Parrot sound tool design: drum tool data

The rarity of tool manufacture in wild parrots is surprising because they share key life history traits with advanced tool-using species including large brains, complex sociality, and prolonged parental care. When it does occur, tool manufacture in parrots tends to be innovative, spontaneous and individually variable but most cases have been in captivity. In the wild, only palm cockatoos (Probosciger aterrimus) have been observed using tools regularly. However, they are unusual because they use tools to enhance their displays rather than for foraging or self-maintenance. Males in northern Australia make two types of tool from sticks and seed pods, which they tap rhythmically against a tree during display. We analysed 256 sound tools retrieved from 70 display trees. Drum sticks (89% of tools) were used more often than seed pod tools; most males manufactured only drumsticks, but some made both types. Individual males differed significantly in the design of their drumsticks including the length, width and mass, but we found no evidence that neighbours copied each other. We discuss the highly individualised preferences for sound tool design in context of the behavioural predispositions behind the rarity of tool manufacture in wild parrots.

Methods
Drum stick tools and seed pod tools were collected from the base of display trees after use by the birds. Drum sticks were measured and weighed, yielding the following parameters: 

Length (mm), measured end to end following the natural contours of the drumming tool,
Mean width (mm), the mean of three measurements including within 1 cm of each end and in the middle of the stick, 
Chord (mm), the shortest distance between the end points of the stick providing an adjusted measure of length accounting for curvature of the stick,
Dry mass (g), the mass of the stick when fully desiccated, 
The number of protruding stubs where smaller branches had been removed by the birds,and
Drum stick curvature was estimated by dividing the stick length by its chord.

There were insufficient seed pod tools for analysis of individual preferences. Standard statistical techniques were used to examine individual differences.