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Many bird species migrate long distances without any food intake and must optimize storage of energy with respect to minimization of aerodynamic drag. To contribute to the understanding of this issue, we investigated, by magnetic resonance imaging (MRI), spatial distributions of body fat during the accumulation process before migration. We collected data from 12 Lesser Whitethroats (Sylvia curruca), 9 European Robins (Erithacus rubecula), 8 Blackcaps (Sylvia atricapilla), and 5 Willow Warblers (Phylloscopus trochilus). On average, each bird was examined 3.2 times. Adipose tissue was visualized using T1-weighted spin-echo MRI at 1.5 T. Fat-containing pixels were identified by an image-segmentation procedure. Data were analyzed with respect to (1) fat distribution within the body, (2) relationship between frontal surface area and fat mass increase, (3) fat mass increase in comparison with increase in total body mass, and (4) fat mass in relation to standardized visual classification of fat deposits. Fat increase was reflected by a larger frontal area, though adipose tissue was not deposited equally along the length of the bird. Slices with largest frontal area showed relatively low fractions of fat. Frontal area increased less than expected from conventional geometrical models, which indicates that the body shape is altered. The increase in total body mass was generally higher than the total fat mass increase, which indicates that other tissue, most likely flight muscle, can metabolize rapidly in correlation with fat accumulation. In Blackcap, total fat mass was not linearly related to standardized fat-deposit classes. Key words: adipose tissue, fat storage, fuel load, magnetic resonance imaging, migration, migratory birds. Plusieurs especes d'oiseaux migrent sur de longues distances sans s'alimenter et doivent optimiser l'accumulation d'energie tout en minimisant la trainee aerodynamique. Afin d'ameliorer notre comprehension sur cette question, nous avons etudie, a l'aide de l'imagerie par resonance magnetique (IRM), la repartition spatiale des graisses corporelles au cours du processus d'accumulation precedant la migration. Nous avons recolte des donnees de 12 Sylvia curruca, 9 Erithacus rubecula, 8 Sylvia atricapilla et 5 Phylloscopus trochilus. En moyenne, chaque oiseau etait examine 3,2 fois. Le tissu adipeux etait observe de l'aide d'IRM en sequence spin echo ponderee en T1 a 1,5 T. Les pixels correspondant aux graisses etaient identifies par une procedure de segmentation des images. Les donnees etaient analysees relativement h (1) la distribution des graisses dans le corps, (2) la relation entre la superficie frontale et l'augmentation de la masse adipeuse, (3) l'augmentation de la masse adipeuse comparativement a l'augmentation de la masse corporelle totale et (4) la masse adipeuse en fonction de la classification visuelle standardisee des depots adipeux. L'augmentation des graisses se refletait par une plus grosse superficie frontale, bien que le tissu adipeux ne se soit pas depose egalement tout le long de l'oiseau. Les tranches avec de plus grosses superficies frontales presentaient des proportions de graisses relativement faibles. La superficie frontale a moins augmente que prevu selon les modales geometriques conventionnels, lesquels indiquent que la forme du corps est alteree. L'augmentation de la masse corporelle totale etait generalement plus importante que l'augmentation de la masse adipeuse totale, ce qui indique que d'autres tissus, probablement des muscles de vol, peuvent metaboliser rapidement en correlation avec l'accumulation de graisses. Chez S. atricapilla, la masse adipeuse totale n'etait pas reliee lineairement aux classes standardisees de depot adipeux.

