Mothers front-load their investment to the egg stage when helped in a wild cooperative bird

In many cooperative societies, including our own, helpers assist with the post-natal care of breeders' young, and may thereby benefit the post-natal development of offspring. Here we present evidence of a novel mechanism by which such post-natal helping could also have hitherto unexplored beneficial effects on pre-natal development: by lightening post-natal maternal workloads, helpers may allow mothers to increase their pre-natal investment per offspring. We present the findings of a decade-long study of cooperatively breeding white-browed sparrow weaver, Plocepasser mahali, societies. Within each social group, reproduction is monopolized by a dominant breeding pair, and non-breeding helpers assist with nestling feeding. Using a within-mother reaction norm approach to formally identify maternal plasticity, we demonstrate that when mothers have more female helpers they decrease their own post-natal investment per offspring (feed their nestlings at lower rates) but increase their pre-natal investment per offspring (lay larger eggs, which yield heavier hatchlings). That these plastic maternal responses are predicted by female helper number, and not male helper number, implicates the availability of post-natal helping per se as the likely driver (rather than correlated effects of group size), because female helpers feed nestlings at substantially higher rates than males. We term this novel maternal strategy 'maternal front-loading' and hypothesize that the expected availability of post-natal help allows helped mothers to focus maternal investment on the pre-natal phase, to which helpers cannot contribute directly. Such cryptic maternally mediated helper effects on pre-natal development may markedly complicate attempts to identify and quantify the fitness consequences of helping.

Prolactin and the regulation of parental care and helping behavior in cooperatively breeding white-browed sparrow weaver societies

In many cooperatively breeding societies non-breeding individuals help to rear the offspring of breeders. The physiological mechanisms that regulate such cooperative helping behavior are poorly understood, but may have been co-opted, during the evolution of cooperative breeding, from pre-existing mechanisms that regulated parental care. Key among these may be a role for prolactin. Here we investigate whether natural variation in circulating prolactin levels predicts both parental and helper contributions to nestling provisioning in cooperatively breeding white-browed sparrow weavers, Plocepasser mahali. In sparrow weaver groups, a single dominant pair monopolize reproduction and non-breeding subordinates help with nestling feeding. We show that: (i) among parents, dominant females feed nestlings at higher rates, make longer provisioning visits and have higher prolactin levels than dominant males; and (ii) among subordinates, engaged in cooperative helping behavior, those within their natal groups feed nestlings at higher rates and have higher prolactin levels than immigrants. Moreover, continuous variation in prolactin levels positively predicts nestling-provisioning rates and mean provisioning visit durations when all bird classes are combined. These relationships are principally driven by differences among bird classes in both circulating prolactin levels and provisioning traits; the more limited within-class variation in prolactin and provisioning traits were not evidently correlated, highlighting a likely role for additional mechanisms in the fine-scale regulation of care. Our findings broadly support the hypothesis that parental care and cooperative helping behavior are regulated by a common underlying mechanism and highlight the need for experimentation to now establish the causality of any role for prolactin.

Geographical variation in breeding chronology of Germain’s swiftlet (Aerodramus inexpectatus germani) in southern Thailand

The Germain’s swiftlet (Aerodramus inexpectatus germani) feeds on flying insects whose distribution and abundance are influenced by climatic conditions, especially rainfall. In turn, insect availability influences the onset of breeding by swiftlets, hence regions with different climatic conditions such as the west and the east coasts of peninsular Thailand should differ in swiftlet breeding chronology. Here we aimed to determine the variation in breeding chronology between the west and east coast colonies and relate this to insect availability. For each side of the peninsula, at least 40 breeding pairs were continuously observed (July 2014 to October 2015), using ‘internet protocol’ infrared cameras installed inside four commercial swiftlet houses. Flying insects were trapped every two weeks using sticky traps at three permanent stations within major foraging habitats. The onset of breeding in the western colonies was earlier than in the eastern colonies. These significant differences were explained by rainfall and food availability that showed synchrony between the nestling feeding period and peaks in insect biomass. Rainfall of 100–300mm month−1 overlapped with peak breeding and peak insect biomass. These findings have significant implications for the sustainability of swiftlet nest harvesting and demonstrate that, to minimise negative reproductive outcomes for swiftlets where nests are harvested, the timing of harvesting should consider regional climatic conditions.

Distribution of the parental care in the Iberian magpie Cyanopica cooki: worker males but lazy helpers?

