Scientists from the University of Exeter and GWCT observed 81 male and 43 female pheasants living wild on an estate in the United Kingdom, and studied the abilities of the groups to be vigilant for predators as well as foraging.
What was found was that females in larger harems were less vigilant than females in smaller harems. Males showed the opposite trend, with vigilance increasing for males with larger harem sizes.
The experts found that individual pheasants adjusted their own vigilance levels depending on the harem sizes they were placed in. With larger harem sizes females would spend more time foraging and less time being vigilant, but the reverse was true for males.
This may go some way to explaining why we see relatively small harem sizes in wild pheasants. Females benefit from being in a large harem as reduction in individual vigilance allows for them to increase time spent foraging, a feature necessary when preparing for the breeding season. Males try to shoulder the burden of standing guard by adjusting their own vigilance levels but with too many females they simply cannot compensate and as a result the safety of the group is compromised due to the lack of collective vigilance.