Earlier research suggests that circulating levels of a potent stress hormone, corticosterone, in the female before conception or egg laying is a dominant factor.
Experts at the reserve said that despite high winds and an intruder osprey in the vicinity, the female osprey had been "showing signs of egg-laying behaviour".
If a cowbird female fails to lay in a warbler nest in time for her egg to hatch with those of the host, she can reset the clock in her favour by killing the first clutch.
The drug works by using high doses of the female hormone progestin, which also is contained in birth-control pills, to block a potentially fertilized egg from becoming implanted in a woman's uterus.