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.