A simple procedure to reduce investigator-induced chick mortality in Yellow-legged Gull ( Larus michahellis ) colonies

The most frequently reported negative effect of human disturbance in gull colonies is an increase in chick mortality. This is primarily caused by the presence of humans startling nearby chicks, prompting them to flee out of their own territories where they are attacked by adults of adjacent territories. This effect may be exacerbated in handled chicks. This study proposed and evaluated a simple, non-invasive procedure designed to keep older Yellow-legged Gull (Larus michahellis) chicks within their territories after handling and release. The procedure consisted of placing the chicks on the ground in a supine position, with their heads positioned behind their bodies in a way that deprived them of vision. This procedure (vision-deprived supine release) is only appropriate for chicks that are at least three weeks old. Results demonstrated that the procedure significantly increased the time chicks remained at the release site. Moreover, more than half of the sampled chicks did not regain their natural standing posture until enough time had passed for the researcher to have retreated to a distance of approximately 150 m or more, a distance considered sufficient to minimize human disturbance. The only potentially negative aspect of this procedure was that a very small proportion of chicks (n = 1) were not able to roll over and regain their natural standing posture. Although it was uncommon, it should be further investigated. Overall, the vision-deprived supine release may substantially contribute to minimizing investigator-induced disturbance during chick handling in gull colonies.