No abstract

Long stretches of sea and desert often interrupt the migration routes of small songbirds, whose fat reserves must be restored before these can be crossed as they provide no opportunity for refuelling. To investigate whether magnetic cues might enable inexperienced migratory birds to recognize a region where they need to replenish their body fat, we caught and held thrush nightingales (Luscinia luscinia) in Sweden just before their first migration and exposed them to a magnetic field simulating that at a migratory stopover in northern Egypt, before the Sahara Desert. We found that this magnetic field stimulated the birds to extend their fat-deposition period, indicating that magnetic cues may help small migratory birds to confront large ecological barriers.

The Paridae family (chickadees, tits and titmice) is an interesting avian group in that species vary in important aspects of their social structure and many species have large and complex vocal repertoires. For this reason, parids represent an important set of species for testing the social complexity hypothesis for vocal communication—the notion that as groups increase in social complexity, there is a need for increased vocal complexity. Here, we describe the hypothesis and some of the early evidence that supported the hypothesis. Next, we review literature on social complexity and on vocal complexity in parids, and describe some of the studies that have made explicit tests of the social complexity hypothesis in one parid—Carolina chickadees, Poecile carolinensis. We conclude with a discussion, primarily from a parid perspective, of the benefits and costs of grouping and of physiological factors that might mediate the relationship between social complexity and changes in signalling behaviour.

No abstract

When birds are attacked by predators the initial take-o¡ is crucial for survival. The strategy in the initial phase of predator evasion is probably a¡ected by factors such as body mass and presence of cover and conspeci¢cs, but it may also be a response to the character of the predator's attack. In choosing an angle of £ight, birds face a trade-o¡ between climbing from the ground and accelerating across the ground. This is, to our knowledge, the ¢rst study investigating whether the attack trajectory of a raptor a¡ects the take-o¡ strategy of the prey bird. First-year male great tits (Parus major) adjusted take-o¡ angle to a model predator's angle of attack. Birds attacked from a steep angle took o¡ at a lower angle than birds attacked from a low angle. We also compared take-o¡s at dawn and dusk but could not ¢nd any measurable e¡ect of the diurnal body mass gain (on average 7.9%) in the great tits on either £ight velocity or angle of ascent.