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In many nuclear families, dependent offspring receive unequal shares of parental investment. Initial overproduction can be adaptive from the perspective of parents, but parents must be able to identify appropriate candidates for favourite status. We studied early nestling development in house sparrows (Passer domesticus), a species in which the loss of some brood members is common, testing the prediction that body mass and carotenoid-rich flange colours are important to nestling success. There was substantial variation in both traits within broods, even only 1 day after hatching. Nestlings low in the within-brood mass hierarchy gained more mass if they displayed more carotenoid-rich flanges than broodmates. Position in the colour hierarchy did not, however, predict mass gain for individuals that were heavier than their broodmates. Nestlings that were heavier or had more carotenoid-rich mouths were also less likely to be the victim of brood reduction. Our results suggest that house sparrow parents use both nestling body size and mouth colour when making allocation decisions. Understanding both how and when offspring traits and parental preferences function is key to understanding how selective pressures act on offspring–parent communication. [ABSTRACT FROM AUTHOR]

Southern Rockhopper Penguins Eudyptes chrysocome chrysocome exhibit marked intra-clutch egg size dimorphism and engage in obligate brood reduction of the first and smaller laid egg, with the rearing of the two nestlings occurring only on rare occasions. Studying the sex ratio of hatchlings within a brood in this species, under this breeding strategy context, is essential to understanding population dynamics. The objectives of this study were to analyze egg size dimorphism within the broods, determine the offspring sex ratio, and investigate if the hatchling sex ratio was determined by hatching order and egg volume to discuss potential biases in the context of brood reduction. We molecularly assessed hatchlings' sex ratio according to the hatching order at Isla Pingüino, Argentina (3 years, n = 68 broods). Sex ratio was not different from the expected 50:50 proportion in all the studied years. In this sense, we found a balanced sex ratio in the first hatchlings, typically the survivors of the brood reduction process. Although not significant, we found a bias toward females in the second hatchlings. Assuming less energy is required to raise a female compared to a male, because of its smaller size; this strategy might enhance the chances of raising two nestlings if conditions are optimal. Southern Rockhopper Penguins at Isla Pingüino appear to exhibit a strategic sex allocation evident in the second but not in the first hatchlings, which might be consistent with its out-of-the-norm exponential growth population. [ABSTRACT FROM AUTHOR]

Altricial birds often display biased preferences in providing parental care for their dependent offspring, especially during food shortages. During this process, such inflexible rules may result in provisioning errors. To demonstrate how parents optimize their provisioning strategies, we proposed a "diagnosis model" of parental care to posit that parents will undergo a diagnosis procedure to test whether selecting against some particular offspring based on phenotype is an optimal strategy. We tested this model in an asynchronous hatching bird, the Azure-winged Magpie Cyanopica cyanus , based on 10 years of data about demography and parental provisioning behaviors. Given their higher daily survival rates, core offspring (those hatched on the first day) merits an investment priority compared with their marginal brood mates (those hatched on later days). However, a marginal offspring also merited a priority if it displayed greater weight gain than the expected value at the early post-hatching days. Parents could detect such a marginal offspring via a diagnosis strategy, in which they provisioned the brood at the diagnosis stage by delivering food to every nestling that begged, then biased food toward high-value nestlings at the subsequent decision stage by making a negative response to the begging of low-value nestlings. In this provisioning strategy, the growth performance of a nestling became a more reliable indicator of its investment value than its hatching order or competitive ability. Our findings provide evidence for this "diagnosis model of parental care" wherein parents use a diagnosis method to optimize their provisioning strategy in brood reduction. [ABSTRACT FROM AUTHOR]

Resource limitations, either due to environmental conditions or constraints on parental provisioning effort, can drive intense competition among offspring. In communal groups, resource availability may increase if parents receive assistance from other group members; however, if those caregivers also produce young, offspring demand may increase at the same time. It is possible, therefore, that the costs of intrabrood competition in large broods may outweigh the benefits of provisioning from additional caregivers. We tested the relationships between group size, brood size, and provisioning rates in the greater ani (Crotophaga major), a communally nesting cuckoo in which multiple breeding pairs and nonreproductive helpers cooperatively raise a shared brood. Crucially, brood and group size can vary independently in this species, allowing us to test changes in each variable separately. Using video footage of 2255 prey deliveries across 10 nests, we found that an increase in the number of adult caregivers within a group did not sufficiently offset a corresponding increase in the number of dependent young within a brood: prey availability per average nestling decreased with brood size, regardless of group size. In larger broods, last-hatched nestlings received significantly less prey than their broodmates, in part due to greater hatching asynchrony that exacerbated competitive asymmetries and facilitated inequality in food allocation. Our results indicate that last-hatched ani nestlings suffer a "double cost" in large broods: they must compete with more nestmates, and suffer disproportionately from asynchronous hatching. These costs may contribute to increased parent–offspring conflict and may constrain group size in communal breeders. [ABSTRACT FROM AUTHOR]

