Large-scale, cross-species comparative analyses on cooperative breeding-where individuals care for the offspring of other group members-are important for understanding sociality and cooperation. However, the datasets that facilitate these analyses are often limited in precision. To advance comparative research on cooperative breeding, we hereby introduce the Cooperative-Breeding Database (Co-BreeD) for birds and mammals. We describe key features of Co-BreeD's structure: (i) integration of complementary datasets, each presenting a biological parameter relevant to cooperative-breeding research; (ii) sample-based (i.e. multiple samples per species linked to an exact sampling location and period); and (iii) open-source. Respectively, these features enable: (a) comprehensive identification of cooperative-breeding species according to the user's chosen definition, (b) linking intra- and inter-specific variation in traits with fine-scale environmental parameters and (c) enabling the research community to correct and expand this database. We present the initial Co-BreeD dataset, which estimates the prevalence of breeding events involving potential alloparents in 460 populations of 324 species, including 6 human populations (No. total = 43,247 breeding events). We conclude by demonstrating: (i) how Co-BreeD can improve comparative research (e.g. by enabling the study of cooperative breeding as a continuous rather than a binary trait); and (ii) that cooperative breeding is probably more prevalent than previously estimated in birds and mammals.

Research on cooperative breeding (a system with the core characteristic of individuals providing care for the offspring of others) is important for understanding sociality and cooperation. However, large-scale comparative analyses on the drivers and consequences of cooperation frequently use considerably inaccurate datasets (e.g. due to inconsistent definitions and outdated information). To advance comparative research on cooperative breeding, we introduce the Cooperative-Breeding Database (Co-BreeD), a growing database of key socio-biological parameters of birds and mammals. First, we describe Co-BreeD’s structure as a (i) sample-based (i.e. multiple samples per species linked to an exact sampling location and period), (ii) peer-reviewed and (iii) updatable resource. Respectively, these curating principles allow for (i) investigating intra- and inter-species variation and linking between fine-scale social and environmental parameters, (ii) accuracy and (iii) continuous correction and expansion with the publication of new data. Second, we present the first Co-BreeD dataset, which estimates the prevalence of breeding events with potential alloparents in 265 samples from 233 populations of 150 species, including 2 human societies (N = 26,366 breeding events). We conclude by demonstrating (i) how Co-BreeD facilitates more accurate comparative research (e.g. increased explanatory power by enabling the study of cooperative breeding as a continuous trait, and statistically accounting for the sampling error probabilities), and (ii) that cooperative breeding in birds and mammals is more prevalent than currently estimated. ### Competing Interest Statement The authors have declared no competing interest.

Extreme reproductive skew occurs when a dominant female/male almost monopolizes reproduction within a group of multiple sexually mature females/males, respectively. It is sometimes considered an additional, restrictive criterion to define cooperative breeding. However, datasets that use this restrictive definition to classify species as cooperative breeders systematically overestimate reproductive skew by including groups in which reproduction cannot be shared by definition (e.g. groups with a single female/male). Here, we review the extent of reproductive sharing in 41 mammal and 37 bird species previously classified as exhibiting alloparental care and extreme reproductive skew, while only considering multi-female or multi-male groups. We demonstrate that in groups where unequal reproduction sharing is possible, extreme reproductive skew occurs in a few species only (11/41 mammal species and 12/37 bird species). These results call for significant changes in datasets that classify species' caring and mating system. To facilitate these changes, we provide an updated dataset on reproductive sharing in 63 cooperatively breeding species. At the conceptual level, our findings suggest that reproductive skew should not be a defining criterion of cooperative breeding and support the definition of cooperative breeding as a care system in which alloparents provide systematic care to other group members' offspring.

Cooperative breeding (i.e. when alloparents care for the offspring of other group members) has been studied for nearly a century. Yet, inconsistent definitions of this breeding system still hamper comparative research. Here, we identify two major inconsistencies, discuss their consequences and propose a way forward. First, some researchers restrict the term 'cooperative breeding' to species with non-breeding alloparents. We show that such restrictive definitions lack distinct quantitative criteria to define non-breeding alloparents. This ambiguity, we argue, reflects the reproductive-sharing continuum among cooperatively breeding species. We therefore suggest that cooperative breeding should not be restricted to the few species with extreme reproductive skew and should be defined independent of the reproductive status of alloparents. Second, definitions rarely specify the type, extent and prevalence of alloparental care required to classify species as cooperative breeders. We thus analysed published data to propose qualitative and quantitative criteria for alloparental care. We conclude by proposing the following operational definition: cooperative breeding is a reproductive system where >5% of broods/litters in at least one population receive species-typical parental care and conspecifics provide proactive alloparental care that fulfils >5% of at least one type of the offspring's needs. This operational definition is designed to increase comparability across species and disciplines while allowing to study the intriguing phenomenon of cooperative breeding as a behaviour with multiple dimensions.

