The Anatomy of the bill Tip of Kiwi and Associated Somatosensory Regions of the Brain: Comparisons with Shorebirds

Three families of probe-foraging birds, Scolopacidae (sandpipers and snipes), Apterygidae (kiwi), and Threskiornithidae (ibises, including spoonbills) have independently evolved long, narrow bills containing clusters of vibration-sensitive mechanoreceptors (Herbst corpuscles) within pits in the bill-tip. These ‘bill-tip organs’ allow birds to detect buried or submerged prey via substrate-borne vibrations and/or interstitial pressure gradients. Shorebirds, kiwi and ibises are only distantly related, with the phylogenetic divide between kiwi and the other two taxa being particularly deep. We compared the bill-tip structure and associated somatosensory regions in the brains of kiwi and shorebirds to understand the degree of convergence of these systems between the two taxa. For comparison, we also included data from other taxa including waterfowl (Anatidae) and parrots (Psittaculidae and Cacatuidae), non-apterygid ratites, and other probe-foraging and non probe-foraging birds including non-scolopacid shorebirds (Charadriidae, Haematopodidae, Recurvirostridae and Sternidae). We show that the bill-tip organ structure was broadly similar between the Apterygidae and Scolopacidae, however some inter-specific variation was found in the number, shape and orientation of sensory pits between the two groups. Kiwi, scolopacid shorebirds, waterfowl and parrots all shared hypertrophy or near-hypertrophy of the principal sensory trigeminal nucleus. Hypertrophy of the nucleus basorostralis, however, occurred only in waterfowl, kiwi, three of the scolopacid species examined and a species of oystercatcher (Charadriiformes: Haematopodidae). Hypertrophy of the principal sensory trigeminal nucleus in kiwi, Scolopacidae, and other tactile specialists appears to have co-evolved alongside bill-tip specializations, whereas hypertrophy of nucleus basorostralis may be influenced to a greater extent by other sensory inputs. We suggest that similarities between kiwi and scolopacid bill-tip organs and associated somatosensory brain regions are likely a result of similar ecological selective pressures, with inter-specific variations reflecting finer-scale niche differentiation

Using the power of comparison to explain habitat use and migration strategies of shorebirds worldwide

Shorebirds, or waders, form an ecologically (but not phylogenetically) homogenous group of birds that, despite this homogeneity, exhibits clear correlated contrasts in habitat use and migration distance between closely related species pairs. In addition, within species there is distinct variation in breeding and wintering latitudes, i.e. migration distance. I examine here such contrasts at different taxonomic levels and evaluate what we can learn about selective forces on habitat selection and the evolution of migration strategies in birds. My primary example is the worldwide migration system of the Red Knot Calidris canutus. These sandpipers breed only on high arctic tundra (65–83°N), but they move south from their disjunct, circumpolar breeding areas to nonbreeding sites on the coasts of all continents (except Antarctica), between latitudes 58°N and 53°S. Due to their specialized sensory capabilities, Red Knots generally eat hard-shelled prey found on intertidal, mostly soft, substrates. As a consequence, ecologically suitable coastal sites are few and far between, so they must routinely undertake flights of many thousands of kilometres. In contrast to prediction, Red Knots at tropical intertidal sites have lower fuelling rates than birds at more southern or northern latitudes. This leads to greater time–stress in the southernmost wintering populations that not only have to cover over 14,000 km in single migrations, but also cannot rely on tropical regions to make refuelling stops. Rapid human-caused losses of the food-base in staging areas during both north- and southward migrations have been demonstrated to have caused rapid declines in several Red Knot populations. Detailed worldwide eco-demographic research on these extreme long-distance migrants, as embodied in, for example, the recently established Global Flyway Network, yields a two-pronged benefit: (1) on the basis of the unintended large-scale experiments carried out by humans, we rapidly come to grips with the selection pressures moulding the migration strategies of migrant birds, and (2) in applied contexts, the work gives instantaneous feedbacks on the conservation consequences of man-made alterations to wetland environments at the relevant global spatial scales.