Bird orientation: external cues and ecological factors

Abstract: Birds are generally capable of accurate orientation. In this thesis I present studies about the external cues and the ecological factors that influences bird orientation. A multitude of environmental cues are used to select and maintain a proper direction. These are visual cues, like the sun and the starry night sky, but other cues like the geomagnetic field, wind and odours also play important roles. The use and integration of external directional information varies between species, sites and occasions. Juvenile pied flycatchers (Ficedula hypoleuca) on their first migration seem to use both visual (solar) and magnetic cues for their orientation, and it was difficult to rank either cue as dominant over the other. The magnetic compass of migrating birds is an inclination compass, meaning that it does not sense polarity but rather the angle of inclination. Snow Buntings (Plectrophenax nivalis) close to the magnetic north pole did not respond to manipulations of the magnetic information, but seemed to be able to use magnetic information at an angle of inclination of close to 89°. The perception of magnetic information in robins (Erithacus rubecula) is dependent on the intensity and wavelengths of light, suggesting a magnetoreception mechanism associated with vision. In late evening, magnetic information seems to be used as a calibration basis for visual cues in four species of migrating North American songbirds. The selection of a direction is not only dependent on the availability and utilisation of external cues. Several experiments (with various bird species) in this thesis show in a consistent way, that the amount of stored fat decides whether migrating birds will orient towards their expected goal or if they will choose another direction. Fat is stored as fuel for the often extensive migratory flights. In particular when a bird faces an ecological barrier, like a sea or desert crossing, without any possibilities to restore energy reserves, birds are reluctant to continue flying without enough fat stores and often show reverse orientation. The concentration of orientation in passerine birds is mainly dependent on the prevailing winds and to a much lesser extent on variation between groups of migrants or between autumn and spring migration. Orientation in roosting flights of swifts (Apus apus) is accurately directed into the headwind direction, probably to avoid excessive drift and site displacement. A more detailed investigation of the orientation behaviour of swifts performing roosting flights revealed a cyclic pattern of adjustments in their orientation in relation to the wind direction. The studies presented in this thesis emphasises the variation and flexibility of orientation in birds.