Adult males of many migratory species arrive at breeding grounds before females and young. In a 34-year study of the masked shrike, Lanius nubicus, an age- and sex-specific pattern of spring arrival was distinguished. Adult males arrived first followed by juvenile males and adult females, whereas juvenile females appeared last. We hypothesized that these differences in migratory strategies would be reflected in a differential response to climate conditions at the wintering grounds. Testing correlations between spring arrival time and winter climate conditions provided strong support to our hypothesis. Adult males' arrival time exhibited high associations with climate conditions in early spring, upon migratory take-off, whereas juvenile males responded mostly to conditions in November, upon autumn arrival in Africa. Adult females responded to both parameters in autumn and early spring, whereas young females' arrival correlated only with a few variables in autumn. GLM models of median spring arrival day for all categories but the young females were highly statistically significant with adjusted R-squared values of 0.81–0.93. The emerging pattern of different associations between timing of spring migration and climate conditions at the wintering grounds sheds new light on existing evolutionary theories regarding age- and sex-specific migratory strategies.

Kin-biased behavior (that is responding differentially to kin and non-kin) is thought to be adaptive in many social interactions. One example of this kin bias is behaving less aggressively toward a relative than a non-relative, a behavior which yields inclusive fitness benefits. However, data are lacking about the ability of animals to weigh their preference for kinship and the density of conspecifics simultaneously and to respond accordingly. Fire salamanders (Salamandra infraimmaculata) larviposit in high densities in ponds. Thus, larvae of different females confront competition and predation by other larvae. We studied whether larvae prefer their kin over particular density or vice versa. We experimentally used a transparent glass aquarium with inner chambers to test the responses of a focal larva toward its siblings and non-siblings. Specifically, we quantified the time a focal larva spent near its siblings or non-siblings, presented in varying densities, and the aggression level it demonstrated. We found that focal larvae spent more time near non-siblings if non-sibling and sibling groups were of equal density. The focal larvae were also more aggressive toward non-siblings. The results may be explained by the cannibalistic nature of these larvae: high density may provide more opportunities for food, especially when non-siblings are present. Further explanations for these findings may include other advantages of staying in a larger group and/or the stronger olfactory and visual stimulation offered by groups compared to a single individual. These findings suggest that larvae make differential responses toward conspecifics depending simultaneously on the level of relatedness and the density of the group. Such responses have important implications for social-aggregation decisions and may especially affect the fitness of cannibalistic species.

Most papers on the physical condition of birds during spring migration focused on food availability preceding migratory take-off. Only a few studies examined the effect of climate conditions at the wintering grounds upon autumn arrival on bird physical condition later on. Here, we hypothesized that environmental conditions upon arrival at the wintering grounds, and not necessarily upon departure, have a crucial carry-over effect on bird spring migration. Using 29,000 observations of the lesser whitethroat, Sylvia curruca, and the eastern Bonelli's warbler, Phylloscopus orientalis, we found temperatures upon arrival at the African wintering grounds to be the only climatic variable correlated with birds' body state upon spring stopover in Israel, six months later. Two different mechanisms could explain these results. One possibility is that high temperatures create favorable conditions for insect activity, which allows rapid recovery from autumn migration and hence successful winter survival and maintenance. Another possible scenario is that harsh conditions, due to the heat and dry environment, cause high mortality, permitting survival of larger individuals which, then, enjoy reduced inter- and intra-specific competition. Whatever the mechanism is, our findings suggest that conditions upon autumn arrival, and not necessarily at the end of winter as traditionally thought, may have a major impact on migrating birds.

