Seasonal body mass changes in Eurasian Golden Plovers Pluvialis apricaria staging in the Netherlands: decline in late autumn mass peak correlates with increase in raptor numbers

Eurasian Golden PloversPluvialis apricariastaging in the Netherlands during the nonbreedingseason show strikingly constant seasonal changes in body mass with a first masspeak in late November and December and a second peak in late April and May. Despitehuge sample sizes, variations in this pattern over successive years in the 1990s and amongage classes were minuscule. However, in contrast to the body mass levels at other times ofthe year, there was a marked decline in the winter peak mass of Golden Plovers from the1970s/early 1980s to 1989–2000. The decrease, by an average of 29 g, was about half theextra mass previously stored in autumn. This additional mass is known to consist of fat andmay be interpreted as an energy store−insurance−for sudden cold spells when a negativeenergy balance forces the birds to move south and stay in front of the frostline. As the rateof the mass increase in September–October showed no change from the 1980s to the 1990s,changes in food availability are unlikely to explain the long-term mass decline. Also, therewere no differences in two factors known to influence energy expenditure and feeding rate,air temperature and rainfall. The one striking environmental change relevant to plovers wasthe steep increase in the occurrence of raptors in the northern Netherlands in the 1980s,notably Peregrine FalconsFalco peregrinusand GoshawksAccipter gentilis. We argue that thehalving of the winter mass peak over a decade is consistent with the hypothesis that underincreased risk of predation, birds lower their body mass in order to reduce individualvulnerability, a reduction that may be traded off against an increased risk of starvation.