Does avian flight initiation distance indicate tolerance towards urban disturbance?

<span style="font-family: 'Times New Roman','serif'; font-size: 10.5pt">Urbanization is dramatically altering biodiversity and ecosystem health worldwide. Birds are highly reactive to environmental change and are thus important indicators of ecological condition at both a global and a local scale. Flight initiation distance (FID) is generally used as a quantitative measure of a bird&rsquo;s tolerance to human-caused disturbance and may indicate how well a species or population has become adapted to chronic environmental stress. Here, we address the following questions, looking at speciesspecific FID values in different habitats along the urban&ndash;rural gradient:<span>&nbsp; </span>(a) Are within-species FID values generally lower in strongly urbanized areas than in more rural areas?<span>&nbsp; </span>And if so, (b) does variation of FID (VFID) indicate species-specific tolerance to urbanization?<span>&nbsp; </span>From 2008 to 2009, we measured FID in coastal bird species at eight sites along the urbanized coast of Xiamen, China, in a total of 254 trials. The results indicate that bird species with a high propensity to disperse and with large population sizes tend to decrease their FID more strongly along the urban&ndash;rural habitat gradient. This pattern was most apparent in the Little Egret (Egretta garzetta). Based on VFID, 17 bird species out of 36 were classified as being tolerant to urban environmental conditions, with FID values showing decreasing trends along the urban&ndash;rural gradient, as in the Little Egret. Our results suggest that VFID may be the relevant measure for analyzing birds&rsquo;tolerance to urbanization and for assessing the speed by which species or populations can adjust or adapt to novel environmental conditions.