Although interspecific competition is suggested to be one of the major forces dictating community structure, interspecific interference competition for nest sites in birds has been reported mainly from observational studies. Here, we asked whether interference by the larger House Sparrow (Passer domesticus) could reduce breeding success and parental behavior in the smaller Great Tit (Parus major) following clutch completion, by experimentally allowing House Sparrows to access half of the Great Tit nest boxes. Significantly more tit pairs failed to raise young in nest boxes that House Sparrows were able to enter during their breeding period compared to those that were not able to do so, because House Sparrows usurped 77.8% of the Great Tit nests. Great Tits also increased the duration of nest defense in the presence of House Sparrows. As the outcome of interference competition may lead to breeding failure, birds should necessarily evolve ways to avoid nest competitors either by selecting nests that restrict access to their larger competitors and/or by initiating breeding earlier. Conservation efforts should be directed toward attaching a metal restrictor plate around the entrance of nest boxes to prevent woodpeckers from enlarging the entrance and larger species from entering nests.

Numerous studies report shifts in bird migration phenology, presumably owing to global warming. However, most studies focus on migration patterns in the Northern Hemisphere. In this study, we investigated associations between weather conditions in African wintering grounds of the lesser whitethroat, Sylvia curruca, and spring arrival time in Eilat, Israel. Using multivariate regression models, we analysed a 30-year dataset in order to examine correlations between median springtime arrival and 46 climate variables of the wintering quarters. The model obtained exhibited a highly statistical fit, involving mean precipitation in February and March with negative effects and number of wet days during November–February. February precipitation levels were also the major factor associated with the interquartile range of arrival time. Interestingly and contrary to published results, annual or seasonal precipitation showed no correlation with spring arrival time, nor did temperature. Moreover, winter in this region falls into dry season with negligible rainfall quantities. Hence, it is unlikely that precipitation effect on habitat productivity is a driving force of migration, as suggested by other studies. Instead, we propose that precipitation in February acts as a cue for the birds, indicating the approach of spring and migration time.

Many plant species contain plant secondary metabolites (PSMs), such as alkaloids, in their tissues for protection against herbivore attack, but PSMs can also be found in floral nectar. Some pollinators have been shown to discriminate against floral nectar with PSMs and consuming PSMs may have negative fitness effects on pollinators. However, only a few studies have investigated the effects of ecologically relevant levels of PSMs on pollinator foraging performance. Here, we addressed the question of whether the natural concentrations of the alkaloids, nicotine and anabasine, found in tree tobacco, Nicotiana glauca, nectar affect foraging performance in Palestine sunbird, Nectarinia osea, pollinators that use the plant's nectar as a food source. We trained foraging sunbirds to discriminate between rewarding and nonrewarding artificial flowers based on colour. We measured sunbird foraging performance through their accuracy at distinguishing the two colours immediately after training (pretreatment), and again the following day after consuming sucrose solutions with or without alkaloids (post-treatment). We also explored other potential effects of PSM consumption by assessing bird activity level and flower visit rate. Birds that consumed alkaloids did not significantly change their activity level or flower visit rate across time (pre- and post-treatment) compared to birds that did not consume alkaloids (no significant time by treatment interaction). However, alkaloid consumption significantly decreased sunbird foraging performance in terms of their accuracy in distinguishing the rewarding colour, potentially due to reduced memory retention and/or other cognitive or physiological impairments following alkaloid consumption. We also found that sunbirds discriminated against higher, in favour of lower, ecologically relevant alkaloid concentrations in the nectar of tree tobacco and that previous exposure to alkaloids reduced overall consumption of alkaloid solutions. Reduced foraging performance due to PSM ingestion could greatly affect a pollinator's foraging efficiency, which could, in turn, affect both pollinator and plant reproductive fitness.

The evolution of traits in hosts may be influenced by their parasites and vice versa and a coevolutionary arms race often develops between the two. As part of such an arms race, the common cuckoo mimics the eggs of its hosts to avoid egg rejection. Traits related to this arms race may also be influenced by climatic conditions, such as temperature, affecting, for example, food availability and, thus, female condition and egg size (therefore may reflect Bergmann's rule or the resource rule). The potential interaction between coevolution and climate has rarely been studied. We investigated whether egg and body size of cuckoos and reed warblers from Britain and Denmark had undergone change between 1868 and 1956, and whether such changes were correlated with climatic factors. Cuckoo egg size decreased during the studied period while warbler egg size remained stable. Hence, cuckoo and warbler eggs have become more similar in size over time. Cuckoo egg volume decreased with increasing annual precipitation, but annual precipitation decreased over time. Warbler egg volume increased with spring temperatures (which could not reflect Bergmann's rule, but may support the resource rule). Hence, it seems that the measured climatic indices did not affect cuckoo egg size but may in part affect warbler egg size. Therefore, the decrease in cuckoo egg size may be the result of the coevolutionary arms race. Body and egg sizes in the cuckoos were negatively correlated whereas warbler body and egg sizes were uncorrelated, suggesting that selection probably acted on egg size directly and not via selection on body size. Taken together, these findings may indicate that climate change, the coevolutionary arms race, or both, affected egg sizes. It is suggested that drawing conclusions regarding the arms race without taking into account other selective pressures (e.g., climate) may confound conclusions regarding parasite-host systems.

