I’m assuming to get her to accept his sperm?Note the male -- that's Limpy Chick! -- grabbing at the feathers on the back of the female's neck at the conclusion of the coitus. We've seen this with each observed copulation.
What purpose might it have?
But roosters grab the hens so they can hang on while they mate.
The kangaroo was neat!
Do females choose a new partner every year?