Shorebirds' seasonal adjustments in thermogenic capacity are reflected by changes in body mass:How preprogrammed and instantaneous acclimation work together

• Published in: Integrative and Comparative Biology
• Main Authors: Vezina, Francois, Dekinga, Anne, Piersma, Theunis
• Format: Article in Journal/Newspaper
• Language: English
• Published: 2011
• Subjects: BASAL METABOLIC-RATE; KNOTS CALIDRIS-CANUTUS; ENDOGENOUS CIRCANNUAL RHYTHMICITY; THYROID-HORMONE CONCENTRATION; DISTANCE MIGRANT SHOREBIRD; SPARROWS PASSER-DOMESTICUS; RUFOUS-COLLARED SPARROWS; COST-BENEFIT-ANALYSIS; PECTORAL MUSCLE MASS; RED KNOTS; Calidris canutus; Red Knot
• Online Access: https://hdl.handle.net/11370/39b5672a-af3b-4569-8e94-a4b62df9dbf1; https://research.rug.nl/en/publications/39b5672a-af3b-4569-8e94-a4b62df9dbf1; https://doi.org/10.1093/icb/icr044; https://pure.rug.nl/ws/files/6766740/2011IntegrCompBiolVezina.pdf

Phenotypic flexibility in shorebirds has been studied mainly in the context of adjustments to migration and to quality of food; little is known on how birds adjust their phenotype to harsh winter conditions. We showed earlier that red knot (Calidris canutus islandica) can acclimate to cold by elevating body mass. This goes together with larger pectoral muscles, i.e., greater shivering machinery, and thus, better thermogenic capacity. Here, we present results of a yearlong experiment with indoor captive knots to determine whether this strategy is part of their natural seasonal phenotypic cycle. We maintained birds under three thermal regimes: constant cold (5 degrees C), constant thermoneutrality (25 degrees C) and natural seasonal variation between these extremes (9-22 degrees C). Each month we measured variables related to the birds' endurance to cold and physiological maintenance [body mass, thickness of pectoral muscles, summit metabolic rate (M(sum)), food intake, gizzard size, basal metabolic rate (BMR)]. Birds from all treatments expressed synchronized and comparable variation in body mass in spite of thermal treatments, with a 17-18% increase between the warmest and coldest months of the year; which appeared regulated by an endogenous driver. In addition, birds living in the cold exhibited a 10% higher average body mass than did those maintained at thermoneutrality. Thickness of the pectoral muscle tracked changes in body mass in all treatments and likely contributed to greater capacity for shivering in heavier birds. Consequently, M(sum) was 13% higher in cold-acclimated birds compared to those experiencing no thermoregulation costs. However, our data also suggest that part of maximal heat production comes from nonshivering processes. Birds facing cold conditions ate up to 25% more food than did birds under thermoneutral conditions, yet did not develop larger gizzards. Seasonal variation in BMR followed changes in body mass, probably reflecting changes in mass of metabolically active tissues. Just as ...