Short communication The effects of force- fledging and premature fledging on the survival of nestling songbirds HENRY M. STREBY, 1† * SEAN M. PETERSON, 1 JUSTIN A. LEHMAN, 1 GUNNAR R. KRAMER, 1 KELLY J. IKNAYAN 1† & DAVID E. ANDERSEN 2 1 Department of Fisheries, Wildlife and Conservation Biology, Minnesota Cooperative Fish and Wildlife Research Unit, University of Minnesota, St. Paul, MN, USA 2 U.S. Geological Survey, Minnesota Cooperative Fish and Wildlife Research Unit, St. Paul, MN, USA Despite the broad consensus that force-fledging of nest- ling songbirds lowers their probability of survival and therefore should be generally avoided by researchers, that presumption has not been tested. We used radiotelemetry to monitor the survival of fledglings of Ovenbirds Seiurus aurocapilla and Golden-winged Warblers Vermivora chry- soptera that we unintentionally force-fledged (i.e. nest- lings left the nest in response to our research activities at typical fledging age), that fledged prematurely (i.e. nest- lings left the nest earlier than typical fledging age), and that fledged independently of our activities. Force-fledged Ovenbirds experienced significantly higher survival than those that fledged independent of our activities, and pre- maturely fledged Ovenbirds had a similarly high survival to those that force-fledged at typical fledging age. We observed a similar, though not statistically significant, pat- tern in Golden-winged Warbler fledgling survival. Our results suggest that investigator-induced force-fledging of nestlings, even when deemed premature, does not neces- sarily result in reduced fledgling survival in these species. Instead, our results suggest that a propensity or ability to fledge in response to disturbance may be a predictor of a higher probability of fledgling survival. Keywords: breeding ecology, fledgling survival, Golden-winged Warbler, observer effects, Ovenbird, Seiurus aurocapilla, Vermivora chrysoptera. Many studies of nesting passerines use different methods late in the nestling stage from those used earlier in the nestling stage to avoid the negative impacts of force- fledging or prematurely fledging young from nests (e.g. Anderson & Anderson 1961, Bjornstad & Lifjeld 1996, Holmes et al. 1996, Payne & Payne 1998, Sillet et al. 2000, Ferretti et al. 2005, Maddox & Weatherhead 2008). Although the terms are often used interchange- ably, we use ‘force-fledging’ to refer to nestlings leaving the nest in response to investigator stimulus, and ‘premature fledging’ as force-fledging that occurs prior to typical fledging age. Although the term fledge technically refers to the developmental stage at which young birds first fly, it is used ubiquitously in the songbird literature to refer to leaving the nest (i.e. fledging from the nest; Gill 1995), and we maintain the latter common defini- tion here. Anecdotal evidence of detrimental effects of force-fledging and premature fledging dates back more than 100 years, when Cole (1910) reported finding ringed nestlings dead outside nests. Cole (1910) subse- quently stated that observing dead nestlings outside nests, regardless of researcher activities, ‘is not an uncommon thing’, and concluded that no causal relationship could be drawn between nestling handling and mortality in those cases. Cole nevertheless concluded that premature- fledging is ‘probably, however, the greatest danger to the birds from our work’. Recently, Pietz et al. (2012) reiter- ated that warning: ‘We echo Cole’s (1910) advice from a century ago that researchers who handle older nestlings (e.g. to measure or band) need to be aware of their possi- ble impacts.’ Yet Pietz et al. (2012) conceded that the fates of force-fledged or prematurely fledged birds are rarely known. We are not aware of any empirical studies of the impacts of force-fledging or premature fledging on songbirds despite widespread attempts to avoid it (e.g. Ezaki 1988, Briskie 1995, Brooke & Nakamura 1998, Confer et al. 2003, Nagy & Holmes 2005, Ardia 2006) on the assumption that it results in reduced fledgling sur- vival (e.g. Hamilton & Martin 1985, Miller & Leonard 2010, Ball & Bayne 2012). We examined the impact of force-fledging at typical fledging age and premature fledging on fledgling survival in Ovenbirds Seiurus aurocapilla and Golden-winged War- blers Vermivora chrysoptera in the western Great Lakes region, USA and Canada. We did not purposefully force any nestlings to fledge, but some broods did not remain in nests after our ringing and transmitter attachment activi- ties, which provided an ideal opportunity to test the assumption that force-fledging and premature fledging caused by investigator activities negatively affect fledgling survival. We compared survival of fledglings that left nests independently of our activities within 3 days of marking, those that force-fledged (nestlings would not stay in nest after handling at typical fledging age) and those that pre- maturely fledged (nestlings would not stay in nest after handling those younger than typical fledging age). † Present address: Department of Environmental Science, Policy and Management, University of California, 130 Mulford Hall, Berkeley, CA 94720, USA. *Corresponding author. Email: streby@berkeley.edu © 2013 British Ornithologists’ Union Ibis (2013), doi: 10.1111/ibi.12051