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 Plovers Pluvialis apricaria staging in the Netherlands during the non-breeding season show strikingly constant seasonal changes in body mass with a first mass peak in late November and December and a second peak in late April and May. Despite huge sample sizes, variations in this pattern over successive years in the 1990s and among age classes were minuscule. However, in contrast to the body mass levels at other times of the year, there was a marked decline in the winter peak mass of Golden Plovers from the 1970s/early 1980s to 1989–2000. The decrease, by an average of 29 g, was about half the extra mass previously stored in autumn. This additional mass is known to consist of fat and may be interpreted as an energy store − insurance − for sudden cold spells when a negative energy balance forces the birds to move south and stay in front of the frostline. As the rate of 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, there were 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 was the steep increase in the occurrence of raptors in the northern Netherlands in the 1980s, notably Peregrine Falcons Falco peregrinus and Goshawks Accipter gentilis. We argue that the halving of the winter mass peak over a decade is consistent with the hypothesis that under increased risk of predation, birds lower their body mass in order to reduce individual vulnerability, a reduction that may be traded off against an increased risk of starvation.

Radio-telemetry observations of the first 650 km of the migration of Bar-tailed Godwits Limosa lapponica from the Wadden Sea to the Russian Arctic

In 1999 and 2000, 45 Bar-tailed Godwits Limosa lapponica were supplied with radio-transmitters during spring staging on the island Texel in the western Wadden Sea. With the use of Automatic Radio Tracking Stations (ARTS) on Texel and in south Sweden, and hand-held receivers on Texel, it was possible to follow the later part of the stopover period on Texel for 34 birds (76%) and the passage over south Sweden for 26 birds (58%). Thus, the method of automatic tracking of overflying migrating shorebirds works successfully where the migration corridor is narrow and predictable, as in the case with late spring shorebird migration from the Wadden Sea towards arctic Russia. The timing of departure from Texel and passage over south Sweden of radio-marked birds, with median dates of 30 May and 2 June respectively, were in agreement with published data on the spring migration of Siberian-breeding Bar-tailed Godwits L. l. taymyrensis. The individual variation in migration dates was larger than expected, with birds passing south Sweden between 25 May and 10 June, indicating that the time-window for departure might be broader than previously thought. There was no clear difference between males and females in timing of migration. The time difference between departure from Texel and passage over south Sweden (average 3.3 days) indicates that most Bar-tailed Godwits do not embark on the long flight towards Siberia directly from the western Wadden Sea, but are more likely to stop in the more easterly portion of the Wadden Sea before the final take-off. This pattern is similar to what has been found in other shorebirds and geese (e.g. Red Knots Calidris canutus and Dark-bellied Brent Geese Branta bernicla) migrating along the same route.

Baseline and Stress-Induced Plasma Corticosterone during Long-Distance Migration in the Bar-Tailed Godwit, Limosa lapponica

The specific roles of corticosterone in promotion of avian migration remain unclear even though this glucocorticosteroid is elevated in many migrating bird species. In general, glucocorticosteroids promote metabolic homeostasis and may elicit effects on feeding and locomotion. Because the migratory stages of refueling and flight are characterized by distinct behaviors and physiology, the determination of corticosterone levels during each stage should help identify potential processes in which corticosterone is involved. We measured baseline levels of corticosterone in bar-tailed godwits (Limosa lapponica) during two distinct stages of migration: (1) immediately after arrival at a false stopover site just short of theWadden Sea and (2) throughout the subsequent 4-wk refueling period on the Wadden Sea. Plasma corticosterone was higher in arriving than in refueling birds. In addition, corticosterone increased with size-corrected body mass during the refueling phase, suggesting that corticosterone rises as birds prepare to reinitiate flight. Therefore, elevated corticosterone appears associated with migratory flight and may participate in processes characterizing this stage. We also performed a capture stress protocol in all birds and found that corticosterone increased in both arriving and refueling godwits. Therefore, the normal course of migration may be typified by corticosterone concentrations that are lower than those associated with stressful and life-threatening episodes.

A single-year comparison of two methods of censusing breeding Red Knot and Sanderling in High Arctic Greenland

A uniquely intense field effort at Zackenberg, NE Greenland, in June–July 2003 made it possible, for the first time, to compare two methods of measuring breeding densities of two notoriously difficult-to-census High Arctic breeding shorebirds (Red Knot Calidris canutus and Sanderling Calidris alba): (1) mapping of displays and other activities of birds in a rapid assessment early in the season, and (2) systematic ‘roping’ of potential breeding areas to disturb and then find incubating birds on, or very close to, their nests. The latter method is particularly relevant to species that rely on crypsis to avoid nest detection. During 16 and 19 June an experienced observer, in a standardized way, mapped all visual observations of Red Knot and Sanderling over a 4.0 km2 study area, which consisted mainly of low-angle mountain slopes between altitudes of 100 and 400 m. The observations were interpreted to represent 8–9 ‘pairs’ of Red Knot and 13–17 ‘pairs’ of Sanderling. Observations nearby allowed for a few additional pairs of Red Knot. Between 17 June and 5 July a team of five observers systematically roped the same study area and found two Red Knot nests and 15 Sanderling nests. Most of the study area remained under daily scrutiny until 19 July, and during these visits we encountered two more families of Red Knots and seven more Sanderling families. Thus, the roping effort yielded a few more Sanderling ‘pairs’ than expected from the early-season survey, but fewer Red Knot. This may imply that either: (1) the early-season rapid assessment particularly overestimated the knot population, and/or (2) relative to Sanderlings, knot nests were heavily depredated before roping, and/or (3) incubating birds escaped notice during roping, and/or (4) some of the local Red Knots may not have started a breeding attempt at all. Further work with radio-tagged individuals is necessary to establish whether we need to invoke non-breeding as a cause of the discrepancy

Sex-ratio and body size of sandpiper chicks at Zackenberg, north-east Greenland in 2003

There is growing evidence that female birds may adaptively bias the sex ratio of their young as a function of environmental conditions. Data on brood sex ratio in shorebirds are scarce, however. In this study we report the brood sex ratios and morphometrics of Arctic sandpipers. Sex was determined in 13–64 chicks of Sanderling Calidris alba, Red Knot Calidris canutus islandica, Dunlin Calidris alpina arctica, and Ruddy Turnstone Arenaria interpres in NE Greenland during the 2003 breeding season. Brood sex ratios were biased significantly towards males in Dunlin and Ruddy Turnstone, but in Dunlin this bias disappeared in chicks older than two days. There was a non-significant bias towards females in Sanderling and Red Knot. Only for bill length in Dunlin hatchlings, there were significant differences between males and females. Surprisingly, in contrast to their parents, male chicks had longer bills than female chicks