Ways to be different: foraging adaptations that facilitate higher intake rates in a northerly-wintering shorebird compared to a low-latitude conspecific

Abstract

At what phenotypic level do closely related subspecies that live in different environments differ with respect to food detection, ingestion and processing? This question motivated an experimental study on rock sandpipers (Calidris ptilocnemis). The species' nonbreeding range spans 20 deg of latitude, the extremes of which are inhabited by two subspecies: C. p. ptilocnemis that winters primarily in upper Cook Inlet, Alaska (61°N) and C. p. tschuktschorum that overlaps slightly with C. p. ptilocnemis but whose range extends much farther south (∼40°N). In view of the strongly contrasting energetic demands of their distinct nonbreeding distributions, we conducted experiments to assess the behavioral, physiological and sensory aspects of foraging and we used the bivalve Macoma balthica for all trials. C. p. ptilocnemis consumed a wider range of prey sizes, had higher maximum rates of energy intake, processed shell waste at higher maximum rates and handled prey more quickly. Notably, however, the two subspecies did not differ in their abilities to find buried prey. The subspecies were similar in size and had equally sized gizzards, but the more northern ptilocnemis individuals were 10-14% heavier than their same-sex tschuktschorum counterparts. The higher body mass in ptilocnemis probably resulted from hypertrophy of digestive organs (e.g. intestine, liver) related to digestion and nutrient assimilation. Given the previously established equality of the metabolic capacities of the two subspecies, we propose that the high-latitude nonbreeding range of ptilocnemis rock sandpipers is primarily facilitated by digestive (i.e. physiological) aspects of their foraging ecology rather than behavioral or sensory aspects.

Energetic solutions of Rock Sandpipers to harsh winter conditions rely on prey quality

“…14% higher than laboratory‐derived estimates (1.22 mg shell/s; Ruthrauff et al . ). However, the latter value was measured in Rock Sandpipers maintained at 14 °C, and birds experienced their lowest metabolic demands under normothermic conditions (Scholander et al .…”

“…Ruthrauff et al . () demonstrated that Rock Sandpipers achieved maximum intake rates at densities ≥ 150 M. balthica /m 2 . Because M. balthica densities in upper Cook Inlet far exceed this threshold (Ruthrauff et al .…”

“…Ruthrauff et al . () suggested that the colonization of upper Cook Inlet by Rock Sandpipers is likely to have occurred within the last 14 000 years, an occurrence facilitated by recent climate warming that both formed Cook Inlet as a physical feature (Schmoll et al . , Reger et al .…”

“…Additionally, numerous physiological (Ruthrauff et al . , ), metabolic (Ruthrauff et al . ), behavioural (Ruthrauff & Eskelin , Ruthrauff et al .…”

“…), behavioural (Ruthrauff & Eskelin , Ruthrauff et al . ) and environmental (Ruthrauff et al . ) factors have been suggested to support the use of Cook Inlet by Rock Sandpipers during winter.…”