Why do eastern curlews Numenius madagascariensis feed on prey that lowers intake rate before migration?

Eastern curlews Numenius madagascariensis spending the nonbreeding season in eastern Australia foraged on three intertidal decapods: soldier crab Mictyris longicarpus, sentinel crab Macrophthalmus crassipes and ghost-shrimp Trypaea australiensis. Due to their ecology, these crustaceans were spatially segregated (=distributed in 'patches') and the curlews intermittently consumed more than one prey type. It was predicted that if the curlews behaved as intake rate maximizers, the time spent foraging on a particular prey (patch) would reflect relative availabilities of the prey types and thus prey-specific intake rates would be equal. During the mid-nonbreeding period (November-December), Mictyris and Macrophthalmus were primarily consumed and prey-specific intake rates were statistically indistinguishable (8.8 versus 10.1 kJ x min(-1)). Prior to migration (February), Mictyris and Trypaea were hunted and the respective intake rates were significantly different (8.9 versus 2.3 kJ x min(-1)). Time allocation to Trypaea-hunting was independent of the availability of Mictyris. Thus, consumption of Trypaea depressed the overall intake rate. Six hypotheses for consuming Trypaea before migration were examined. Five hypotheses: the possible error by the predator, prey specialization, observer overestimation of time spent hunting Trypaea, supplementary prey and the choice of higher quality prey due to a digestive bottleneck, were deemed unsatisfactory. The explanation for consumption of a low intake-rate but high quality prey (Trypaea) deemed plausible was diet optimisation by the Curlews in response to the pre-migratory modulation (decrease in size/processing capacity) of their digestive system. With a seasonal decrease in the average intake rate, the estimated intake per low tide increased from 1233 to 1508 kJ between the mid-nonbreeding and pre-migratory periods by increasing the overall time spent on the sandflats and the proportion of time spent foraging.