Philopatry to stopover site and changes in body condition of migrating Reed Warblers Acrocephalus scirpaceus were studied in Bet Shean Valley, Israel, where warblers were netted throughout the year. Although the majority of birds were seen only once, the proportion of transients seen twice or more in different years is comparable to the figure for summer residents returning between years, indicating a high degree of philopatry among transients. Transients get heavier with longer duration of stay, up to about 15 days, after which body mass increase appears to level off at about 3 g. Change in body condition, taken to be body mass divided by wing length, also was noted, albeit of less significance. The mean date of arrival in the autumn of birds in their first year was about 20 days later than that of older birds. Reed Warblers use their time effectively to replenish their body mass and improve their condition before starting the dangerous crossing of the Sahara Desert.
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Vol. 102 • No. 2