As you know if you read my blog, Trivers Willard is an important theoretical construct which has been tested numerous times. TW works in some species, not in others, and overall, that should be predictable (accroding to TW).
It turns out that finches control the sex of their offspring, and do so in a way that TW would predict, apparently. There is a paper in Science that I'll probably eventually get to writing up for you, and in the mean time, here's a quick news report from Scientific American.
See if you can figure out how Trivers Willard is working here, and why the important theoretical aspect of this research is glossed in this news report.