Spectrographic analysis of the Advertising call of Western grebes showed seasonal and geographic variation, individual differences, and variation between birds of different sex, color phase, and pairing status. In each of these cases, I determined: 1) how the calls dif fered structurally, and 2) whether birds responded to playbacks of these differences in the field. Advertising is used by unpaired males to attract females. Unpaired males distinguish and ignore the calls of other males and paired females, but respond to calls of unpaired females of the same color phase. Paired birds also continue to use the Advertising call in a variety of contexts involving contact between individuals in large breeding groups. This means that unpaired males must somehow avoid confusing such calls with those of potential mates. Playback experiments showed that Western grebes accomplish this by varying call bout-length and by calling alternately back and forth. After pairing, Western grebes shift to shorter bouts (1-2 calls/bout) and reply only to their mate's call. Seasonal variation in call-bout length is then just a by-product of pairing status changes. Advertising thus is a very flexible "open signal", whose meaning depends on both its fine-grained structure and its social contexts.