Aerodynamics and behaviour of moult and take-off in birds

Moult is a costly period of the annual cycle for most birds, during which most or all of the feathers, representing up to 30% of the lean dry mass of a bird, are shed and replaced (Jenni and Winkler, 1994; Murphy, 1996). Assessment of the costs and benefits of the processes involved in moult are crucial to explaining the diversity of moult patterns (timing, duration, extent, sequence), their adaptive radiation across avian taxa, and the ways in which moult affects and interacts with other important stages of the annual cycle such as breeding and migration. In this paper we briefly review the costs of moult, and stress the importance of biomechanical considerations associated with flight when making such a cost–benefit analysis. In particular, we focus on the impact of moult on the most demanding mode of avian flight—that is, take-off from the ground—and we also indicate the factors that constrain take-off flight ( Figure 1 ). The ability to take-off quickly and at a steep angle of ascent is likely to have a direct influence on fitness and survival: for example, increased take-off performance aids predatory avoidance (Cresswell, 1993; Grubb and Greenwald, 1982; Page and Whitacre, 1975; Witter et al., 1994). For this reason we wish to highlight the necessity for behavioural and evolutionary biologists to consider and quantify biomechanical costs and constraints in order to gain a comprehensive view of the factors influencing important life-history strategies in birds. Figure 1 The effect of moult on flight. (a) Lappett-faced vulture Torgos tracheliotus making a running take-off into a strong headwind. This species is the largest African vulture, with a body mass of around 8 kg. It undergoes a very slow and irregular moult, and this individual is missing one distal primary on each wing (J.M.V.R., Kenya, 1982). (b) Lesser black-backed gull Larus fuscus making a sharp turn in gusty winds despite moult of secondary feathers and at least one distal primary, and considerable disruption to the trailing edge and abrasion of tail feathers (J.M.V.R., North Yorkshire, 1985). https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781003210801/d008fc4e-848f-44f2-a981-3bf5a273442c/content/fig8_1.jpg/>