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.
Highlights
Competition for food resources occurs naturally between organisms living in communities [1], and between close-related species sharing the same geographic area for breeding and/or foraging [2,3]
This type of segregation can usually be explained by three factors: (1) Anatomic differences between males and females [11,12,13,14], most frequently related to size of seabird species [12,14], influencing flight speed, foraging range and flapping frequency, as well as diving depth and duration induced by body mass [15]; (2) Divergent parental roles, influencing nest fidelity; and (3) nutritional requirements [13]
Generalized Additive Mixed Models (GAMMs) showed a good predictive capacity, explaining 32.2%, 42.1% and 35.1% of the deviance in the probability of birds to switch between foraging and travelling behavioural modes (Table 2). Foraging probability of both female and male brown booby (BRBO) increased with decreasing ocean mixed layer thickness (OMLT) and increasing gradient in SST (GSST) and gradient in OMLT (GOMLT) (Fig 3)
Summary
Competition for food resources occurs naturally between organisms living in communities [1], and between close-related species sharing the same geographic area (sympatry) for breeding and/or foraging [2,3]. Intra-specific competition can exist in large seabird colonies, especially during breeding seasons when adult seabirds are constrained by central place foraging [4,10], with breeders of some species segregating sexually This type of segregation can usually be explained by three factors: (1) Anatomic differences between males and females (sexual dimorphism) [11,12,13,14], most frequently related to size of seabird species [12,14], influencing flight speed, foraging range and flapping frequency, as well as diving depth and duration induced by body mass [15]; (2) Divergent parental roles, influencing nest fidelity; and (3) nutritional requirements [13]. Many RSD sulid species show sexual segregation, this is not always the case [14,19,20,21], perhaps due to harsher environmental conditions at their colony surroundings, resulting in fish stock depletion, or when broods are larger and demand a higher foraging effort [19]
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