The trade-off between parents feeding themselves and their young is an important life history problem that can be considered in terms of optimal behavioral strategies. Recent studies on birds have tested how parents allocate the food between themselves and their young. Until now the effect of food consumption by parent birds on their food delivery to their young as well as other parental activities has rarely been studied. I have previously shown that parent Palestine sunbirds (Nectarinia osea) will consume nectar and liquidized arthropods from artificial feeders. However, they will only feed their young with whole arthropods. This provided a unique opportunity to experimentally manipulate the food eaten by parents independent of that fed to their offspring. Here, I hypothesized that parents invest in their current young according to the quality of food that they themselves consume. Breeding pairs with two or three nestlings were provided with feeders containing water (control), sucrose solution (0.75 mol) or liquidized mealworms mixed with sucrose solution (0.75 mol). As food quality in feeders increased (from water up to liquidized mealworms mixed with sucrose solution): 1) Parents (especially females) increased their food delivery of whole arthropod prey to their young. 2) Only males increased their nest guarding effort. Nestling food intake and growth rate increased with increasing food quality of parents and decreasing brood size. These results imply that increasing the nutrient content of foods consumed by parent sunbirds allow them to increase the rate at which other foods are delivered to their young and to increase the time spent on other parental care activities.

In monogamous species, conspecific aggression is a primary tactic to deter intruders. Alternative reproductive strategies, such as extra-pair copulations or intra-specific brood parasitism that peak at the pre-egg laying period and are practised by intruders, pose a significant threat to the monogamous pair. Here, we addressed the question whether male and female Palestine Sunbirds (Nectarinia osea) demonstrate different aggression levels toward conspecific intruders during the pre-egg laying period. We further asked whether indirect measures of certainty of paternity affect male aggression toward such intruders. For that, we measured the aggression of both partners toward intruders while using stuffed birds. We conducted two experiments on a monogamous and territorial Palestine Sunbird population. In the first experiment, we exposed breeding pairs to a conspecific or a heterospecific intruder and measured their aggression toward it. During the second experiment, we exposed breeding pairs to an intruder, before and after temporarily removing the local male. The local male demonstrated higher aggression than the local female toward a male decoy. Toward a female decoy, the local male demonstrated lower aggression than the local female. Following their release, the removed males did not change their aggression toward the male decoy, but rather they decreased their aggression toward the female decoy. These results suggest that the indirect male’s certainty of paternity may be an important factor of the male’s interests when he faces a risk of potential intra-specific brood parasitism. Furthermore, these findings suggest that each gender is more sensitive to its similar intruder gender, and acts according to its interests in terms of its aggression toward the intruder.

Plant secondary metabolites (PSMs), such as alkaloids, are often found in many parts of a plant, including flowers, providing protection to the plant from various types of herbivores or microbes. PSMs are also present in the floral nectar of many species, but typically at lower concentrations than in other parts of the plant. Nectar robbers often damage floral tissue to access the nectar. By doing so, these nectar robbers may initiate an increase of PSMs in the floral nectar. It is often assumed that it takes at least a few hours before the plant demonstrates an increase in PSMs. Here, we addressed the question of whether PSMs in the floral tissue are immediately being released into the floral nectar following nectar robbing. To address this research question, we investigated whether there was an immediate effect of nectar robbing by the Palestine Sunbird (Nectarinia osea) on the concentration of nectar alkaloids, nicotine and anabasine, in Tree Tobacco (Nicotiana glauca). We found that the concentration of anabasine, but not nicotine, significantly increased in floral nectar immediately following simulated nectar robbing. These findings suggest that nectar robbers could be ingesting greater amounts of PSMs than they would if they visit flowers legitimately. As a consequence, increased consumption of neurotoxic nectar alkaloids or other PSMs could have negative effects on the nectar robber.

