No abstract

We examined the variation in element number in the perch-coo of the collared dove and its possible causes. A negative correlation was present between body weight of an individual and the occurrence of two instead of three elements in one coo. In birds showing them, coos consisting of two elements were more common towards the end of longer coo bouts. An analysis of the spectro-temporal structure of three-element coos showed a decrease in duration of the third element in long coo-bouts. This, and a comparison of the spectrotemporal parameters of three- and two-element coos, suggests that reduction in element number is due to replacing the third element by silence. Possible causes might be muscular exhaustion and insufficient respiration, which might explain why two element coos are more common in lighter, and presumably weaker, birds. However, declining motivation cannot be ruled out as an alternative explanation. Four-element coos were produced by one male. Their occurrence was not linked to specific positions in a bout. The spectro-temporal structure of these coos suggest that they result from a programming error causing a 'stutter' to proceed a normal coo. Two different mechanisms may thus underlie variation in element number in the collared dove.

Coo development and its differentiation was studied between the second and eighteenth month of life in captive collared dove males (N = 9) and females (N = 6). During this period eight vocal characteristics were analyzed to determine the nature of age-related changes in the spectrotemporal structure of the perch-coo. At the onset males and females produced coos with the same structure, with exception of the fundamental frequency. In males, age-related changes in fundamental frequency, stereotypy, and coo length were limited. Early stabilization of these parameters may be related to maturation of the syrinx. Vocal activity, bout length, modulation percentage and sound percentage increased, which might be caused by an age-dependent increase in testosterone. The number of harmonics decreased, which might be related to some kind of 'motor practice'. Similar to song development in songbirds, perch-coos of male doves develop gradually into more conspicuous vocalizations, showing some plasticity in form development. In contrast, in female doves none of the vocal parameters changed with age. As a result, the structure of adult female coos continued to resemble that of juveniles, as has also been shown for female song in songbirds. For several vocal parameters the developmental trajectories of male and female coos began to diverge at the age of about one year. Two to three months later the sex differences became significant. Two vocal parameters showed an age-related change in inter-individual variation: vocal activity and modulation percentage of males. These findings support the hypothesis that frequency modulations may play an important role in intra-specific communication, conveying messages about a senders sex and age.

No abstract

Irma Adriaanse helped in data collection. Herman Berkhoudt kindly lent us the stroboscope. Frans van Berge Henegouwen assisted in preparing the figures. Jeffrey Cynx, Katharina Riebel, J.P. Mailman, Jaap Dubbeldam, and Hans Slabbekoorn provided helpful comments.