Long-term Reproduction (1984–2013), Nestling Diet, and Eggshell Thickness of Peregrine Falcons (Falco peregrinus) in Yellowstone National Park

Peregrine Falcons (Falco peregrinus) were extirpated from Yellowstone National Park (YNP) by 1970 as a result of widespread use of DDT (dichloro-diphenyl-trichloroethane) throughout North America from the late 1940s to the early 1970s. DDT, and its primary metabolite DDE (dichloro-diphenyl-dichloroethylene), caused eggshell thinning and impaired reproduction in Peregrine Falcons and other raptors. Restoration of Yellowstone’s Peregrine Falcon population began with nationwide restrictions placed on the use of DDT in 1972, coupled with the release of 36 captive-raised juveniles in YNP and the dispersal of 644 captive-raised juvenile Peregrine Falcons released within 260 km of YNP. We monitored Peregrine Falcon reestablishment and reproductive success in YNP (nesting success, productivity, and brood size) from 1984–2013. Productivity was defined as the number of young reaching ≥28 d per territorial pair. Brood size referred to the number of young reaching ≥28 d per successful pair. From 2010–2013, we collected and analyzed prey remains and eggshell fragments from nine Peregrine Falcon territories across YNP. We documented a substantial increase in the number of occupied territories from one in 1984 to 32 by 2007, as well as high nesting success (74%), productivity (1.62 young/territorial pair), and brood size (2.18 young/successful pair) during 1984–2013. Nesting success, productivity, and brood size were at or above the target values identified by U.S.F.W.S. and those found for the Rocky Mountain/Great Plains region during the 2003 national survey. Peregrine Falcon eggshells collected at the nine eyries were 4% thinner than pre-1947 measurements (pre-DDT) and presumably indicate low DDE concentrations. Prey remains were dominated by birds (97% of individuals), mostly terrestrial species (63%) including American Robins (Turdus migratorius), Franklin’s Gulls (Leucophaeus pipixcan), and Mountain Bluebirds (Sialia currucoides).