Certain loud calls made by female red junglefowl and Lapland longspurs are given most frequently immediately after egg laying, when a copulation should have the highest probability of fertilizing the next egg to be laid. In these species there is considerable male-male interaction for access to fertilizable females, and males are attracted to or follow females making these calls. Based on our interpretation of these calls, we develop a general hypothesis to predict the pattern of occurrence of fertility advertisement both within and among bird species. We hypothesize that certain loud calls given by female birds before and during the egg-laying period are designed to advertise fertility and thereby incite male-male competition. This hypothesis predicts that calls advertising female fertility should most often occur soon after an egg is laid (i.e. during the period of highest fertility) but may also occur at any time during the female's fertilizable period. Such calls are unlikely to be given by females in strictly monogamous species (especially those with long-term pair bonds) because in these species each female usually mates with only a single male and extra-pair copulations are avoided. Although reports of loud female calls associated with copulation are rare in the literature, the 18 examples we found (including junglefowl and longspurs) were predominantly (15/18) in species adopting mating systems other than strict monogamy, and these calls were most commonly and disproportionately reported in multi-male mating systems. This form of “estrus” in birds may be widespread because few species appear to be strictly monogamous, but it will be difficult to study because the period of high fertility for female birds is so short.