The use of tongue spots for ageing and wing length for sexing skylarks Alauda arvensis:A critical evaluation

The Skylark Alauda arvensis is a European passerine species lacking plumage dimorphism and having acomplete post-nuptial moult in adults, and a complete post-juvenile moult in young birds. Tongue spotswere thought to allow age discrimination after moult. Based on data from a study population in theNetherlands, where the age of many birds was known, we show that tongue spots are not useful forageing Skylarks. Some nestlings lack tongue spots and half of all birds had tongue spots when threeyears or older. Regarding sex discrimination, males and females can usually be distinguished from abimodal wing length distribution, but the threshold and amount of overlap in wing length between sexesvary greatly between studies. Here, we show that 98% of all individuals from our Dutch breedingpopulation could be sexed correctly by wing length. However, during migration and winter, whenpopulations from other regions co-occur in the same area, molecular analyses revealed that the overlapin wing length between sexes increased. Therefore, ringers need to be cautious when assigning sex,and small males especially are often misclassified. For example, based on criteria used in France, 14%of Dutch male Skylarks, which are partial migrants, would be misclassified as females