I\.~)L.AN, V., Jr. & KPTrEKSON, E. D. 1991: Experiments on winter-site attachment in young dark-c\.ed juncos. Ethology 87, 123-133.
AbstractSite attachment in young birds has sometimes been considered to take place during a sensitive phase as the result ot an imprinting-like process. We conducted two four-year-long experiments o n N intcr-site attachment in young migratory dark-eyed juncos (lunm hyenzalis). In both, we caught birds over a range of d a m , displaced them to large outdoor enclosures, and released them near the end o t winter. In the first experiment, release was at the capture site and all returns next winter were to t l i x site. Thc return rate did not vary with date of capture and confinement; therefore attachment cciuld h a w been completrd either before capture or after release. T h e absence of returns to the ct)tifinement site suggests that attachment to a place requires that the individual live free there. In the second experiment, release was at the confinement site, and many subjects remained there, probably until they migrated northa,ard. Returns next winter were to both the capture site (the majority) and thc confinement-release 5ite. The fact that a few juncos formed a bond to the confinement-release sire i n this experiment reinforces the view that living free is necessary for sire attachment. Individuals caught and confined earlier in winter tended to be less likely to return than those caught later. T h e dihtribution of returns according to date of capture indicates that attachment was completed at dittcrent dates in different subjects, which is not consistent with imprinting during a clearly defined sensitive period. However, young juncos hatch and migrate at various dates. Therefore o u r experiniriit does not rule out the existence of a brief sensitive phase for site attachment, its dates varying among individuals because age and/or timing of migration vary.