The magnetic map sense and its use in fine-tuning the migration programme of birds.

The Earth's magnetic field is one of several natural cues, which migratory birds can use to derive directional ("compass") information for orientation on their biannual migratory journeys. Moreover, magnetic field effects on prominent aspects of the migratory programme of birds, such as migratory restlessness behaviour, fuel deposition and directional orientation, implicate that geomagnetic information can also be used to derive positional ("map") information. While the magnetic "compass" in migratory birds is likely to be based on radical pair-forming molecules embedded in their visual system, the sensory correlates underlying a magnetic "map" sense currently remain elusive. Behavioural, physiological and neurobiological findings indicate that the sensor is most likely innervated by the ophthalmic branch of the trigeminal nerve and based on magnetic iron particles. Information from this unknown sensor is neither necessary nor sufficient for a functional magnetic compass, but instead could contribute important components of a multifactorial "map" for global positioning. Positional information could allow migratory birds to make vitally important dynamic adaptations of their migratory programme at any relevant point during their journeys.

Magnetic activation in the brain of the migratory northern wheatear (Oenanthe oenanthe).

Behavioural and neurobiological evidence suggests the involvement of the visual and trigeminal sensory systems in avian magnetoreception. The constantly growing array of new genetic approaches becoming available to scientists would bear great potential to contribute to a generally accepted understanding of the mechanisms underlying this ability, but would require to breed migratory birds in captivity. Here we show that the transcontinental night-migratory Northern Wheatear (Oenanthe oenanthe), which is currently the only migratory songbird successfully being bred in reasonable numbers in captivity, shows magnetic-field-induced neuronal activation in the trigeminal brainstem areas receiving their input through the ophthalmic branch of the trigeminal nerve. In addition, preliminary data indicate night vision-triggered activation in the anterior visual forebrain. This brain area could represent the same brain region, which has previously been named "Cluster N" and shown to be involved in processing magnetic compass information in European Robins. Thus, based on brain activation data, both visually and trigeminally mediated magnetic senses known from other birds seem to exist in Northern Wheatears. This makes this species a potentially excellent model species for future genetic research on magnetoreception in migratory birds.

Migrating songbirds tested in computer-controlled Emlen funnels use stellar cues for a time-independent compass.

This paper investigates how young pied flycatchers, Ficedula hypoleuca, and blackcaps, Sylvia atricapilla, interpret and use celestial cues. In order to record these data, we developed a computer-controlled version of the Emlen funnel, which enabled us to make detailed temporal analyses. First, we showed that the birds use a star compass. Then, we tested the birds under a stationary planetarium sky, which simulated the star pattern of the local sky at 02:35 h for 11 consecutive hours of the night, and compared the birds' directional choices as a function of time with the predictions from five alternative stellar orientation hypotheses. The results supported the hypothesis suggesting that birds use a time-independent star compass based on learned geometrical star configurations to pinpoint the rotational point of the starry sky (north). In contrast, neither hypotheses suggesting that birds use the stars for establishing their global position and then perform true star navigation nor those suggesting the use of a time-compensated star compass were supported.

A mathematical expectation model for bird navigation based on the clock-and-compass strategy.

We present here a mathematical formula for the directional distribution of migratory birds if they use a vector navigation/clock-and-compass strategy to find their winter quarters. It is based on mathematical expectation theory and shows that a simple parabola can describe the expected geographical spread of clock-and-compass birds as a function of migratory distance. Predictions based on this model are then tested against all same autumn ringing recoveries of first-season Pied Flycatchers, Ficedula hypoleuca, ringed in Scandinavia and European Robins, Erithacus rubecula, ringed in Sweden and Finland and recovered north of the Sahara Desert. We find that the predictions of our analytical model fit the ringing recovery distribution of freely migrating conspecifics extremely well.

Modelling migration: the clock-and-compass model can explain the distribution of ringing recoveries.

The aim of the present study was to test whether the directional distributions found in ringing recoveries could be explained if night-migrating passerines use only a simple clock-and-compass strategy during autumn migration. I developed a mathematical model that predicts the expected directional distributions as a function of distance covered by birds using a clock-and-compass strategy. The predictions were compared with data from natural migration: all ringing recoveries of pied flycatchers, Ficedula hypoleuca, ringed in Scandinavia and recovered elsewhere in Europe and North Africa within the same autumn. The predictions of the model correlated strongly with the distribution of ringing recoveries. This suggests that at least young pied flycatchers, and perhaps night-migrating passerines in general, use a simple clock-and-compass strategy to reach their wintering area. If so, all the prerequisites (a compass and an internal clock) that they need to orient during migration seem to be known at present, at least at the behavioural level, and navigational abilities or any elusive map-sense are not needed for migration. Copyright 1998 The Association for the Study of Animal Behaviour.

Redstarts, Phoenicurus phoenicurus, can orient in a true-zero magnetic field.

I tested the migratory orientation of redstarts in a true-zero magnetic field to elucidate the importance to this species of access to either magnetic or celestial cues. I also tested the validity of the assumption on which all funnel experiments are based: that what we observe in an orientation funnel reflects what the bird would do if actually migrating. In a set of funnel experiments, I tested 47 night-migrating redstarts caught during their first autumn migration. Each bird was tested once under each of four experimental conditions in a semi-randomized block-design. Upon completion of the final funnel tests the birds were fitted with light indicators and released. The results showed that redstarts can find the migratory direction on the basis of access to either celestial or magnetic cues. Thus, when celestial cues were available they could orient in a true-zero magnetic field. In addition, a starry sky facilitated high migratory activity as well as a clearly directed orientation. The release experiments showed that the vanishing bearings on nights with no wind were in good agreement with the direction of the funnel activity, but that the vanishing bearings were strongly influenced even by light wind. Copyright 1998 The Association for the Study of Animal Behaviour. Copyright 1998 The Association for the Study of Animal Behaviour.