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In sexually colour dimorphic bird species, males can exhibit phenotypic variation, with males breeding in either dull female-like plumage or brightly coloured plumage. Two contradictory hypotheses predict that the male phenotype variation can influence the female investment in a given breeding attempt. Whereas females usually prefer males in bright coloured plumage, the “differential allocation hypothesis” predicts that females should invest more in their reproductive output when mating with them; while the “compensatory investment hypothesis” predicts that females should invest more when mating with non-preferred males. To test those predictions, we analysed reproductive data for two consecutive breeding seasons (2018–2019 and 2019–2020) of Lined Seedeaters Sporophila lineola. S. lineola is a socially monogamous songbird species in which males exhibit two breeding phenotypes, a black-and-white plumage being the most common, and a less common female-like brownish plumage. Our findings show that females mated with brownish males have a higher reproductive investment (i.e., egg volume) than those mated with black-and-white males. Despite the lower investment of females in egg volume, our results showed that black-and-white males have a higher nestling survival, producing more hatchings and fledglings per season than brownish males, which could suggest a higher fitness. Our findings indicate that investment allocation on the eggs and offspring survival rates can be attributed to the plumage colour of males.

In species in which adults of the two sexes show long-term association, males often engage in acts of assistance (“services”) aimed at females and/or their young in various behavioural contexts that are not reciprocated. We conducted a quantitative review of the primate literature on sex differences in involvement in protection against predation. We found that males were more likely to be at the front of group progressions, especially in contexts of increased risk, were more vigilant, sometimes to the point of becoming sentinels, were more active in mobbing, and were far more likely to counter-attack felids and monkey-eating raptors than females did. Our evaluation showed that these services may be an expression of male parental care or aimed at bonded partners, but we also encountered many cases in which non-sires, unrelated to either females or immatures, provide them. We hence investigate explanations invoking group augmentation or costly signalling. Currently, available data on male anti-predation services in nonhuman primates do not allow us to distinguish among them, although costly signalling better explains data on birds and humans. We develop predictions that will allow more detailed tests of the various hypotheses for this understudied phenomenon.

Most bird species will defend a territory during the breeding period to assure the availability of resources for their reproductive success. Migratory birds abdicate their territory ownership during the non-breeding period, posing a challenge for the following breeding season. Here we investigated the territorial behavior of male Lined Seedeaters Sporophila lineola, an intra-tropical migrant, during the breeding season (December–May) 2018/2019 in south-eastern Brazil. The Lined Seedeater is a sexually color-dimorphic species that inhabits open areas and feeds on seeds. We followed 18 color-banded individuals, during the period in which they had an active nest. For each individual, we recorded the locations in which they exhibited any potential territorial behavior, with the aid of a handheld GPS. We then estimated the size and shape of the territories using a Kernel Density Estimator. The breeding territories had on average 0.59 ± 0.24 ha, ranging from 0.21 to 0.91 ha in area. Males exhibited agonistic behavior whenever another male intrudes on their territories, especially if in the vicinity of their nests. Our observations indicate that male Lined Seedeaters defend small territories of exclusive use during the breeding season, but forage over a wider home range shared with other conspecifics. Therefore, Lined Seedeaters and other members of Sporophila seem to exhibit home ranges that are much larger than their breeding territories. Empirical studies are needed to understand the influence of territory size and quality on reproductive fitness.

The Lined Seedeater (Sporophila lineola) is a small intra-tropical migrant songbird. However, little is known about its breeding and wintering grounds, or migratory habits. To investigate potentially distinct breeding populations and the migratory schedule of Lined Seedeaters we analysed its spatial and temporal distribution using published breeding records, museum vouchers, and data from citizen science projects (eBird and WikiAves). Our findings suggest that there are three main breeding areas: northern Argentina and north-western Paraguay, south-eastern Brazil, and north-eastern Brazil, and that the breeding season seems to be restricted to November through May, with slight differences in timing among these three areas. The species winters in the northern part of South America (mostly in grassland areas) and maybe also in the Amazonia. Moreover, rainfall predicts the latitudinal and longitudinal movements of Lined Seedeaters, with the migratory movements associated with an increase in rainfall. Taken together, these results provide a first comprehensive overview on the migration of Lined Seedeater, calling for further empirical field studies. Understanding intra-tropical migratory patterns is paramount to comprehend the potential impacts of environmental change in tropical ecosystems.

