Johan Bäckman

Begging affects parental effort in the Pied flycatcher, Ficedula hypoleuca

Abstract It has been suggested that nestlings use begging to increase their share of parental resources at the expense of current or future siblings. There is ample evidence that siblings compete over food with nestmates by begging, but only short-term effects of begging on parental provisioning rates have been shown. In this study, we use a new experimental design to demonstrate that pied flycatcher (Ficedula hypoleuca) nestlings that beg more are able to increase parental provisioning rates over the major part of the nestling period, thus potentially competing with future siblings. Parents were marked with microchips so that additional begging sounds could be played back when one of the parents visited the nest. By playing back begging sounds consistently at either male or female visits, a sex difference in provisioning rate that lasted for the major part of the nestling period was induced. If each parent independently adjusts its effort to the begging intensity of nestlings, begging may also be the proximate control mechanism for the sexual division of labour.

Convergent patterns of long-distance nocturnal migration in noctuid moths and passerine birds

Vast numbers of insects and passerines achieve long-distance migrations between summer and winter locations by undertaking high-altitude nocturnal flights. Insects such as noctuid moths fly relatively slowly in relation to the surrounding air, with airspeeds approximately one-third of that of passerines. Thus, it has been widely assumed that windborne insect migrants will have comparatively little control over their migration speed and direction compared with migrant birds. We used radar to carry out the first comparative analyses of the flight behaviour and migratory strategies of insects and birds under nearly equivalent natural conditions. Contrary to expectations, noctuid moths attained almost identical ground speeds and travel directions compared with passerines, despite their very different flight powers and sensory capacities. Moths achieved fast travel speeds in seasonally appropriate migration directions by exploiting favourably directed winds and selecting flight altitudes that coincided with the fastest air streams. By contrast, passerines were less selective of wind conditions, relying on self-powered flight in their seasonally preferred direction, often with little or no tailwind assistance. Our results demonstrate that noctuid moths and passerines show contrasting risk-prone and risk-averse migratory strategies in relation to wind. Comparative studies of the flight behaviours of distantly related taxa are critically important for understanding the evolution of animal migration strategies.

Orientation scatter of free-flying nocturnal passerine migrants: components and causes

We investigated the variation in concentration of orientation among nocturnally migrating passerine birds. Using tracking radar, we recorded flight tracks of birds during spring and autumn migration and, with the aid of concurrent wind recordings, we calculated heading directions. The concentrations of track and heading directions were compared between seasons and also between different categories of migrants that were defined by flight speed and wing beat frequency. Wind drift was a dominant cause of the large scatter of track directions, especially for autumn migration. When wind effects were compensated for, we found only small differences in the concentration of heading directions between different categories of migrants. This shows that between-group variation is not a major source of the overall variation in orientation when groups are distinguished on the basis of airspeed and wing beat frequency. Although the total concentration of heading directions was almost exactly the same for spring and autumn migrants, there was an element of partial compensation for wind drift in spring but not in autumn. When we removed the effect of this compensatory behaviour by considering situations with low wind speeds, the concentration of headings during spring tended to exceed that during the autumn. This suggests a more accurate orientation of the birds during spring than in autumn, when a large proportion consists of naive migrants on their first migratory journeys. The high concentration of heading directions of free-flying migrants are in clear contrast to the widely scattered distributions generally observed in orientation experiments with caged birds.

Magnetic information calibrates celestial cues during migration

Migratory birds use celestial and geomagnetic directional information to orient on their way between breeding and wintering areas. Cue-conflict experiments involving these two orientation cue systems have shown that directional information can be transferred from one system to the other by calibration. We designed experiments with four species of North American songbirds to: (1) examine whether these species calibrate orientation information from one system to the other; and (2) determine whether there are species-specific differences in calibration. Migratory orientation was recorded with two different techniques, cage tests and free-flight release tests, during autumn migration. Cage tests at dusk in the local geomagnetic field revealed species-specific differences: red-eyed vireo, Vireo olivaceus, and northern waterthrush, Seiurus noveboracensis, selected seasonally appropriate southerly directions whereas indigo bunting, Passerina cyanea, and grey catbird, Dumetella carolinensis, oriented towards the sunset direction. When tested in deflected magnetic fields, vireos and waterthrushes responded by shifting their orientation according to the deflection of the magnetic field, but buntings and catbirds failed to show any response to the treatment. In release tests, all four species showed that they had recalibrated their star compass on the basis of the magnetic field they had just experienced in the cage tests. Since release tests were done in the local geomagnetic field it seems clear that once the migratory direction is determined, most likely during the twilight period, the birds use their recalibrated star compass for orientation at departure. Copyright 2000 The Association for the Study of Animal Behaviour.