Birds can save distance and time on their migratory journeys by following great circles rather than rhumblines, but great-circle routes require more complex orientation with changing courses. Flight directions at different places along the route and in relation to the destination can be used to test whether birds migrate along great circles or rhumblines. Such data have indicated great-circle migration among shorebirds at high latitudes, but no critical tests have been made for passerines. Using tracking radar on board the icebreaker Oden in August 2005, we recorded westerly flight directions of passerine migrants over the Chukchi Sea. The main sector of migratory directions was 237-311[degrees] centered on a mean heading direction of 274[degrees]. The most likely species to participate in this westward trans-Beringia migration, mainly departing from Alaska, were Eastern Yellow Wagtail (Motacilla tschutschensis), Arctic Warbler (Phylloscopus borealis kennicotti), Northern Wheatear (Oenanthe oenanthe), and Bluethroat (Luscinia svecica); all except the Bluethroat were recorded from the ship. Observed flight directions agreed with predicted great-circle courses but not with rhumbline courses for three of these four species with winter quarters in Southeast Asia; no definite conclusion could be drawn for the Northern Wheatear (wintering in East Africa). These results support great-circle migration among passerines traveling between Alaska and Old World winter quarters, though the long-distance precision and orientation mechanisms are still unknown. The relative importance of different evolutionary causes--such as circumvention of geographic barriers, retracing of ancient colonization ways, or distance reduction by great-circle migration--to complex bird migration routes with changing courses remains to be understood. Key words: Arctic birds, bird migration, great-circle orientation, trans-Beringia migration. Les oiseaux peuvent reduire la distance et la duree de leurs periples migratoires en suivant une orthodromie plutot qu'une loxodromie. Cependant, les orthodromies requierent une orientation plus complexe lors des changements de cap. Les directions de vol en differents points du trajet migratoire peuvent etre utilisees pour evaluer si les oiseaux migrent le long d'orthodromies ou de loxodromies. De telles donnees ont devoile des migrations par orthodromie chez les oiseaux de rivage a de hautes latitudes mais aucun test decisif n'a ete mene pour les passereaux. A. l'aide d'un radar du brise-glace Oden, nous avons enregistre les directions de vol vers l'ouest des passereaux migrant au-dessus de la roer de Chukchi en aout 2005. Le principal secteur de directions migratoires etait 237-311[grados], centre sur un azimut moyen de 274[grados]. Les especes les plus susceptibles de realiser cette migration trans-Beringie vers l'ouest, principalement a partir de l'Alaska, etaient Motacilla tschutschensis, Phylloscopus borealis kennicotti, Oenanthe oenanthe et Luscinia svecica. Toutes ces especes, a l'exception de L. svecica, ont ete observees a partir du bateau. Les directions de vol observees correspondaient aux trajectoires orthodromiques predites et non aux trajectoires loxodromiques pour trois de ces quatre especes ayant leurs quartiers d'hiver dans le sud-est de l'Asie; aucune conclusion nette n'a pu etre tiree pour O. oenanthe (qui hiverne en Afrique de l'Est). Ces resultats supportent l'hypothese de migration par orthodromie chez les passereaux qui voyagent entre l'Alaska et les quartiers d'hiver de l'Ancien Monde, bien que la precision sur de longues distances et les mecanismes d'orientation demeurent inconnus. L'importance de differentes causes d'evolution--tel que le contournement des barrieres geographiques, la reconstitution d'anciennes voies de colonisation ou la reduction de la distance migratoire en suivant une orthodromie--relativement aux roles migratoires complexes comportant des trajectoires changeantes reste a etre comprise.

Migratory animals show a suite of adaptations to cope with their journeys. These include not only morphological features for efficient locomotion and storage of energy but also behavioral adjustments to [...]

Eleonora's Falcon (Falco eleonorae) breeds in the Mediterranean region and is highly adapted for catching small birds on passage migration between Eurasia and their African winter quarters, which they feed their young. We studied gliding flight behavior of Eleonora's Falcon at a breeding colony located on a small island southwest of Sardinia, Italy. Gliding and soaring flight performance was measured using an optical range finder and evaluated against flight mechanical theory. The male falcon does the majority of hunting and usually sets off from the colony to hunting areas located at high altitude over the open sea to catch prey. To lower the cost of transport and maximize the energy gain from hunting, we show that the birds use vertical winds for soaring when available. The occurrence of rising air changes with wind direction. At north-north-westerly winds (on-shore), slope lift is available outside the nesting cliffs, and at south-southeasterly winds thermals that form over the island drift out over the sea. Our observations demonstrated the flexibility of flight behavior in relation to the wind situation, and birds thereby make full use of available soaring conditions.