Simple Summary: In some species of birds, asynchronous hatching occurs when incubation begins before completion of clutch, resulting in age differences between nestlings. According to the brood reduction hypothesis, which explains the reason for asynchronous hatching, selective survival is permitted only for older nestlings when availability of food is unpredictable. Due to the feature of food distribution, passerines are not likely to practice selective feeding of older nestlings. The essential purpose of this study is to determine whether the brood reduction hypothesis can explain asynchronous hatching in passerines. Infrared cameras were installed inside nest boxes to determine whether great tit (Parus major) parents, which show asynchronous hatching, practices selective feeding of older nestlings. The results of this study showed that great tit parents did not practice selective feeding of older nestlings, highlighting that using the brood reduction hypothesis to explain asynchronous hatching of great tits is not reasonable. Further research is needed in order to explain the adaptiveness of asynchronous hatching. The brood reduction hypothesis, which explains asynchronous hatching in birds, as an adaptation that enables selective survival of older nestlings when availability of food is unpredictable. This study was conducted in order to determine whether the brood reduction hypothesis can explain asynchronous hatching in passerines. Infrared cameras were installed inside nest boxes where great tits (Parus major) were attempting to reproduce in order to determine whether the parents practiced selective feeding of older nestlings. According to the results of the study, no significant difference was observed between the hatching order and the average number of feedings per nestling. In addition, when examining the distribution of food according to hatching order over time, every 30 min, beginning at 9 a.m., selective distribution of food to older nestlings was not observed. In conclusion, use of the brood reduction hypothesis, which supports selective provision of beneficial feeding of older and larger nestlings, to explain the asynchronous hatching of passerines is problematic, thus conduct of future studies focusing on other hypotheses in order to explain the asynchronous hatching of this passerine bird will be necessary. [ABSTRACT FROM AUTHOR]

I examined sex allocation of offspring in northern flickers, Colaptes auratus , a woodpecker with biparental care and sexual size dimorphism. The analysis of data involving 8321 nestlings that were sexed over a span of 16 years revealed no significant bias towards the smaller sex (females) at the population level (all fledglings pooled), counter to the Fisherian hypothesis of equal investment at the end of parental care. At the level of the brood, high-quality parents (older and in better body condition) did not produce more of the expensive sex (males) counter to the Trivers–Willard hypothesis. Brood sex ratios were somewhat repeatable among females but did not vary significantly with year, clutch size or hatching date probably because the main food of flickers, ground-dwelling ants, is relatively invariable. Mother flickers allocated more of the cheaper sex to last-hatched offspring in asynchronous broods, consistent with the 'intrabrood sharing-out hypothesis' but not the 'competitive advantage hypothesis'. The female-bias in last-laid eggs was present in complete broods, which had no nestling mortality, suggesting that primary sex ratio manipulation was involved. However, a small shift in the direction of female-bias occurred as a result of a greater mortality of the more costly (male) nestlings. Female flickers may therefore attempt to minimize maladaptive brood reduction by allocating more of the cheaper sex to positions later in the laying sequence. • Population level offspring sex ratio of northern flickers did not differ from parity. • High-quality parents did not produce more males (the expensive sex). • Flickers allocated more females to last-hatched offspring within asynchronous broods. • Female bias in last-laid eggs was present in broods without nestling mortality. • Sex allocation was consistent with the 'intrabrood sharing-out hypothesis'. [ABSTRACT FROM AUTHOR]

