Functional Variation in the Feeding Apparatus of Charadriiform Shorebirds With Emphasis on Bill-Tip Organs and Micro...

Shorebirds within Charadriiformes show substantial variation in where, what, and how they eat. Foraging is aided by a complex feeding apparatus that may include a bill-tip organ with sensory pits and micro tongue tip spines. The morphology of these structures varies among species, but their evolutionary patterns have not been previously mapped. Here we present a comparative morphological analysis of 20 shorebird species across five families, using morphometric data from digital photographs and scanning electron microscopy to reconstruct the evolutionary history of bill-tip organ morphology and micro tongue tip spines. While sensory pits occurred in all species, well-developed bill-tip organs for remote touch were characteristic of Scolopacidae, with pit number and bill length varying among species. Sensory pit numbers were best explained by a Brownian motion model of trait evolution, suggesting increases and decreases of pit numbers across time and high pit numbers in the most recent common ancestor of this clade. Species possessing tongue spines were generally smaller and exhibited a shorter distance between the tongue tip and mandible tip (e.g., the black-fronted dotterel, Thinornis melanops), supporting a functional role in biofilm grazing. The presence of micro tongue tip spines was best explained by an equal-rates model of trait evolution, with the trait evolving twice with no evidence of subsequent loss: once in the most recent common ancestor shared by calidrid sandpipers (Calidris spp.), shanks (Tringa spp.), the terek sandpiper (Xenus cinereus), and the ruddy turnstone (Arenaria interpres), and again in a plover lineage. The phylogenetic distribution of sensory pits and tongue spines showed functional adaptations to diverse foraging strategies. These findings provide new insights into the evolutionary diversification of the shorebird feeding apparatus.