Persistent laterality in soaring flight is elicited by demand

Abstract

Efficient neural processing for executing difficult behavioural tasks is achieved through lateralisation, the favouring of one side. Flight, however, strongly selects for bilateral symmetry, and a persistent turning-side bias could be in theory costly. How soaring birds reconcile lateralised efficiency with symmetric flight is unresolved. We followed 248 individuals of three soaring species, the European honey buzzard, the golden eagle and the white stork, from their first flight for a median of 1.3, 2.2 and 1.7 years per individual respectively, through more than 1.5 million circling manoeuvres. We modelled the turning side of every circle as a difficulty-conditioned process with individual random effects. Laterality was individual: each bird held a stable, repeatable preferred side. Side-bias in circling widened about threefold from the easiest to the hardest circles, and on the days a bird visibly favoured one side, that side was its own seven times in ten. The bias was established early and changed little with age, and its mechanical trace, while present on a single manoeuvre, did not grow into a measurable daily or seasonal cost. Across the three species, lateralisation and symmetric flight coexist: the bias stays faint until difficulty draws it out, yet remains too small to compound.