Although 90 % of all bird species are monogamous, many species practice alternative reproductive strategies as extra-pair copulations, intra-specific brood parasitism, and quasi-parasitism. In territorial monogamous species, both partners hold and defend the territory from intruders. Often, the intruders are males and usually the local male banishes the intruders. Indeed, many studies focused on the response of the local male toward intruder males. However, the benefits and costs associated with the responses of the local male toward intruder females have been largely overlooked. Focusing mainly on alternative reproductive strategies, we developed a model to predict the aggression a monogamous male may demonstrate toward an intruder female during the pre-egg laying stage of his local female partner. This model demonstrates that the intensity of aggression that the local male shows toward an intruder female depends on the extra-pair copulations that his local female partner may perform. Further, the aggression also depends upon intra-specific brood parasitism and quasi-parasitism that might be carried out by the intruder female. Our approach suggests that when considering mating strategies, there is a need to assess how these three alternative reproductive strategies may affect the local male's aggression toward intruder females.

Floral color is an important cue that converged in many ornithophilous flowers and can be used by nectarivorous birds to make foraging decisions. Wild ornithophilous flowers are frequently red, although they are more often yellow in Israel. The Palestine sunbird (Nectarinia osea) is the only nectarivorous bird in Israel and surrounding Mediterranean areas. Given the prevalence of yellow flowers in their habitats (along with sunbirds' expected sensitivity increase in this region of color vision), we predicted that Palestine sunbirds prefer yellow food sources over red. We examined sunbird foraging behavior when they were presented simultaneously with a yellow and red feeder, each containing the same quantity and quality of food. We investigated whether sunbirds had a side bias in the color preference experiment, but also in a separate experiment where both feeders were white. Sunbirds did not exhibit a significant color bias, while they did have a significant preference for a particular side of the cage. Location appears to be a more important cue than color to Palestine sunbirds, likely because location can offer information on the most rewarding plants and recently depleted flowers. However, color may still provide useful information that could influence foraging decisions in different contexts.

Winter habitat use and the magnitude of migratory connectivity are important parameters when assessing drivers of the marked declines in avian migrants. Such information is unavailable for most species. We use a stable isotope approach to assess these factors for three declining African-Eurasian migrants whose winter ecology is poorly known: wood warbler Phylloscopus sibilatrix, house martin Delichon urbicum and common swift Apus apus. Spatially segregated breeding wood warbler populations (sampled across a 800 km transect), house martins and common swifts (sampled across a 3,500 km transect) exhibited statistically identical intra-specific carbon and nitrogen isotope ratios in winter grown feathers. Such patterns are compatible with a high degree of migratory connectivity, but could arise if species use isotopically similar resources at different locations. Wood warbler carbon isotope ratios are more depleted than typical for African-Eurasian migrants and are compatible with use of moist lowland forest. The very limited variance in these ratios indicates specialisation on isotopically restricted resources, which may drive the similarity in wood warbler populations' stable isotope ratios and increase susceptibility to environmental change within its wintering grounds. House martins were previously considered to primarily use moist montane forest during the winter, but this seems unlikely given the enriched nature of their carbon isotope ratios. House martins use a narrower isotopic range of resources than the common swift, indicative of increased specialisation or a relatively limited wintering range; both factors could increase house martins' vulnerability to environmental change. The marked variance in isotope ratios within each common swift population contributes to the lack of population specific signatures and indicates that the species is less vulnerable to environmental change in sub-Saharan Africa than our other focal species. Our findings demonstrate how stable isotope research can contribute to understanding avian migrants' winter ecology and conservation status.

A number of mosquito species avoid predator-inhabited oviposition sites by detecting predator-released kairomones. In the laboratory, we found that when offered de-ionized water and de-ionized water conditioned with Notonecta maculata, gravid Anopheles gambiae females preferentially oviposited into the former. We then conducted further experiments using two chemical components found in Notonecta-conditioned water, chemically pure n-tricosane and/or n-heneicosane, that was previously shown to repel oviposition by Culiseta longiareolata. These hydrocarbons failed to deter oviposition by An. gambiae females. Thus, different mosquito species may rely on distinct chemical cues to avoid predators. Identification and chemical characterization of such kairomones could facilitate innovative, environmentally sound mosquito control.