How are the killdeer male and female different?

Follow them around, and see which one lays the eggs! :)

Males, in general, have more dark feathers around their bills and eyes. Also, their bills are usually thicker near their head and sometimes their heads are shaped a little differently, usually "thicker" in the forehead. The same criteria for sexing adults is also the same for chicks.

Behavior wise, adult males tend to be more confrontational towards other killdeer, especially other males. They make the "kill-dee" sounds more than females and are generally more noisy and high strung.