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Stop-over and Migration Ecology of the Whimbrel: Fall 2009 Season Report
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Reliable estimates of adult survival for many shorebird species are lacking. We used Cormack–Jolly–Seber (CJS) methods to provide an estimate of apparent, or local, survival (φ) of a population of whimbrels, Numenius phaeopus, breeding in the subarctic Churchill, Manitoba region. We used data collected in two time‐periods: 1973–1976 and 2010–2014. We also quantified nest‐site fidelity in 2010–2014 to provide context to our apparent survival estimates because mark‐recapture analyses cannot distinguish between mortality and permanent emigration. The most parsimonious CJS model did not include effects of sex or time on apparent adult survival in either period (φ = 0.76 ± 0.13 SE; φ = 0.75 ± 0.04 SE, 1973–1976 and 2010–2014, respectively). Additionally, observations of marked whimbrels between 2010 and 2019 (n = 124) showed that 61 of the 105 marked individuals (58.1%) were resighted. These estimates of return rates are, as expected, much lower than estimates of apparent survival. The median year‐to‐year distance between nests (n = 139) in 2010 to 2014 was 198 m ± 88 SE and did not differ significantly (p = 0.84) between females (x̄ = 721.9 m ± 119.8 SE) and males (x̄ = 720.3 m ± 83.1 SE). If our apparent survival estimate is indicative of true survivorship, then adult mortality during the non‐breeding season has remained constant over the last five decades, implying that the recent decline in whimbrel populations may stem largely from reduced fecundity, including egg or juvenile survival, rather than primarily from reduced adult survival.
Size-selection of crabs is different among the wader species. The lower acceptance threshold can be explained with the optimal foraging model of size selection. Handling time increases with prey size, but the weight increase is more. Therefore, smaller prey is less profitable and is ignored if the feeding rate rises. For bird species taking crabs out of their burrows the upper limit is set by the decreasing accessibilty. Width of gape is also a limiting factor. Whimbrel Numenius phaeopus is the main predator. Most species prey upon small crabs, but since this size is also the commonest, the prey risk increases with size. Only 15% of crabs are taken out of its burrown.-from Author whimbrel Numenius phaeopus predation on crabs
Numenius phaeopus feeding on crabs took food with a low rate of return because half of the mass was inorganic, and 30% of the organic mass within the skeleton itself was not digestible. Whimbrel is able to metabolize 99% of the fat, 74% of the protein and 1% of the carbohydrates. Apparent and true metabolizable energy content of crabs was 7 and 8 kJ g-1 dry mass, respectively. When more food was available, the bird became more selective by refusing a greater part of the skeleton. As a consequence, the relative amount of flesh in the food increased. Food digestibility decreased when more food was ingested per day, probably due to a third factor: intake rate increased with the amount of food taken and a higher intake rate reduced prey digestibility. However, since the decrease in digestibility was less than the increase in intake rate, ingesting food at the higher rate was a profitable strategy. -from Authors
Whimbrels Numenius phaeopus are able to consume 2-3 mg
ash-free dry mass per second feeding over the short term, but when the intake
rate is that high, they have to pause to allow for digestion. If these digestive
pauses are taken into account, the maximum crude intake rate is reduced to
1 mg s-l. This maximum intake rate does not increase during the season,
even though the total daily food consumption in April is 50% higher than in
February and March (24 and 36 g ash-free dry mass d- 1, respectively). The
increased consumption is achieved by feeding for a greater part of the time
at the highest intake rates. In February and March, the intake rate of Whimbreis
is reduced iflow water falls before 8.00 h, and it is only high when low
water falls between 9.00 and ca. 15.00 h. This diurnal pattern disappears in
April, when a high intake rate is also achieved at night. The body mass increases
at a rate of 1.1 % per day prior to migration. A rate of gain in body
mass greater than the one observed hardly seems possible because of physiological
constraints (the limit in crude intake rate set by the capacity of the
gut) combined with physical limits, namely the duration of emersion period.
A possible limiting effect due to heat or salt stress could not be demonstrated
convincingly.
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Throughout the year, the muddy coast of the Guianas in northeastern South America is a very important staging area for North American breeding shorebirds. Highest numbers are found along the coast of Suriname, where up to 1.3 million birds currently occur during the southbound migration peak. Lower numbers occur in French Guiana (760,000) and Guyana (no recent counts). Three coastal wetlands in Suriname have been designated as Western Hemisphere Shorebird Reserve Network sites of Hemispheric Importance. Current shorebird numbers are lower than in the 1970s and 1980s. In Suriname, numbers of at least five species have decreased by 40-80%. In French Guiana, at least seven species show a negative trend. The government of Suriname has made significant advances with the conservation of shorebirds. Most of the coast has been designated as multiple use management areas (MUMA), while two important shorebird areas are protected as nature reserves. Moreover, since 2002, all shorebird species are fully protected by law (not the case in French Guiana and Guyana). However, notwithstanding their protected status, shorebirds are still hunted and trapped in high numbers (several tens of thousands each year). Strict enforcement of the Game Law rules is essential, and the success of an education and awareness campaign initiated in 2006 needs to be build upon.