[nullc]

The article seemed to suggest that non-cross pairs are not observed at all, but they needed more genetic testing to determine if tan/tan never happens.

Even if the tan/tan is still physiologically possible it may be the case that they still won't mate even if there are no white present at all due to required mating signals being missing.

[jojobas]

From a different article:

So are females most strongly attracted to the tough, macho, white-striped males? Actually, no. Lab studies have found that females of either morph prefer the tan-striped males. White-striped females, more pushy than their tan-striped sisters, grab the tan-striped bachelors right away, so these pairs form more quickly than the opposite combination. Males of both morphs tend to prefer the white-striped females, but those females quickly hook up with tan-striped males if they can, so eventually the leftover birds will form pairs consisting of white-striped males and tan-striped females.

https://www.audubon.org/news/the-fascinating-and-complicated...

Granted, the question of homo-zygotic white offspring viability is not even touched.

Edit: yet another article tentatively says the double-white sparrows to exist, but in far lesser number than expected, they suspect some genetic disadvantage.

https://www.cell.com/current-biology/fulltext/S0960-9822(15)...

[nullc]

Thanks! Very interesting!