In many species of cooperative breeding birds, breeders and helpers participate in the parental care with different food provision rules. Normally, helpers feed nestlings less frequently and with smaller quantities of food than breeders. But studies analysing the reaction of feeders to nestling demand are scarce and rarely measure the quantity of food that is actually delivered. In this study, we analysed the provisioning effort of breeders and helpers in the Iberian Magpie, Cyanopica cooki, and how this effort varies with brood demand. We did so by measuring the nestling feeding rate and the biomass supply of each individual. In this way, we obtained a more accurate measurement of the investment assumed by each individual belonging to each status. We found that breeding males visited the nest more often than both breeding females and helpers (mean = 2.24; 0.85 and 1.58, respectively). Furthermore, breeding males delivered more biomass in each feeding visit to the nest than those from other statuses. Breeders, both male and female, increased their parental effort (i.e., provisioning rate and biomass) when brood demand was higher (i.e., more siblings and older nestlings), whereas helpers contributed differently to the nest, but depending on the two types of helpers occurring in this species. Differences in the possible benefits obtained by breeders and helpers may explain these different strategies. In addition, male and female breeders (but not helpers) reduce the feeding rate throughout the breeding season. Thus, in the Iberian magpie, breeders and helpers reveal different patterns of investment depending on nestling food demand.

Experimental immune activation using a mild antigen decreases reproductive success in free-living female Dark-eyed Juncos (Junco hyemalis)

Seasonal animals time breeding so that offspring rearing coincides with favorable conditions. Offspring rearing is energetically demanding; therefore, additional energetic challenges during this life-history stage may allocate energy away from offspring care, decreasing reproductive success. Activation of the immune system may be one such energetic challenge, and may have a disproportionately higher impact on reproductive success earlier in the breeding season when resources are less abundant and thermoregulatory demands are greater. We monitored nestling growth and survival in incubating female Dark-eyed Juncos (Junco hyemalis (L., 1758)) injected with a mild antigen to stimulate antibody production and induce an energetic challenge. We found nests of treated females were more likely than controls to fail prior to 6 days post hatch, coinciding with timing of peak antibody production. No effect of season was detected. Offspring mass did not differ between treatments prior to failure, suggesting that failure was potentially due to differences in behaviour other than nestling feeding. Our findings indicate a trade-off between immunity and nest survival that is not affected by time of season. Based on the results of our study, we suggest that future research be directed toward how immune activation influences behaviours, including nest guarding and predator aggression, and mediates this trade-off.

Parent-absent calls are related to nestling reaction time and parental food allocation in the spotless starling

Absent repeat calls (ARC) are produced by nestlings of some bird species when parents are not at the nest, and play a role in sibling interactions and parental investment. We explored if individual traits influencing begging also determine ARC in the spotless starling (Sturnus unicolor), and whether this behaviour explains nestling feeding success. We video-taped natural broods and examined the effects of experimental feeding in this behaviour. Experimentally fed chicks stopped calling and received fewer feedings. Among un-fed chicks, absence calls were more frequent in smaller nestlings. We found a positive relationship between nestling reaction time to parental arrival and food acquisition: chicks that reacted first received more feedings that slower chicks. ARC performance was also positively related to reaction time: chicks that produced more calls also reacted first to parents. These results suggest that ARC may have important effects on resource allocation and family interaction networks.

Colony Development and Density-Dependent Processes in Breeding Grey Herons

The density-dependent processes that limit the colony size of colonially breeding birds such as herons and egrets remain unclear, because it is difficult to monitor colonies from the first year of their establishment, and the most previous studies have considered mixed-species colonies. In the present study, single-species colonies of the Grey Heron (Ardea cinerea) were observed from the first year of their establishment for 16 years in suburban Tokyo. Colony size increased after establishment, illustrating a saturation curve. The breeding duration (days from nest building to fledging by a pair) increased, but the number of fledglings per nest decreased, with colony size. The reproductive season in each year began earlier, and there was greater variation in the timing of individual breeding when the colony size was larger. The prolonged duration until nestling feeding by early breeders of the colony suggests that herons at the beginning of the new breeding season exist in an unsteady state with one another, likely owing to interactions with immigrant individuals. Such density-dependent interference may affect reproductive success and limit the colony size of Grey Herons.

Nest intrusions in relation to breeding status in the spotless starling

Nest intruders are common in many avian species. In the spotless starling (Sturnus unicolor), a passerine closely related to the European starling (Sturnus vulgaris), it has been recently shown that visits to alien nests were in most cases related to get familiarity and personal information about prospected nest boxes and that collecting public information seems also to be involved in the intruding behaviour. In the present study we investigate whether nest intrusions are related to breeding status in both males and females. Individuals that had bred previously in the colony were detected as frequently as those without previous breeding experience intruding nests and the proportion of intruders that were owners and those that had not a nesting site during the current year was balanced. Males that were actively breeding and those that were not breeding intruded with similar frequencies in alien nests, while most females were not actively breeding when they were observed intruding a nest box. The nests more frequented by male intruders were those showing the highest female nestling feeding rates, but the frequency of female intrusions was not similarly affected by female owner feeding rates. The results suggest that some of the intrusions by males are related to nest acquisition although in general males seem to intrude to obtain other breeding resources, presumably extra-pair matings. Female intruders with a nesting territory but not currently breeding could be sampling alternative nesting sites while female floaters that intruded nests could be searching for nesting sites or attempting to lay parasitic eggs. Birds behaving as intruders at some moment of their stay in the breeding colony raised more fledglings that birds never observed performing this behaviour.