The Streby Lab
Actionable Science for Evolutionarily Rational Conservation and Management
Force fledging nestling songbirds is a death sentence
Read the Methods sections of studies involving nest monitoring and one of the most common sentences will be something like "We did not approach nests closely during the late nestling stage to avoid force fledging broods." Explanations for this caution are rarely given, but the obvious assumption is that nestlings that leave the nest in response to human activity will experience such negative consequences that force-fledging should be avoided, even at the cost of valuable data.
During our studies of full-season productivity in Ovenbirds, Golden-winged Warblers, Wood Thrushes, and Hermit Thrushes we marked nestlings with bands and radio transmitters during the late nestling stage. We made considerable efforts to keep marked nestlings in the nest after we handled them, assuming force fledging was a death sentence, but some birds just wouldn't stay put. Although our sample of force-fledged birds was intentionally small, we had enough Ovenbirds and Golden-winged Warblers to assess fledgling survival and to test this common assumption. Interestingly, force-fledged birds experienced significantly higher survival than birds that fledged "naturally".
We do not suggest dragging nestling songbirds from their nests as a conservation strategy to improve productivity. Rather, our results suggest birds that fledge in response to disturbance might just be ready to fledge, and a propensity or ability to force fledge might be a predictor of higher potential survival, rather than a fatal mistake. It is possible that force fledging birds from nests high above the ground, over water, or in other dangerous locations, might have different outcomes. However, we unintentionally force fledged five nestling Wood Thrushes from mid-canopy nests, and all five survived beyond the lifespan of our radio transmitters. Our results and observations suggest many studies are missing out on valuable late-nestling-stage data all because of an invalid assumption.