Function of Manx Shearwater Calls in Mate Attraction

Playback experiments were used to study the role of vocalization in mate attraction by Manx shearwaters, a nocturnal seabird. Pre-breeding birds were exposed to test calls of male and female shearwaters calling from the range of normal locations: above the colony as in a flyover call, on the ground in the colony, or from within the nesting burrow. Both female and male test calls increased the frequency of male calling but only male calls increased the frequency of female calling. These findings suggest that male calls 1) facilitated competing responses from other males that are attempting to attract females to their burrows and 2) stimulated calling in available females that require more directional information to find the advertising male. Male calling location, inside or outside the burrow, probably reflected a compromise between being audible to females and being susceptible to predation. Because the call was more widely broadcast, male test calls played from outside the burrow caused more females to land in the colony than the same call played from inside the burrow. However in a simultaneous choice test between a burrow and an outside male test call, males calling from inside did not lose females to the adjacent outside male. Thus calling from inside the burrow in an area where another male is calling outside maximized female attraction while minimizing risk of predation. The male call played from inside the burrow attracted as many females as a pair call from inside but the response of females to the pair was less sustained, indicating that duetting pairs-inhibited the response of unpaired females. Male and female calls emanating from above the colony suppressed the rate of female flyover calls suggesting that during this test condition other birds stopped calling in an attempt to orient to the source of the test call.