Natural Resources, School of


Document Type


Date of this Version



Iowa Bird Life 73: 120-127.


Copyright 2003 Iowa Ornithologists’ Union


Forests in NE Iowa are highly fragmented, which may affect sustainability and dynamics of bird populations. We studied Wood Thrush (Hylocichla mustelina) nesting success at the Mines of Spain Recreation Area near Dubuque, IA during 2001. We also monitored movements of juveniles using radio transmitters during the birds’ initial dispersal from the nest site, and we used DNA analyses to determine their gender. Seventy percent of the nests (n=10) were parasitized by Brown-headed Cowbirds (Molothrus ater). The daily nest success rate was 0.9612 (SE=0.0189), and daily juvenile survival was 0.9703 (SE=0.0169). Three juveniles dispersed 270 m (female), 746 m (male), and 980 m (male). Females in our simulation model built 2.90 nests per year (SD = 1.02), and 0.63 fledglings per female were alive at the end of the breeding season. We used a simulation model to predict annual population growth for this small breeding population as 0.546 (95% CI = +/-0.025). During 2001, this population can certainly be classified as a “sink,” which reinforces the need for more information on regional juvenile and adult movements.