Foraging behaviour and habitat-use drives niche segregation in sibling seabird species

Trophic Niche Differentiation in Two Sympatric Nuthatch Species (Sitta yunnanensis and Sitta nagaensis)

Sympatric closely related species may experience interspecific trophic competition due to ecological similarity; they may isolate in terms of diet or habitat use as a strategy to avoid competition. The body tissues of consumers contain stable isotope signatures information that can be applied to infer their dietary information. In this study, δ13C and δ15N stable isotopes were analyzed to determine the dietary information and trophic niches of sympatric coexisting Sitta yunnanensis and Sitta nagaensis. The results showed that the food sources of S. yunnanensis and S. nagaensis were from six orders, including Orthoptera, and the cumulative contribution rate was 99.97%, with the two species eating similar diets but at different rates. The larger δ13C of S. yunnanensis indicates that it had a wider range of habitats for feeding, while the difference in δ15N values was not significant (p > 0.05), indicating that both species feed on similar nutrient levels. As determined by Bayesian ellipses, the isotopic niches of S. yunnanensis and S. nagaensis were differentiated; the isotopic niche width of S. yunnanensis is 2.69‱2, which was larger than that of S. nagaensis (0.73‱2), indicates that differentiation between the two species in diet or habitat use reduced competition. Trophic niche differentiation and differences in foraging proportions may be the principal resource allocation mechanisms behind S. yunnanensis and S. nagaensis coexistence.

Habitat partitioning among sympatric tinamous in semiarid woodlands of central Argentina

Sympatric, phylogenetically related and morphologically similar species that overlap in their distributions at a regional scale display different patterns of co-occurrence at local assemblages. Occurrence of each species at local scales might be the result of interspecific competition for limiting resources. However, these patterns could also arise from species-specific habitat preferences along the abiotic or land use gradients. To assess the role of these mechanisms, we investigated niche partitioning among sympatric tinamou species occurring in semiarid woodlands of central Argentina. We used occupancy models incorporating habitat characteristics and interspecific interactions, while accounting for detectability, to examine the spatial overlap among elegant crested tinamous (Eudromia elegans), brushland tinamous (Nothoprocta cinerascens), and nothura tinamous (Nothura darwinii and N. maculosa) across a wide regional scale. In addition, we investigated time partitioning among these species by estimating the degree of overlap in their daily activity patterns. The regional distribution of the three species was influenced by the gradient in plant productivity and vegetation structure, in agreement with their broad habitat requirements. We also found that the occurrence of each species was presumably affected by the presence of one or two predator species. Models including interactions among tinamou species found weak negative and positive interactions among species pairs, suggesting that co-occurrence patterns were mainly driven by species-specific habitat use rather than interspecific competition. The three species were diurnal, showing two main peaks of activity, and overlapped widely in their overall diel activity, although subordinate species tended to shift their activity patterns to reduce encounters with the dominant tinamou species, suggesting some segregation in this niche dimension. Projected changes in rainfall seasonality and warmer conditions in this region could benefit elegant crested tinamous over the other two species, although climate and land use changes will likely have a negative impact on all tinamou species.

Marked differences in foraging area use and susceptibility to predation between two closely-related tropical seabirds

Ecological theory predicts that closely-related species must occupy different niches to coexist. How marine top predators achieve this during breeding, when they often gather in large multi-species colonies and are constrained to central-place foraging, has been mostly studied in productive temperate and polar oceans with abundant resources, but less so in poorer, tropical waters. Here, we track the foraging movements of two closely-related sympatric seabirds-the white-tailed and red-tailed tropicbirds Phaethon lepturus and P. rubricauda-breeding on Aldabra Atoll, Seychelles, to investigate potential mechanisms of niche segregation and shed light on their contrasting population trends. Combining data from GPS, immersion, depth and accelerometry loggers, we show that the two species have similar behaviour at sea, but are completely segregated spatially, with red-tailed tropicbirds flying further to feed and using different feeding areas than white-tailed tropicbirds. Using nest-based camera traps, we show that low breeding success of both species-which likely drives observed population declines-is caused by high nest predation. However, the two species are targeted by different predators, with native avian predators mainly targeting red-tailed tropicbird nests, and invasive rats raiding white-tailed tropicbird nests when they leave their eggs unattended. Our findings provide new insight into the foraging ecology of tropicbirds and have important conservation implications. The extensive range and spatial segregation highlight the importance of considering large-scale protection of waters around tropical seabird colonies, while the high level of nest predation provides evidence in support of rat eradication and investigating potential nest protection from native avian predators.

