Transformations at High Latitudes: Why Do Red Knots Bring Body Stores to the Breeding Grounds?

Abstract

Brown Pelicans (Pelecanus occidentalis) plunge dive into water to capture fish with highly distensible pouches. During prey capture, the pouch may expand to hold 11 L of water and the mandibular rami may bow from a resting position of 5 cm to over 15 cm. We compared mineralization of two bending regions of the mandible with a nonbending region, and examined cross-sectional morphology to determine if mineral content and shape play a role in mandibular bending. A rostral bending zone, adjacent to the mandibular symphysis, possessed only about 20% mineral content, significantly less than the lateral bending zone and the rigid caudal mandible, both comprised of over 50% mineral content. Additionally, the rostral zone was solid in cross section, facilitating bending, whereas the lateral zone was made up of bones joined by connective tissue. This latter morphology permits movement between the bones even though there was relatively high mineralization. We suggest that mineral content may be an overlooked component to cranial kinesis in birds.

Body condition of shorebirds upon arrival at their Siberian breeding grounds

“…On theoretical grounds, shorebirds, of which many species undertake similarly long or even longer migrations than geese, are less likely to use this strategy, because of their much smaller size (Klaassen 2003). Empirical studies comparing isotope signatures of eggs with body stores accumulated during migration (Morrison and Hobson 2004) and chicks with those of feathers grown by their parents on wintering or spring staging areas (Klaassen et al 2001), have confirmed that arctic shorebirds generally produce eggs mostly from nutrients collected after arrival on the breeding grounds ('income breeders'; (Klaassen et al 2001;Morrison and Hobson 2004;Morrison et al 2005).…”

“…Instead of being channelled into eggs, nutrient stores carried to the breeding grounds may also be metabolised during the period directly after arrival, when food availability is limited and unpredictable owing to weather and snow conditions (Morrison 1975;Sandberg 1996;Baker et al 2004;Morrison et al 2005). Such stores may be important for survival and reproduction: unusually cold early summers in 1972 and 1974 caused extensive mortality of adult Red Knots Calidris canutus in northern Greenland and Canada (Morrison 1975;Boyd and Piersma 2001), and birds departing from Iceland with below-average mass suffered more than heavier birds (Morrison 2006;Morrison et al 2007).…”

“…Such stores may be important for survival and reproduction: unusually cold early summers in 1972 and 1974 caused extensive mortality of adult Red Knots Calidris canutus in northern Greenland and Canada (Morrison 1975;Boyd and Piersma 2001), and birds departing from Iceland with below-average mass suffered more than heavier birds (Morrison 2006;Morrison et al 2007). In 1999, when snow melt was late at Alert, Canada, arrival masses were significantly lower than the long-term mean and many shorebirds postponed breeding or did not breed at all (Morrison et al 2005).…”

“…Red Knots and Ruddy Turnstones Arenaria interpres arrived at Ellesmere Island (Canada) with relatively large fat and muscle stores (Morrison and Davidson 1990;Morrison et al 2005). During the post-arrival period these stores declined in size while the digestive system, heart and liver increased, indicating that a key function of the stores was to enable a physiological transformation into suitable breeding condition at a time of the year when food was scarce (Morrison et al 2005).…”

“…In preparation for migratory flight, long-distance migrant shorebirds change their body composition: digestive organs not needed during the flight are reduced in size to reduce the weight, while flight muscles gain in mass and fat stores are built up. Upon arrival the digestive organs regain their original size (Piersma and Gill 1998;Piersma et al 1999;Battley et al 2000;Morrison et al 2005). These transformations involve rapid changes in protein and fat contents.…”