EFFECTS OF MICROHABITAT, FLOCKING, CLIMATE AND MIGRATORY GOAL ON ENERGY-EXPENDITURE IN THE ANNUAL CYCLE OF RED KNOTS

We quantify seasonal changes in the maintenance energy requirements of Red Knots (Calidris canutus islandica). This subspecies breeds on the tundra of northeast Canada and north Greenland, migrates through Iceland and spends the winter in the coastal regions of western Europe. Maintenance Metabolism (M(maint)) is defined as Basal Metabolic Rate plus extra costs for thermoregulation at environmental temperatures below the thermoneutral zone. M(maint) of Red Knots resting in different microhabitats was estimated on the basis of measurements with heated taxidermic mounts, which were calibrated with forced convection against postabsorptive live birds resting over a range of air temperatures (i.e., against their Standard Metabolic Rate). Based on a physically realistic regression model for heat loss, we established the relationships between the electric power consumption of the mounts and three critical weather variables affecting dry heat loss: air temperature, wind speed and global solar radiation. Observations of Red Knots' use of different microhabitats (including their occurrence in flocks of different bird density) and orientation into the wind were collected on the wintering and on the breeding grounds. At lower standard operative temperatures on the coastal wintering grounds they foraged in tighter flocks and more often faced into the wind, saving 8% compared to solitary birds standing with their flanks exposed. We then used (1) microhabitat-specific equations, (2) long-term meteorological data sets, and (3) estimates of habitat use and wind orientation of free-living Red Knots at the different locations and times of the year, to reconstruct the seasonal patterns in M(maint) in the field. Average predicted M(maint) varied between 2.93 W in January and 1.64 W in August on the Dutch wintering areas. The maximum monthly M(maint) in winter was higher than that reached on the Canadian breeding grounds (2.28 W, or 0.78 times the Dutch January cost) and on the Icelandic staging grounds (2.27 W, or 0.77 x the Dutch cost in January, in spring, and 1.98 W, or 0.68 x, in fall). Based on 31 years of weather data from the Dutch Wadden Sea in the period 1960-1991 (assuming that Red Knots have not changed their behavior), the long-term overwinter average of M(maint), was 2.57 W, with an average monthly minimum of 1.87 W and a maximum of 3.05 W. The west-central coast of France, 900 km more to the south, offers energetically cheaper conditions (0.76 x values for the Dutch coast in January) in the nonbreeding season than the Wadden Sea. If islandica knots moved on to West Africa during the nonbreeding season they would incur a saving of 1. 13 W on M(maint), and pay an extra 0.13-0.22 W to cover the cost of travel.