Timing edit
The lengthening of the days is the main physiological cue for migration. These changes are related to hormonal changes in the birds. Before migrating, many birds exhibit increased activity or Zugunruhe (German for migratory restlessness), which Johann Friedrich Naumann first identified in 1795. Physiological changes that occur during this time include increased fat deposition. Zugunruhe can occur even in birds kept in cages without any environmental cues, such as g. reduction in day length and temperature) have demonstrated how circannual endogenous programs regulate bird migrations. [77] Birds in cages exhibit a preferential flight direction that mimics the direction they would migrate in the wild; they alter this preference direction about the same time as their wild conspecifics do. [78].
Satellite tracking of 48 Asian houbaras (Chlamydotis macqueenii) over several migrations revealed that this species times the departure of its spring migration according to the local temperature. Interestingly, departure reactions to temperature varied among subjects but were individually repeatable (across several years of tracking). This implies that temperature use by individuals serves as a cue for population-level climate change adaptation. Put another way, many migratory birds are expected to leave earlier in the year for their summer or winter destinations as a result of global warming. [79].
Males in polygynous species with significant sexual dimorphism typically return to breeding grounds earlier than females. This is termed protandry. [80][81].
Pfeilstörche edit
Some of the earliest evidence of long-distance stork migration was found in 1822 when a white stork with an arrow made of central African hardwood was discovered in the German state of Mecklenburg. [12][13][14] The term “Arrow stork” in German, “Pfeilstorch,” was used to describe this bird. Since then, around 25 Pfeilstörche have been documented.
Physiology and control edit
Even in non-migratory bird species, the ability to control migration, as well as its timing and response, appears to be a primitive trait that is controlled by genetics. A far more complicated phenomenon, the capacity to navigate and orient oneself during migration may involve both endogenous programs and learning. [75][76].
FAQ
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