The aerodynamic and mechanical effects of elongated tails in the scarlet-tufted malachite sunbird: measuring the cost of a handicap

Abstract

Experimental manipulations of tail length have shown that the long tail of a male scarlet-tufted malachite sunbird, Nectarinia johnstoni, handicaps its ability to fly and its efficiency at catching aerial insects. Here the possible aerodynamic effects of increased tail length are examined. Two experimental groups of birds had their tails either lengthened or shortened; in one control group the tail was kept the same length but had feather splints added; and in another control group extra mass was added in the form of rings on the legs. Birds with shortened tails spent longer flying and hawked more efficiently than before the manipulation; birds with lengthened tails spent less time flying and were less efficient at hawking. Males with feather splints added also spent less time flying. Ringing had no effect on the birds' behaviour. These results correspond well with the predictions made if the influence of tail length mainpulation was primarily on the amount of drag produced by the tail and on the moment of inertia of the tail. These two effects mean that flight is more costly and manoeuvres become more difficult with increasing tail length. To compensate for the influence of the long tail the birds may increase their wingspan: the increase in span predicted from theory corresponds well with the relationship between tail length and wing length observed in the field. © 1992 The Association for the Study of Animal Behaviour.