The opening of milk bottles by birds: Evidence for accelerating learning rates, but against the wave-of-advance model of cultural transmission.

One of the most widely cited cases of cultural transmission in animals is the opening of milk bottles by British birds. Bottle opening was first reported in Swaythling in 1921 and its spread from that date to 1947 was mapped by Fisher and Hinde (1949). Using data from Fisher and Hinde, this paper tests two quantitative models of cultural transmission: (1) the logistic model describing the cumulative number of bottle opening sites in the Belfast area and in the whole of the UK, and (2) the linear wave-of-advance model describing the progressive spread over time of bottle opening from its presumed single point of origin, Swaythling. For both the UK and Belfast, the logistic provides a poorer fit than alternative accelerating functions: the positive exponential and the reverse S-shaped hyperbolic sine respectively yield the best corrected fits to the Belfast and UK data. Neither functions have the terminal deceleration phase typical of the logistic, but both have an accelerating phase consistent with the cultural assumption of an auto-catalytic increase in the rate of spread over time. For both Belfast and the whole UK, the wave-of-advance model can clearly be rejected, even with the addition of a second source of innovation in County Durham. The results support the view that bottle opening originated from several independent sites, but spread through an accelerating process that could have included direct and/or indirect social influences.