An unusual mating market: nest building in red bishops

A rather surprising biological market was described by Markus Metz, Georg Klump and Thomas Friedl. Males of red bishops, a South-African weaver bird, attract females with elaborate nests. The females prefer fresh and green nests, so males have to put in quite a bit of effort to out-compete others with newly built nests. The males build more nests when the demand by females goes up.

What does biological market theory add to sexual selection theory? Markus Metz and colleagues can explain this much better than I ever could:

But why apply biological market theory to the red bishop’s mating system? Traditionally, mating systems are investigated based on standard sexual selection theory, which can explain — based on the operational sex ratio (OSR; Emlen and Oring 1977) — why males are territorial and/or polygynous, or why females alone care for eggs and chicks. Biological market theory can also explain selective pressure through partner choice; however, it actually does not add explanatory power compared to standard sexual selection theory if the trading classes are males and females and females choose among males. This is because in such a mating system, the OSR is equal to the supply–demand ratio and, therefore, applying sexual selection theory based on OSR would essentially lead to the same conclusions as applying biological market theory based on supply/demand ratios. In the red bishop’s mating system, however, females do not choose primarily among males, but rather among nests provided by males. Individual males offer a different number of nests, with the number of nests offered by a male varying in time. Thus, the supply–demand ratio in the red bishop’s mating market is not equal to the OSR, but is defined as the ratio of the number of available nests to the number of females searching for nests at a given time. (p. 1370)