Cooperative breeding and density regulation in small island populations of the Seychelles warbler

The faunas of tropical islands are particularly rich in endemic species, and constitute a disproportionately large proportion of the global biodiversity (Collar & Stuart 1985; Stattersfield & Capper 2000). In the last few centuries island faunas have become among the most threatened in the world, mainly because of anthropogenic effects such as human disturbance and the introduction of predators or competitor species (Stattersfield & Capper 2000). In contrast to species from temperate regions, we have little knowledge of the processes that regulate populations of tropical bird species, despite that fact that this knowledge is crucial to their conservation. In this thesis I studied population regulation in small isolated populations of the cooperatively breeding Seychelles warbler (Acrocephalus sechellensis). As this is a vulnerable species, direct manipulation of density through permanent removal of birds was not possible. However, translocations carried out as a part of a conservation project gave me the possibility to study density dependence of reproduction and survival in newly established populations. In addition I investigated the proximate mechanisms involved in helping behaviour and the long-term fitness consequences of group living.

No evidence of immediate fitness benefits of within-season divorce in monogamous birds

Individuals of socially monogamous species can correct for suboptimal partnerships via two secondary mating strategies: divorce and extra-pair mating, with the former potentially providing both genetic and social benefits. Divorcing between breeding seasons has been shown to be generally adaptive behaviour across monogamous birds. Interestingly, some pairs also divorce during the breeding season, when constraints on finding a new partner are stronger. Despite being important for a comprehensive understanding of the evolution of social monogamy, whether within-season divorce is adaptive and how it relates to extra-pair mating remains unknown. Here, we meta-analysed 90 effect sizes on within-season divorce and breeding success, extracted from 31 studies on 24 species. We found no evidence that within-season divorce is adaptive for breeding success. However, the large heterogeneity of effect sizes and strong phylogenetic signal suggest social and environmental factors—which have rarely been considered in empirical studies—may play an important role in explaining variation among populations and species. Furthermore, we found no evidence that within-season divorce and extra-pair mating are complementary strategies. We discuss our findings within the current evidence of the adaptiveness of secondary mating strategies and their interplay that ultimately shapes the evolution of social monogamy

Experimental evidence for helper effects in a cooperatively breeding cichlid

Neolamprologus pulcher is a cooperatively breeding cichlid fish, in which helpers stay in their natal territory and help with brood care, territory defense, and maintenance. In this study we investigated helper effects by an experimental group size reduction in the field. After this manipulation, focal helpers in reduced groups tended to feed less, and small helpers visited the breeding shelter significantly more often than same-sized helpers in control groups. No evidence was found that remaining helpers compensated for the removed helpers by increasing territory defense and maintenance behavior. Breeders, however, did show a lower defense rate, possibly caused by an increase in brood care effort. Survival of fry was significantly lower in removal than control groups, which provides the first experimental proof in a natural population of fish that brood care helpers do effectively help. The data suggest that in small, generally younger, helpers, kin selection may be an important evolutionary cause of cooperation. Large helpers, however, who are generally older and less related to the breeders than small helpers are suggested to pay to be allowed to stay in the territory by helping. All group members benefit from group augmentatio

Cross-lags and the unbiased estimation of life-history and demographic parameters

Biological processes exhibit complex temporal dependencies due to the sequential nature of allocation decisions in organisms' life cycles, feedback loops and two-way causality. Consequently, longitudinal data often contain cross-lags: the predictor variable depends on the response variable of the previous time step. Although statisticians have warned that regression models that ignore such covariate endogeneity in time series are likely to be inappropriate, this has received relatively little attention in biology. Furthermore, the resulting degree of estimation bias remains largely unexplored. We use a graphical model and numerical simulations to understand why and how regression models that ignore cross-lags can be biased, and how this bias depends on the length and number of time series. Ecological and evolutionary examples are provided to illustrate that cross-lags may be more common than is typically appreciated and that they occur in functionally different ways. We show that routinely used regression models that ignore cross-lags are asymptotically unbiased. However, this offers little relief, as for most realistically feasible lengths of time-series conventional methods are biased. Furthermore, collecting time series on multiple subjects—such as populations, groups or individuals—does not help to overcome this bias when the analysis focusses on within-subject patterns (often the pattern of interest). Simulations, a literature search and a real-world empirical example together suggest that approaches that ignore cross-lags are likely biased in the direction opposite to the sign of the cross-lag (e.g. towards detecting density dependence of vital rates and against detecting life-history trade-offs and benefits of group living). Next, we show that multivariate (e.g. structural equation) models can dynamically account for cross-lags, and simultaneously address additional bias induced by measurement error, but only if the analysis considers multiple time series. We provide guidance on how to identify a cross-lag and subsequently specify it in a multivariate model, which can be far from trivial. Our tutorials with data and R code of the worked examples provide step-by-step instructions on how to perform such analyses. Our study offers insights into situations in which cross-lags can bias analysis of ecological and evolutionary time series and suggests that adopting dynamical models can be important, as this directly affects our understanding of population regulation, the evolution of life histories and cooperation, and possibly many other topics. Determining how strong estimation bias due to ignoring covariate endogeneity has been in the ecological literature requires further study, also because it may interact with other sources of bias

Habitat geometry does not affect levels of extrapair paternity in an extremely unfaithful fairy-wren

