, 1996;Thorup and Rabøl, 2001;Fransson et al, 2005), and suggest that geomagnetic information might act as an external synchronizer affecting hormonal secretions during migration, leading to adaptive fuelling behaviour along the migratory route (Henshaw et al, 2008). In order to investigate the effect of geomagnetic information on corticosterone secretion in migratory birds we here repeated our earlier studies and also measured baseline corticosterone levels and the adrenocortical response in juvenile thrush nightingales trapped in southeast Sweden during autumn migration and exposed to either the ambient magnetic field of southeast Sweden or the magnetic field of northern Egypt.…”

Changes in migration phenology and biometrical traits of Reed, Marsh and Sedge Warblers

“…This species does not defend territories at stopover sites, but is territorial on wintering grounds [33]. Marsh Warblers travel large distances between re-fueling sites and stop only a handful of times during migration [43]. Populations originating Northwest of the Carpathian Basin pass through the region during autumn migration [42].…”

Evidence for a navigational map stretching across the continental U.S. in a migratory songbird

“…In some species, juvenile birds travel alone (5), departing the breeding grounds during their first migration in the fall (6) without the guidance of experienced adults (7). They apparently rely on an innate orientation program for guidance toward their speciesspecific wintering areas (1), which can apparently steer them very precisely over thousands of kilometers (8). In the absence of guidance by experienced conspecifics, this program is considered of critical importance for juvenile birds to survive the migration and ultimately breed (9).…”