Adaptive body regulation in the life history of birds

Abstract: Birds have been flying on this Earth for around 150 million years and in response to fluctuating environments birds have evolved flexible body regulation. This thesis has investigated costs and adaptations in bird body regulation in different life history stages, namely moult and migration. Moult, the shedding and replacement of feathers, did surprisingly not impair tree sparrows Passer montanus flight ability indicating that they compensate physiologically for the wing area reductions experienced during moult. This compensatory aptness was then shown as tree sparrows developed a larger flight muscle relative to body mass when wing area was reduced during natural moult. This pattern was established in an experiment where birds reduced their body mass concurrently with enlarging flight muscle size in response to wing area reductions. In addition, group size affects birds' risk perception and tree sparrows were also found to regulate their bodies according to how large group they were living in, in accordance with theory on predation risk in birds. These experiments show potentially adaptive and novel organ flexibility patterns in birds. Compensatory regulation was then tested in the field, but tree sparrows in the wild did not exhibit the same patterns as birds in the experiments. Physiological constraints and group size-effects probably explain these results. Migration is also an important life history stage and birds en route must also regulate their bodies to environmental demands. Sea and desert often interrupt migration and large energy stores are needed to cross such barriers. Yet, large energy stores incur costs, and that large fuel loads impair take-off ability was demonstrated. Hence, birds do often only accumulate modest amount of fuel. This illustrates an important problem for migrating birds, specifically, how can naïve birds know a priori that they are about to cross a barrier and consequently must fuel up extensively. Thrush nightingales Luscinia luscinia are trans-Saharan migrants and were therefore held in Sweden and subjected to experiments where only the magnetic field was changed. They undertook a simulated magnetic journey from Sweden to northern Egypt and were then allowed to refuel for five days prior to the “desertcrossing”. Compared to control birds, thrush nightingales were found to accumulate larger fat deposits when subjected to the magnetic treatment, showing, for the first time, that birds can incorporate geomagnetic cues in their fuelling decisions. This rapid flexibility and the ability to incorporate external cues in refuelling probably reflect the paramount importance and long history of flight in birds.