A New Giant Petrel (Macronectes, Aves: Procellariidae) from the Pliocene of Taranaki, New Zealand †

A new species of giant petrel, Macronectes tinae sp. nov., is described from the Pliocene deposits of South Taranaki, New Zealand. The holotype is a near complete skull and the paratype a fragmentary left humerus; both come from the Tangahoe Formation, dating from the late Pliocene (Piacenzian or “Waipipian”; age estimated as ca. 3.36–3.06 Ma). The new species of giant petrel is the first fossil Macronectes ever reported. It is morphologically similar to the two present-day Macronectes spp., but it was a smaller bird. The skull is diagnosed by its overall smaller size, a proportionately longer apertura nasi ossea, and potentially by a shorter os supraocciptale. The humerus is diagnosed from both species by a proportionately less deep shaft, a more prominent medial portion of the epicondylus ventralis, and a larger and fusiform fossa medialis brachialis. The Tangahoe Formation is proving to be a remarkable source of marine vertebrate fossils and an important piece of the puzzle in understanding the evolution and biogeography of seabirds.

The role of allochrony in influencing interspecific differences in foraging distribution during the non-breeding season between two congeneric crested penguin species

Mechanisms promoting coexistence between closely related species are fundamental for maintaining species diversity. Mechanisms of niche differentiation include allochrony which offsets the peak timing of resource utilisation between species. Many studies focus on spatial and temporal niche partitioning during the breeding season, few have investigated the role allochrony plays in influencing interspecific segregation of foraging distribution and ecology between congeneric species during the non-breeding season. We investigated the non-breeding migrations of Snares (Eudyptes robustus) and Fiordland penguins (Eudyptes pachyrhynchus), closely related species breeding between 100-350 km apart whose migration phenology differs by two months. Using light geolocation tracking, we examined the degree of overlap given the observed allochrony and a hypothetical scenario where the species commence migration simultaneously. We found that Fiordland penguins migrated to the Sub-Antarctic Frontal Zone and Polar Frontal Zone in the austral autumn whereas Snares penguins disperse westwards staying north of the Sub-Tropical Front in the austral winter. Our results suggest that allochrony is likely to be at the root of segregation because the relative profitability of the different water masses that the penguins forage in changes seasonally which results in the two species utilising different areas over their core non-breeding periods. Furthermore, allochrony reduces relatively higher levels of spatiotemporal overlap during the departure and arrival periods, when the close proximity of the two species' colonies would cause the birds to congregate in similar areas, resulting in high interspecific competition just before the breeding season. Available evidence from other studies suggests that the shift in phenology between these species has arisen from adaptive radiation and phenological matching to the seasonality of local resource availability during the breeding season and reduced competitive overlap over the non-breeding season is likely to be an incidental outcome.

Spatial segregation in a sexually dimorphic central place forager: Competitive exclusion or niche divergence?

Sexual competition is increasingly recognized as an important selective pressure driving species distributions. However, few studies have investigated the relative importance of interpopulation versus intrapopulation competition in relation to habitat availability and selection. To explain spatial segregation between sexes that often occurs in non-territorial and central place foragers, such as seabirds, two hypotheses are commonly used. The 'competitive exclusion' hypothesis states that dominant individuals should exclude subordinate individuals through direct competition, whereas the 'niche divergence' hypothesis states that segregation occurs due to past competition and habitat specialization. We tested these hypotheses in two populations of an extreme wide-ranging and sexually dimorphic seabird, investigating the relative role of intrapopulation and interpopulation competition in influencing sex-specific distribution and habitat preferences. Using GPS loggers, we tracked 192 wandering albatrosses Diomedea exulans during four consecutive years (2016-2019), from two neighbouring populations in the Southern Ocean (Prince Edward and Crozet archipelagos). We simulated pseudo-tracks to create a null spatial distribution and used Kernel Density Estimates (KDE) and Resource Selection Functions (RSF) to distinguish the relative importance of within- versus between-population competition. Kernel Density Estimates showed that only intrapopulation sexual segregation was significant for each monitoring year, and that tracks between the two colonies resulted in greater overlap than expected from the null distribution, especially for the females. RSF confirmed these results and highlighted key at-sea foraging areas, even if the estimated of at-sea densities were extremely low. These differences in selected areas between sites and sexes were, however, associated with high interannual variability in habitat preferences, with no clear specific preferences per site and sex. Our results suggest that even with low at-sea population densities, historic intrapopulation competition in wide-ranging seabirds may have led to sexual dimorphism and niche specialization, favouring the 'niche divergence' hypothesis. In this study, we provide a protocol to study competition within as well as between populations of central place foragers. This is relevant for understanding their distribution patterns and population regulation, which could potentially improve management of threatened populations.