Evaluating the potential effects of capturing and handling on subsequent observations of a migratory passerine through individual acoustic monitoring

Ringing is the most common technique used for individual marking of passerine birds, informing understanding of many aspects of their behaviour and ecology. Birds caught for ringing may also be substantially handled before release (e.g. to obtain biometric data, blood or feather samples), and all such procedures may affect the subsequent behaviour of a captured individual. Previous field studies that have assessed the potential effects of capturing and handling birds have nevertheless, to date, lacked an entirely independent unhandled control. In this study, we used individual acoustic monitoring to compare the apparent survival and re-encounter probabilities of male tree pipits Anthus trivialis that were captured with mist nets and intensively handled (measured, blood-sampled and ringed) with those of a control group of unhandled birds tracked solely by their vocalization. There were no differences between handled and unhandled birds in apparent survival rates, either in simple annual estimates, or in jointly estimated within- and between-breeding season survival estimates. Re-encounter probabilities in the latter analysis peaked at the start of the breeding season, when song activity would be expected to be greatest, but were higher for handled than unhandled birds in mid- to late May. Apart from demonstrating that tree pipits were not affected in the long-term by capture and associated handling, our study confirms that individual acoustic monitoring is a valuable non-intrusive method for long-term monitoring studies.