Year-round at-sea distribution and trophic resources partitioning between two sympatric Sulids in the tropical Atlantic

Abstract

In the oligotrophic tropical marine environment resources are usually more patchily distributed and less abundant to top predators. Thus, spatial and trophic competition can emerge, especially between related seabird species belonging to the same ecological guild. Here we studied the foraging ecology of two sympatric species–brown booby (BRBO) Sula leucogaster (breeding) and red-footed boobies (RFBO) Sula sula (non-breeding)–at Raso islet (Cabo Verde), across different seasons. Sexual segregation was only observed during Jun-Oct, when RFBO were present, with larger females BRBO remaining closer to the colonies, while males and RFBO travelled further and exploited different habitats. Overall, species appeared to prefer areas with specific oceanic features, particularly those related with oceanic currents and responsible for enhancing primary productivity in tropical oceanic areas (e.g. Sea Surface Height and Ocean Mixed Layer Thickness). Female BRBOs showed high foraging-site fidelity during the period of sympatry, while exploiting the same prey species as the other birds. However, during the months of co-existence (Jun.-Oct.), isotopic mixing models suggested that female BRBO would consume a higher proportion of epipelagic fish, whereas female RFBO would consume more squid compared to the other birds, possibly due to habitat-specific prey availability and breeding energy-constraints for BRBO. We conclude that divergent parental roles, environmental conditions, habitat preference and competition could be mechanisms simultaneously underlying sexual segregation for BRBO during a period of co-existence, while inter-specific foraging differences appear to be more affected by habitat preference and different breeding stages. These results support previous statements that BRBO can adapt their foraging ecology to different circumstances of environmental conditions and competition, and that marine physical features play an important role in foraging decisions of boobies.

Sexual segregation in the foraging distribution, behaviour, and trophic niche of the endemic Boyd’s shearwater (Puffinus lherminieri boydi)

“…Our results are in line with previous studies on other monomorphic seabirds, such as the wedge-tailed shearwater Ardenna paci ca (Peck and Congdon 2006), the little auk Alle alle (Welcker et al 2009), and the Manx shearwater Pu nus pu nus (Gray and Hamer 2001), where females took longer foraging trips, subsequently driving a male-biased provisioning with males delivering food to the chick at a higher rate, and showing a greater contribution to overall chick feeding. Peck and Congdon (2006) argued that the sex-speci c foraging of wedge-tailed shearwaters were identical to the results obtained from SSD species, supporting the occurrence of inter-sexual competition at the FA, with the larger sex outcompeting the smaller one from the closest or more pro table regions (Paiva et al 2017;Almeida et al 2021). Despite the spatial segregation (i.e., low spatial overlap) and sex-speci c differences in foraging, we do not have enough data to support the occurrence of inter-sexual competition at the foraging grounds.…”

“…Firstly, the sexual size dimorphism (SSD), where one sex is anatomically larger than the other (González-Solís et al 2000; Phillips et al 2004; Weimerskirch et al 2006), in uencing ight performance, foraging range, or diving depth (Gilardi 1992; Lewis et al 2005; Weimerskirch et al 2009; Paiva et al 2017Paiva et al , 2018. Here, the smaller sex, or the less e cient forager could then be outcompeted by the larger sex, or by the more e cient forager, and forced to forage in less pro table waters or in remoter areas, as a way to avoid competition within foraging grounds, i.e., 'inter-sexual competition' hypothesis (González-Solís et al 2000; Paiva et al 2018;Pereira et al 2018;Almeida et al 2021). Secondly, divergent parental roles, should shape nest attendance and/or provisioning rates.…”

“…To the best of our knowledge only two studies tracked the movements and trophic ecology of Boyd's shearwater using light-sensing geolocators (Zajková et al 2017;Ramos et al 2020). Additionally, the trophic ecology of Boyd's shearwater during the breeding season is also less known when compared to its closest-related counterpart, the Macaronesian shearwater Pu nus baroli (Neves et al 2012;Ramos et al 2015), and other larger breeding seabirds of Cabo Verde (Cerveira et al 2020;Almeida et al 2021). Here, we tracked the foraging movements of Boyd's shearwater using high-precision mini-GPS loggers and studied the trophic ecology during the breeding season at Raso islet, Cabo Verde.…”

“…Phillips et al 2011), boobies(Cherel et al 2008;Almeida et al 2021), giant petrels(Phillips et al 2011), penguins(Forero et al 2002;Bearhop et al 2006), shags(Bearhop et al 2006), and shearwaters(Navarro et al 2009;Ramos et al 2009) during the breeding season. Yet, there are some studies that reported sex-related differences in the isotopic niche of breeding monomorphic species, such as the common tern(Nisbet et al 2002), thin-billed prion(Quillfeldt et al 2008), and the northern(Cleasby et al 2015) and Australasian gannets(Ismar et al 2017).…”

“…Nevertheless, the gannets were the only species that also exhibited different foraging patterns, with little spatial overlap of FA and exploiting areas with distinct oceanographic conditions(Cleasby et al 2015;Ismar et al 2017). Resource partitioning is expected to be greater when resources are scarcer or when larger individuals outcompete the smaller co-speci cs from using the same resources(Young et al 2010;Phillips et al 2011;Almeida et al 2021…”