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Fledging in the common swift, Apus apusweight-watching with a difference
1EGI, Department of Zoology, University of Oxford, U.K.
Insights
Young common swifts must transition from nestling to flight. Studies show they reduce body mass before fledging, optimizing flight efficiency for their aerial life.
Area of Science:
- Ornithology
- Animal Ecology
- Avian Biology
Background:
- Fledging in common swifts (Apus apus) requires a rapid shift from a sedentary nestling phase to sustained aerial life without post-fledging parental care.
- This transition necessitates physiological and morphological adaptations for immediate independent flight.
Purpose of the Study:
- To investigate the physiological and morphological changes in young common swifts during the pre-fledging period.
- To determine how factors like brood size and resource availability influence these changes and subsequent flight capability.
Main Methods:
- Brood size manipulations and varying resource conditions were used across different breeding seasons.
- Observations focused on chick states, body mass, wing length, and wing loading at fledging.
Main Results:
- Chicks in poorer condition (underfed) exhibited delayed fledging, lower body mass, and shorter wing length.
- Nestlings consistently lost mass in the week before fledging, irrespective of feeding conditions.
- Despite mass loss, nestlings had higher body mass than adults and fledged young, with similar wing loadings at fledging.
Conclusions:
- Young common swifts undergo a 'slimming' process, reducing body mass and potentially water content before fledging.
- This mass reduction, coupled with increased wing length, enhances lift:drag ratio, optimizing flight efficiency for their aerial lifestyle.
- These adaptations prepare fledglings for the demands of continuous flight and survival independent of parental care.
Abstract:
Fledging in the common swift involves young having to switch suddenly and completely from the sedentary lifestyle of a nestling to continuous flight with no post-fledging care. Using manipulations of brood size and the contrasting resource conditions in different breeding seasons, I observed fledging under a range of chick states. Poorly fed chicks took longer to fledge, and had lower body mass and short wing length at fledging. Despite this, maximum body mass of nestlings always exceeded those of adults and fledged young. Under all conditions, nestlings lost mass for 1 week prior to fledging, and at fledging they had similar wing loadings. I suggest that in their last days in the nest young swifts use up the fat stores that insured them against irregular parental provisioning as well as losing water from the drying out of feathers. Reductions in body mass associated with increases in wing length before fledging will affect flight efficiency by increasing the lift:drag ratio. This is interpreted as 'slimming' down to the optimal body mass which would improve the lift:drag ratio and set fledgings up for their life on the wing.