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Grasshopper warblers Locustella naevia have been monitored in the study area since 1984. As the population over the years increased from one singing male to several, it became apparent that males do not arrive all at once. Some males arrive at the study area in the last days of April or in the first days of May. Other males arrive throughout May. A few turn up as late as in the beginning of June, more or less a month after the first and at a time when the earliest arriving males sing less frequently (indicating that they are breeding). The aim of the present study was to investigate whether grasshopper warbler males arriving early to the study area were in better physical condition at the time of departure from the winter quarters than males arriving late. As an indicator of condition I used the mean length of growth bars in the tail feathers. One growth bar (one light and one dark bar) corresponds to the growth of the feather during 24 hours. The validity of using measurements of growth bars as an indicator of condition is summarized by Grubb (2006). The basic assumption is that the greater the average growth bar width, the better the bird’s nutritional condition while growing the feather. Grasshopper warblers usually have growth bars in the tail feathers that are visible when holding the bird in the hand, for example during ringing (Figure 1). Grasshopper warblers nesting in Europe undertake a complete moult in the wintering grounds (Moult -WC) in Africa, before returning to the breeding grounds (Svensson 1992). Kennerley & Pearson (2010) also states that “the majority of birds moult completely in tropical Africa”. Most birds undergo a complete moult in October to January, but in rare cases some birds continue to moult in February and March (Demongin 2016). Thus the growth bars on tail feathers, seen on returning birds in spring, reflect the nutritional condition of the bird during the tail-growth period at the wintering grounds, although not necessarily the condition just prior to departure. Kennerley and Pearson also states “In the nominate race, some adults have a partial post-breeding moult prior to migration, usually confined to the body and head feathers, but sometimes involving the tertials and central tail feathers” Similar facts are stated by Demongin (2016). This makes the central tail feathers less reliable for growth bar measurement in this study, see under Materials and methods. I measured the Abstract ORNIS SVECICA 27: 139–143, 2017

ornis svecica

In the Grashopper Warbler Locustella naevia, males with better nutritional condition at the winter ground arrive earlier than other males at the breeding ground