Altricial birds face the challenge of defending their offspring against potential threats in a fixed location: their nest. In species with biparental care, both parents have an equal interest in defending their nest, although the sexes may differ in their investment in anti-predatory behaviours. We conducted a field experiment to investigate sex-asymmetry in anti-predatory behaviours in a breeding population of Lined Seedeaters Sporophila lineola, a small passerine that exhibits sexual dimorphism in adult plumage colour, in south-eastern Brazil. In Lined Seedeaters females are known to be solely responsible for nest building and incubation. Males help to feed the nestlings and are expected to assume the main role in performing anti-predatory behaviours. We presented stuffed models of nest predators (toucans Ramphastidae) and non-predators (pigeons Columbidae) at active nests and measured the number of alarm calls per minute, number of visits during the trial and latency to resume nest visits after the trial. Our results show that birds emitted alarm calls at higher rates when facing a predator than a control stimulus. Moreover, males generally emitted more alarm calls than females, irrespective of the treatment. Both parents did not visit the nest during the predator trials and they took longer to return to the nest after the model had been concealed than during the control trials. Although males seem to take the main role when it comes to responding to intruders (predator and non-predator), further studies, testing a bigger array of predatory stimuli, are needed to better test and understand sex asymmetry in risk-taking behaviours.

Wild birds use social knowledge to avoid being deceived., Many species give deceptive warning calls, enabled by the high risk of ignoring them. In Siberian jays, a territorial, group-living bird, individuals give warning calls toward perched predators and mob them. However, intruding neighbors can emit these warning calls in the absence of predators to access food, but breeders often ignore these calls. Playback field experiments show that breeders flee sooner and return later after warning calls of former group members than those of neighbors or unknown individuals. Thus, breeders respond appropriately only to warning calls of previous cooperation partners. This mechanism facilitates the evolution and maintenance of communication vulnerable to deceptive signaling. This conclusion also applies to human language because of its cooperative nature and thus, its vulnerability to deception.

The social paper wasp Agelaia pallipes is known to eat carrion and scavenge on vertebrates. There are few records of wasps predating vertebrates, including an attack on an adult hummingbird and the predation of bird nestlings. During a project monitoring reproductive behaviour of a neotropical songbird, the Lined Seedeater Sporophila lineola in south-eastern Brazil, we recorded the predation of a four-day-old nestling by a social paper wasp. In the video, the adult female bird attempted to visit the nest prior to the predation. The male could be seen with its crest feathers erect after a wasp left the nest, when the nestling was presumably already dead. When we arrived at the nest to remove the camera, we found the nestling dead, and did not observe the parents in the vicinity. We also registered two other dead nestlings in a different nest with similar wounds. However, the conclusive cause of death of those nestlings is unknown. Nest predation is a major selective pressure in birds, and insects are rarely assumed to play a notable role in this process. Further research is needed to better understand the nature of the relationship between wasps and birds.

Many prey species mob predators to drive them away, thereby reducing their immediate and future predation risk. Given that mobbing is risky, it may also serve as an opportunity for males to advertise their phenotypic quality to females; however, this idea remains untested. We tested this hypothesis with a field experiment in south-eastern Brazil that assessed the response of sexually dimorphic bird species to models of two diurnal owls: a ferruginous pygmy owl (Glaucidium brasilianum), which mainly eats small birds, and a burrowing owl (Athene cunicularia), which mainly eats invertebrates and thus poses a low risk to birds. Across 19 bird species, the mobbing intensity was higher when facing the less-dangerous owl, and more males engaged in predator mobbing than females. The mobbing intensity of males was higher with a larger number of conspecific females present. This finding indicates that males may use mobbing to display their phenotypic quality to females, suggesting that predator mobbing may be influenced by sexual selection. Predation is an important evolutionary force, often leading to an evolutionary arms race between predators and their prey. A puzzling form of prey-predator interactions is predator mobbing. In a wide range of species, prey individuals approach predators and show characteristic visual and acoustic displays. The primary function of mobbing is to drive the predator away; however, it may also serve as an opportunity to advertise phenotypic quality to conspecifics. Field experiments showed that the mobbing intensity of males increased with the number of conspecific females in the audience, suggesting that female choice may influence the evolution of mobbing behavior.