Differences in foraging ecology determine variation in visual fields in ibises and spoonbills (Threskiornithidae)

Abstract

Variations in visual field topography among birds have been interpreted as adaptations to the specific perceptual challenges posed by the species’ foraging ecology. To test this hypothesis we determined visual field topography in four bird species which have different foraging ecologies but are from the same family: Puna Ibis Plegadis ridgwayi (probes for prey in the soft substrates of marsh habitats), Northern Bald Ibis Geronticus eremita (surface pecks for prey in dry terrestrial habitats), African Spoonbill Platalea alba and Eurasian Spoonbill Platalea leucorodia (bill‐sweeps for prey in shallow turbid waters). All four species employ tactile cues provided by bill‐tip organs for prey detection. We predicted that the visual fields of these species would show general features similar to those found in other birds whose foraging is guided by tactile cues from the bill (i.e. bill falling outside the frontal binocular field and comprehensive visual coverage of the celestial hemisphere). However, the visual fields of all four species showed general features characteristic of birds that take food directly in the bill under visual guidance (i.e. a narrow and vertically long binocular field in which the projection of the bill tip is approximately central and with a blind area above and behind the head). Visual fields of the two spoonbills were very similar but differed from those of the ibises, which also differed between themselves. In the spoonbills, there was a blind area below the bill produced by the enlarged spatulate bill tip. We discuss how these differences in visual fields are related to the perceptual challenges of these birds’ different foraging ecologies, including the detection, identification and ingestion of prey. In particular we suggest that all species need to see binocularly around the bill and between the opened mandibles for the identification of caught prey items and its transport to the back of the mouth. Our findings support the hypothesis that sensory challenges associated with differences in foraging ecology, rather than shared ancestry or the control of locomotion, are the main determinants of variation in visual field topography in birds.

Reducing bycatch in gillnets: A sensory ecology perspective

“…Each species probably has a unique ''world view'' with respect to the overall sector of the world from which it can extract information at any one instant, while within that field of view there will be differences in the distribution of spatial resolution and of colour vision. It has been shown that these differences in world view may be correlated with significant differences in the foraging ecology and foraging behaviour of birds of the same genus (Guillemain et al, 2002), family (Martin and Portugal, 2011) or order (Martin and Prince, 2001). Thus generalisations across species groups, such as those listed in Tables 1 and 2 should be made with caution.…”

“…Surprisingly, visual fields do not appear to be directly associated with the perceptual challenges associated with the control of locomotion; this seems to be achieved within the constraints imposed by other factors (Martin, 2012(Martin, , 2014. Differences in visual fields can be finely tuned and are known to occur between species within the same genus which differ in their foraging techniques (Guillemain et al, 2002), as well as within families (Martin and Portugal, 2011).…”

The subtlety of simple eyes: the tuning of visual fields to perceptual challenges in birds

“…Thus, among the ibises (Threskiornithidae), significant differences in visual fields are found between species whose foraging ecology involves surface pecking in dry terrestrial habitats (northern bald ibis Geronticus eremita) and species which probe in the soft substrates of marsh habitats (puna ibis Plegadis ridgwayi) [15]. However, among the herons (family Ardeidae) within the Ciconiiformes, there is a marked similarity in visual field configurations even among species from different genera.…”

“…Therefore, determining a causal relationship between eye structure and behavioural function may be problematic. Nevertheless, subtle behavioural differences may provide a crucial underpinning to the partitioning of resources between species within the same or similar environments, and are thus likely to be reflected in aspects of sensory capacity and information processing [14,15].…”

Visual abilities in two raptors with different ecology

“…While some species search for food when flying at high altitude, others search from a perch or by walking on the ground (Del Hoyo and Elliot, 1994), and we suggest that raptor species with different foraging ecology might differ in their visual fields, eye and head movements (O'Rourke et al, 2010a,b) and perhaps in their visual acuity. Significant differences in the vision of closely related birds that differ in their foraging behaviour have been described in other species (Guillemain et al, 2002;Martin and Piersma, 2009;Martin and Portugal, 2011).…”