A polar system of intercontinental bird migration.

Studies of bird migration in the Beringia region of Alaska and eastern Siberia are of special interest for revealing the importance of bird migration between Eurasia and North America, for evaluating orientation principles used by the birds at polar latitudes and for understanding the evolutionary implications of intercontinental migratory connectivity among birds as well as their parasites. We used tracking radar placed onboard the ice-breaker Oden to register bird migratory flights from 30 July to 19 August 2005 and we encountered extensive bird migration in the whole Beringia range from latitude 64° N in Bering Strait up to latitude 75° N far north of Wrangel Island, with eastward flights making up 79% of all track directions. The results from Beringia were used in combination with radar studies from the Arctic Ocean north of Siberia and in the Beaufort Sea to make a reconstruction of a major Siberian–American bird migration system in a wide Arctic sector between longitudes 110° E and 130° W, spanning one-third of the entire circumpolar circle. This system was estimated to involve more than 2 million birds, mainly shorebirds, terns and skuas, flying across the Arctic Ocean at mean altitudes exceeding 1 km (maximum altitudes 3–5 km). Great circle orientation provided a significantly better fit with observed flight directions at 20 different sites and areas than constant geographical compass orientation. The long flights over the sea spanned 40–80 degrees of longitude, corresponding to distances and durations of 1400–2600 km and 26–48 hours, respectively. The birds continued from this eastward migration system over the Arctic Ocean into several different flyway systems at the American continents and the Pacific Ocean. Minimization of distances between tundra breeding sectors and northerly stopover sites, in combination with the Beringia glacial refugium and colonization history, seemed to be important for the evolution of this major polar bird migration system.

Nocturnal passerine migrants fly faster in spring than in autumn: a test of the time minimization hypothesis

It has been suggested that time selection and precedence in arrival order are more important during spring than autumn migration. Migrating birds are expected to fly at faster airspeeds if they minimize duration rather than energy costs of migration, and they are furthermore expected to complete their journeys by final sprint flights if it is particularly important to arrive at the destination before competitors. We tested these hypotheses by tracking-radar studies of nocturnal passerine migrants during several spring and autumn seasons at Lund (56°N) and Abisko (68°N) at the southern and northern ends of the Scandinavian Peninsula, respectively. The samples from these two sites represent migrants that are mostly rather far from (Lund) or close to (Abisko) their breeding destinations. We found that the birds were flying at clearly faster airspeeds in spring than in autumn at both study sites, with spring speeds exceeding autumn speeds by, on average, 16%, after taking effects of wind conditions and vertical flight speeds into account. This difference in speeds could not be explained by seasonal differences in body mass or wing morphology and thus supports the hypothesis of time-selected spring migration. There was also a significantly larger seasonal difference in airspeed at Abisko than at Lund, suggesting that the birds may have shown an inclination to sprint on their final spring flights to the breeding destinations, although this possible extra sprint effort was modest.

Orientation of nocturnally migrating Swainson's thrush at dawn and dusk: Importance of energetic condition and geomagnetic cues

Abstract We studied the early morning cage orientation of nocturnally migrating Swainsons Thrushes (Catharus ustulatus) during three fall migration seasons. The results were compared with earlier free-flight release tests under starry skies and were found to be consistent with continuation of migratory flights in the expected seasonally appropriate direction. Energetic condition proved decisive: fat birds chose directions in accordance with migration across the Gulf of Mexico, whereas lean birds oriented away from the coast, possibly in search of habitats suitable for refuelling. Whereas the orientation of fat Swainsons Thrushes was affected by experimental shifts of the magnetic field, the response during morning tests was larger than expected. A parallel series of orientation cage experiments performed during evening twilight showed a response to deflected magnetic fields that was close to the expected shift, which suggests a difference in integration of directional information between early morning and evening twilight activity. However, within-individual response to deflected magnetic fields was of the same magnitude during both morning and evening tests.

