Facultative adjustment of pre-fledging mass loss by nestling swifts preparing for flight

Jonathan Wright, Shai Markman, Shaun M. Denney

Research output: Contribution to journalArticlepeer-review


Nestling birds often maintain nutritional reserves to ensure continual growth during interruptions in parental provisioning. However, mass-dependent flight costs require the loss of excess mass before fledging. Here we test whether individual variable mass loss prior to fledging is controlled through facultative adjustments by nestlings, or whether it reflects physiologically inflexible developmental schedules. We show that in the face of natural and experimental variation in nestling body mass and wing length, swifts always achieve very similar wing loadings (body mass per wing area) prior to fledging, presumably because this represents the optimum for flight. Experimental weights (approx. 5% body mass) temporarily attached to nestlings caused additional reductions in mass, such that final wing loadings still matched those of control siblings. Experimental reductions in nestling wing length (approx. 5% trimmed from feather tips) resulted in similar additional mass reductions, allowing wing loadings at fledging to approach control levels. We suggest that nestlings may assess their body mass relative to wing area via wing flapping and special 'push-ups' (on the tips of extended wings) performed in the nest. Thus, by facultatively adjusting body mass, but not wing growth, nestling swifts are always able to fledge with aerodynamically appropriate wing loadings.

Original languageEnglish
Pages (from-to)1895-1900
Number of pages6
JournalProceedings of the Royal Society B: Biological Sciences
Issue number1596
StatePublished - 2006
Externally publishedYes


  • Aerodynamics
  • Apus apus
  • Body mass
  • Common swift
  • Mass-dependent flight costs
  • Wing length

ASJC Scopus subject areas

  • General Biochemistry, Genetics and Molecular Biology
  • General Immunology and Microbiology
  • General Environmental Science
  • General Agricultural and Biological Sciences


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