Density of potential mates has often been proposed to explain the enormous variation in extrapair paternity. However, density is often confounded by other ecological factors that might affect extrapair paternity in their own way. Furthermore, extrapair mating shows strong phylogenetic inertia, making both meaningful intra- and interspecific comparisons difficult. An extreme way to change density is through habitat fragmentation that reduces connectivity between territories. Recently, habitat connectivity was hypothesized to explain the surprising discovery of a virtually monogamous species among the world's most unfaithful bird genus. The monogamous Malurus coronatus lives in narrow riparian strips that limit contact with neighbors to both extreme ends of territories, whereas Malurus species with high levels of extragroup paternity typically live in high-connected habitat in which they are surrounded by neighbors. Here, we test the habitat geometry hypothesis by comparing levels of extragroup paternity of Malurus elegans living in fragmented low-connected habitat and in high-connected habitat. We found that M. elegans does not have lower levels of extragroup paternity in low-connected habitat (68%) than in high-connected habitat (56% of offspring), indicating that connectivity does not limit opportunities for extragroup paternity. Furthermore, there was no evidence that females in low-connected habitat gained extragroup paternity further away or from less sires or that they were more likely to be closely related to their social mate. We conclude that behavioral plasticity in response to density-dependent cost and benefits of mating behavior does not explain intrageneric variation in extragroup paternity in Malurus

Rescue behaviour in a social bird: removal of sticky 'bird-catcher tree' seeds by group members

Rescue behaviour is a special form of cooperation in which a rescuer exhibits behaviours directed towards averting a threat to an endangered individual, thereby potentially putting itself at risk. Although rescue behaviour has been well-documented in experimental studies on rats and ants, published cases in other non-human animals are rare. Here, we report observations of rescue behaviour in the cooperatively breeding Seychelles warbler (Acrocephalus sechellensis). In this species, individuals sometimes become entangled in seed clusters of 'bird-catcher trees' (Pisonia grandis). Just one or a few of these sticky seeds can prevent Seychelles warblers to fly and may lead to mortality. In four cases, individuals were observed displaying behaviour aimed at removing sticky seeds from the feathers of an entangled individual belonging to their group. Intriguingly, the rescuing individuals engaged in this behaviour despite potentially risking entanglement. To our knowledge, this is the first recorded case of rescue behaviour in birds.MH is supported by a NWO VENI fellowship (863.15.020), LB is supported by an Australian Research Council DECRA fellowship (DE130100174

R package HIPHOP: parentage assignment using bi-allelic genetic markers

R package hiphop is a method for parentage assignment using bi-allelic genetic markers like SNPs (Single Nucleotide Polymorphism). It has widespread application (paternity and maternity assignment in a variety of mating systems) and outperforms conventional methods where closely related individuals occur in the pool of possible parents. The method compares the genotypes of offspring with any combination of potentials parents and scores the number of mismatches of these individuals at bi-allelic genetic markers (e.g. Single Nucleotide Polymorphisms). It elaborates on a prior exclusion method based on the Homozygous Opposite Test (HOT; Huisman 2017 ) by introducing the additional exclusion criterion HIPHOP (Homozygous Identical Parents, Heterozygous Offspring are Precluded; Cockburn et al., 2021). Potential parents are excluded if they have more mismatches than can be expected due to genotyping error and mutation, and thereby one can identify the true genetic parents and detect situations where one (or both) of the true parents is not sampled. Package 'hiphop' can deal with (a) the case where there is contextual information about parentage of the mother (i.e. a female has been seen to be involved in reproductive tasks such as nest building), but paternity is unknown (e.g. due to promiscuity), (b) where both parents need to be assigned, because there is no contextual information on which female laid eggs and which male fertilized them (e.g. polygynandrous mating system where multiple females and males deposit young in a common nest, or organisms with external fertilisation that breed in aggregations). For details: Cockburn, Andrew, Peñalba, Joshua V., Jaccoud, Damian, Kilian, Andrzej, Brouwer, Lyanne, Double, Michael C., Margraf, Nicolas, Osmond, Helen L., Kruuk, Loeske E.B., and van de Pol, Martijn (2021) Hiphop: improved paternity assignment among close relatives using a simple exclusion method for biallelic markers. Molecular Ecology Resources, 21 (6). pp. 1850-1865. https://doi.org/10.1111/1755-0998.1338

Social context-dependent provisioning rules in red-winged fairy-wrens do not vary with signals of increased chick need

Individuals should adjust investment in parental care in order to maximize current and future reproductive success. In cooperative breeders, where helpers assist with raising offspring, larger groups may allow for a reduction in investment (load lightening) of each individual. Additionally, the type of individual, and thus the social context, can play an important role in individual investment. Less attention has been paid to how provisioning rules vary across ecological contexts, although theory suggests that individuals can only afford to reduce their investment when nestling starvation is unlikely, thus under mild conditions. Here, we tested whether previously reported provisioning rules based on social context vary with ecological conditions, by experimental manipulation of signals of chick need, in the cooperatively breeding red-winged fairy-wren, Malurus elegans. Previous work in this species has shown that all group members load-lighten with additional male helpers, but not in response to the number of female helpers in the group (additive care). We show that experimental begging playback resulted in all individuals increasing their provisioning rates, indicating that our treatment was perceived as increased chick need. However, in contrast to our prediction that load lightening should only occur when individuals can afford to do so, signals of increased chick need did not stop individuals from reducing their investment with an increasing number of male helpers in the group. These results suggest that despite some flexibility in parental effort, individuals use strict rules with respect to group composition, suggesting that individual provisioning effort is based on multiple integrated cues, and responses to changes in the environment are highly context dependent.This work was supported by an Association for the Study of Animal Behaviour research grant to K.J.M., and by an Australian Research Council DECRA fellowship to L.B. (DE130100174)