Nest-attenders in the Pied Flycatcher (Ficedula hypoleuca) During Nestling Rearing: A Possible Case of Prospective Resource Exploration

Abstract Visits to nest holes by birds other than their owners is a familiar phenomenon for students of breeding biology. In this study, we evaluate that behavior using a transponder reading system. Eighty-five males and females were fitted with transponders at the end of the incubation period or just after hatching. Nest boxes were fitted with transponder readers from just after hatching until all nestlings fledged. That system revealed 123 visits by birds to nest boxes other than their own, a visit being defined as at least one visit to a separate nest box on a separate day. Males were more often detected at other nests than females (53% of males vs. 29% of females visited) and males on average made more visits than females did (4.8 vs. 2.5 visits). However, both males and females devoted time to visiting other nests while still feeding nestlings. That behavior is more common than previously suspected and is consistent with birds prospecting for future nest sites or investigating patch reproductive success.

Flight speeds of swifts (Apus apus): Seasonal differences smaller than expected

We have studied the nocturnal flight behaviour of the common swift (Apus apus L.), by the use of a tracking radar. Birds were tracked from Lund University in southern Sweden during spring migration, summer roosting flights and autumn migration. Flight speeds were compared with predictions from flight mechanical and optimal migration theories. During spring, flight speeds were predicted to be higher than during both summer and autumn due to time restriction. In such cases, birds fly at a flight speed that maximizes the overall speed of migration. For summer roosting flights, speeds were predicted to be lower than during both spring and autumn since the predicted flight speed is the minimum power speed that involves the lowest energy consumption per unit time. During autumn, we expected flight speeds to be higher than during summer but lower than during spring since the expected flight speed is the maximum range speed, which involves the lowest energy consumption per unit distance. Flight speeds during spring were indeed higher than during both summer and autumn, which indicates time-selected spring migration. Speeds during autumn migration were very similar to those recorded during summer roosting flights. The general result shows that swifts change their flight speed between different flight behaviours to a smaller extent than expected. Furthermore, the difference between flight speeds during migration and roosting among swifts was found to be less pronounced than previously recorded.

Compensation for wind drift by migrating swifts

We investigated the orientation in relation to wind of common swifts, Apus apus, during nocturnal spring and autumn migration. Swifts are highly adapted to a life in the air, showing wind-dependent orientation during nocturnal roosting flights, and may be expected to be more efficient in their wind drift/compensation behaviour than nocturnal passerine migrants, which are usually subjected to full or partial wind drift. A tracking radar at Lund in southern Sweden was used to record the orientation of common swifts (identified by their characteristic radar echo signature) on nocturnal migration flights and to measure wind conditions at the altitudes at which the birds were flying. Comparing track and heading directions under easterly and westerly winds revealed that the swifts shifted their heading distinctly into the wind, with the result that track directions were similar under the different wind conditions. As this pattern of complete compensation for drift from cross-winds occurred during both spring and autumn migration, there were no indications of differences between age classes. In addition, we found an effect of side-winds on equivalent airspeeds, with swifts increasing their airspeed with increasing wind speed. Such a response has been theoretically predicted as part of an optimal behaviour for counteracting wind drift but has hereto not been empirically demonstrated. There was also a positive correlation between overall wind speed and equivalent airspeed, making it difficult to interpret whether the swifts respond to the total wind speed rather than specifically to the side-wind effect. Our results suggest that important differences may exist between species in their capacities to orient in relation to the wind and that the swift may be particularly efficient in adjusting heading direction and airspeed to obtain complete compensation for wind drift during high-altitude nocturnal migratory flights