Factors influencing prey capture success and profitability in Australasian gannets (Morus serrator)

A social foraging trade-off in echolocating bats reveals that they benefit from some conspecifics but are impaired when many are around

Social foraging is very common in the animal kingdom. Numerous studies have documented collective foraging in various species and many reported the attraction of various species to foraging conspecifics. It is nonetheless difficult to quantify the benefits and costs of collective foraging, especially in the wild. We examined the benefits and costs of social foraging using on-board microphones mounted on freely foraging Molossus nigricans bats. This allowed us to quantify the bats' attacks on prey and to assess their success as a function of conspecific density. We found that the bats spent most of their time foraging at low conspecific densities, during which their attacks were most successful in terms of prey items captured per time unit. Notably, their capture rate dropped when conspecific density became either too high or too low. Our findings thus demonstrate a clear social foraging trade-off in which the presence of a few conspecifics probably improves foraging success, whereas the presence of too many impairs it.

A bioenergetics approach to understanding sex differences in the foraging behaviour of a sexually monomorphic species

Many animals show sexually divergent foraging behaviours reflecting different physiological constraints or energetic needs. We used a bioenergetics approach to examine sex differences in foraging behaviour of the sexually monomorphic northern gannet. We derived a relationship between dynamic body acceleration and energy expenditure to quantify the energetic cost of prey capture attempts (plunge dives). Fourteen gannets were tracked using GPS, time depth recorders (TDR) and accelerometers. All plunge dives in a foraging trip represented less than 4% of total energy expenditure, with no significant sex differences in expenditure. Despite females undertaking significantly more dives than males, this low energetic cost resulted in no sex differences in overall energy expenditure across a foraging trip. Bayesian stable isotope mixing models based on blood samples highlighted sex differences in diet; however, calorific intake from successful prey capture was estimated to be similar between sexes. Females experienced 10.28% higher energy demands, primarily due to unequal chick provisioning. Estimates show a minimum of 19% of dives have to be successful for females to meet their daily energy requirements, and 26% for males. Our analyses suggest northern gannets show sex differences in foraging behaviour primarily related to dive rate and success rather than the energetic cost of foraging or energetic content of prey.

Breeding stage, not sex, affects foraging characteristics in masked boobies at Rapa Nui

Abstract

Sexual segregation in foraging occurs in some species and populations of boobies (Sulidae), but it is not a general pattern. Sexual segregation in foraging may occur to avoid competition for food, and this competition may intensify during specific stages of breeding. We examined sexual segregation in foraging in relation to breeding stage in masked boobies Sula dactylatra at Rapa Nui by tracking simultaneously incubating and chick-rearing birds using GPS recorders (n = 18) and collected a total of 11 regurgitate samples. Stable isotope analyses (δ13C and δ15N) of whole blood samples were carried out in 20 birds. There were no differences in foraging trip parameters or diet between females and males. Both sexes traveled farther and for longer while incubating than while rearing chicks. Isotopic niches (δ13C and δ15N) overlapped to some degree among all groups at all times, but the lowest overlap between sexes occurred during incubation. While preying on ephemerally distributed flying fish, vertical or horizontal competition avoidance may be almost impossible, and thus females and males share their foraging grounds. Since birds were tracked simultaneously, shorter foraging trips of chick-rearing birds must be an effect of the constraints of provisioning the chick. Differences observed in δ15N and δ13C values between sexes may be caused by subtle differences in their foraging behaviors, or by differences in physiology linked to breeding. Our findings suggest that local oceanography and its inherent food distribution are determinants for sexual segregation in foraging patterns in masked boobies and possibly also other booby species.

Significance statement

In some animals, females and males forage on different areas or prey on different species to avoid competition for food resources. In boobies (Sula sp.), some studies show evidence of sexual segregation in foraging and others do not. Here, we tested if sexual segregation in foraging occurred in masked boobies on the Pacific island of Rapa Nui by studying simultaneously incubating and chick-rearing birds. We found no evidence of sexual segregation on foraging behavior or diet. We discuss that the difference between this and other studies in boobies may be an effect of the local prey availability. When the prey community is more diverse and heterogeneously distributed, each sex may access different resources and thus sexual foraging segregation will occur. In contrast, in areas like Rapa Nui where prey resources are distributed ephemerally, sexual segregation in foraging will not be useful and is thus less likely to occur.

Environmental influences on foraging effort, success and efficiency in female Australian fur seals

Understanding the factors which influence foraging behaviour and success in marine mammals is crucial to predicting how their populations may respond to environmental change. The Australian fur seal (Arctocephalus pusillus doriferus, AUFS) is a predominantly benthic forager on the shallow continental shelf of Bass Strait, and represents the greatest biomass of marine predators in south-eastern Australia. The south-east Australian region is experiencing rapid oceanic warming, predicted to lead to substantial alterations in prey diversity, distribution and abundance. In the present study, foraging effort and indices of foraging success and efficiency were investigated in 138 adult female AUFS (970 foraging trips) during the winters of 1998–2019. Large scale climate conditions had a strong influence on foraging effort, foraging success and efficiency. Foraging effort and foraging success were also strongly influenced by winter chlorophyll-a concentrations and sea-surface height anomalies in Bass Strait. The results suggest increasing foraging effort and decreasing foraging success and efficiency under anticipated environmental conditions, which may have population-level impacts.

Consistent sociality but flexible social associations across temporal and spatial foraging contexts in a colonial breeder

When the consequences of sociality differ depending on the state of individual animals and the experienced environment, individuals may benefit from altering their social behaviours in a context-dependent manner. Thus, to fully address the hypotheses about the role of social associations it is imperative to consider the multidimensional nature of sociality by explicitly examining social associations across multiple scales and contexts. We simultaneously recorded > 8000 associations from 85% of breeding individuals from a colony of Australasian gannets (Morus serrator) over a 2-week period, and examined gregariousness across four foraging states using multilayer social network analysis. We found that social associations varied in a context-dependent manner, highlighting that social associations are most prevalent during foraging (local enhancement) and in regions expected to provide clustered resources. We also provide evidence of individual consistency in gregariousness, but flexibility in social associates, demonstrating that individuals can adjust their social behaviours to match experienced conditions.