Do body condition and plumage during fuelling predict northwards departure dates of Great Knots Calidris tenuirostris from north‐west Australia?

Abstract

It is often assumed that strong selection pressures give rise to trade‐offs between body condition and time in long‐distance migrating birds. Birds that are ‘behind schedule’ in fuel deposition or moult should delay departure, and this should result in a negative correlation between initial condition and departure date. We tested this hypothesis in the Great Knot Calidris tenuirostris migrating from north‐west Australia to eastern Asia en route to Siberia. Great Knot gain mass and moult into breeding plumage before leaving northern Australia in late March and early April, and fly 5400–6000 km to eastern China and Korea. We radiotracked 27 individuals (17 males and ten females) to determine departure dates; 23 migrated and four remained in Australia. We characterized body condition at capture using body mass, predicted pectoral muscle mass (based on ultrasound estimates of the size of the pectoral muscles) and breeding plumage scores. Residual condition indices were uncorrelated, indicating that at the individual level, variation in one fuelling component was not strongly associated with variation in the other components. Birds that did not depart had lower residual body mass and breeding plumage indices than those that did migrate; these four birds may have been subadults. Neither sex, size nor the condition indices explained variation in departure date of migrants. Reasons for this are explored. Departure dates for northward migrating waders indicate that the migration window (span over which birds depart) decreases with proximity to the northern breeding grounds. We suggest that migration schedules become tighter as birds get nearer to the breeding grounds. Thus the lack of a relationship between condition and departure date in Great Knots may reflect the fact that the departure episode under study is the first one in sequence and is still 4–8 weeks before breeding.

Small home ranges and high site fidelity in red knots (Calidris c. canutus) wintering on the Banc d’Arguin, Mauritania

“…Each bird was scanned every 6.6 min. This receiver set-up records signals within a 1-km radius (van Gils et al , 2005Battley et al 2004). During the time that the automatic stations were operating signals were received from 14 individuals.…”

“…Radio signals were collected by two 24-h operating automated radio tracking stations (ARTS), supplemented by regular checks with mobile antennae and receivers. The ARTS (Telemetrics, Arnhem, The Netherlands; see Green et al 2002;Battley et al 2004) were erected at Ebel Kheaiznaya and Baie d'Aouatif ( Fig. 2a) and were operating constantly from 18 to 27 and 18 to 28 December 2003, respectively.…”

High-tide habitat choice: insights from modelling roost selection by shorebirds around a tropical bay

“…This approach prevented us from using triangulation, but increased the area that we could monitor. Technical details on the array are given by Battley et al (2004). Each transmitter frequency was scanned four times each 20 s; the complete cycle of 52 frequencies (including test frequencies) took 17.3 min.…”

An ontogenetic perspective on individual differences

“…In iteroparous organisms, for whom each life-history event may occur repeatedly throughout life, this dictates that each occurrence of an event will be coupled with predictable phenotypic changes [41]. For example, migratory preparation in birds involves mass gain, hypertrophy of flight muscles and atrophy of digestive organs [42,43]. Importantly, birds do not maintain their migratory phenotypes throughout adulthood, but instead cycle through successively different phenotypes during each annual cycle [40].…”