Sibling cannibalism has rarely been documented in the resource-based context of sibling rivalry. The solitary wasp Isodontia harmandi (Pérez) (Hymenoptera: Sphecidae) has an exceptional style of nursery 'communal brood cells', in which multiple larvae gregariously develop while relying on maternal prey provisioning. In this species there are several distinct features; e.g. cannibalism-driven brood reduction, single-sex broods, and mass provisioning. Rearing of the wasp larvae in artificial nest chambers provides the novel opportunity to study larval interactions under controlled conditions. To determine the factors that influence the occurrence of cannibalism and its timing, we conducted a rearing experiment using sib and non-sib pairs under two prey-availability treatments, and recorded the occurrence of cannibalism and the processes of larval growth and prey consumption over a 96-hr period. Cannibalism occurred more frequently in the low-prey treatment than in the high-prey treatment, and in female pairs than in male pairs. Cannibalism tended to occur earlier when the amount of prey remaining decreased rapidly. However, cannibalism did not occur only when all prey had been completely consumed, implying that it was not triggered directly by prey exhaustion. Cannibals were always larger than their victims, whose growth rates had often slowed before they were eaten. These results strongly imply that wasp larvae commit sibling cannibalism in the context of resource-based sibling rivalry, ensuring the double benefit of nutritional gains and reduced resource competition. We also investigated the possibility that wasp larvae monitor the size difference and/or body condition of brood-mates as an indicator of prey availability. [ABSTRACT FROM AUTHOR]

Aggression represents the backbone of dominance acquisition in several animal societies, where the decision to interact is dictated by its relative cost. Among siblings, such costs are weighted in the light of inclusive fitness, but how this translates to aggression patterns in response to changing external and internal conditions remains unclear. Using a null-model-based approach, we investigate how day-to-day changes in food provisioning affect aggression networks and food allocation in growing red kite (Milvus milvus) nestlings, whose dominance rank is largely dictated by age. We show that older siblings, irrespective of age, change from targeting only close-aged peers (close-competitor pattern) when food provisioning is low, to uniformly attacking all other peers (downward heuristic pattern) as food conditions improve. While food allocation was generally skewed towards the older siblings, the youngest sibling in the nest increased its probability of accessing food as more was provisioned and as downward heuristic patterns became more prominent, suggesting that different aggression patterns allow for catch-up growth after periods of low food. Our results indicate that dynamic aggression patterns within the nest modulate environmental effects on juvenile development by influencing the process of dominance acquisition and potentially impacting the fledging body condition, with far-reaching fitness consequences.

Sex allocation theory predicts that species with reversed sexual size dimorphism frequently overproduce the cheapest sex at the brood level. A sex ratio deviation from parity is frequently documented among the offspring of raptors and is largely produced by sex-biased survival or parental adjustment as a response to environmental variability or demographic conditions. The Eleonora's falcon is a long-distance migrant wintering in Madagascar and southeast Africa and breeding colonially primarily in the Mediterranean Sea and the Canary Islands. It feeds on migratory passerines and thus depends greatly on the autumn migration flow and the prevailing winds. Being sexually size dimorphic (i.e. females are larger than males) and depending greatly on the migration, the species is a good case for investigating whether brood sex ratio variation is adaptive. In the present study, we examined the proportion of males and females reared during 2009–2020 in a falcon colony in southeast Crete (Greece) in relation to specific ecological and biological attributes. Female nestlings were shown to be costlier to produce as they are heavier than males. Overall, an unbiased sex ratio was observed across the years, although a consistent trend was recorded towards females early in the breeding season and first-hatched chicks and males late in the breeding season and third-hatched chicks. Our results provide strong evidence that parental condition, habitat quality and a brood reduction effect were the significant drivers for the observed sex ratio patterns. The overproduction of the costlier sex and a high survival of the cheaper one during food-rich years should be regarded as an adaptive evolutionary strategy of the species for maximizing its fitness returns per clutch and for maintaining a balanced offspring sex ratio in the population. • The overall sex ratio in 2009–2020 was 1:1. • The sex ratio was biased towards females in early and first-hatched chicks. • The sex ratio was biased towards males in late and third-hatched chicks. • Parental condition and habitat quality are significant drivers for sex ratio. • Sex ratio adjustment is an adaptive evolutionary strategy. [ABSTRACT FROM AUTHOR]

Brood reduction is a frequent outcome in asynchronously hatching birds. In these species, first-hatched nestlings get a disproportionately larger share of the food delivered by parents, often leading the youngest nestling to starve to death soon after hatching. However, we still do not know the proximate mechanisms of such brood reduction. Starvation of the smallest nestling may be the outcome of scramble competition among nestmates (due to brood size hierarchies) or, alternatively, be caused by parents that ignore the persistent begging from their smallest nestling. To determine whether parents or scramble competition among siblings induces brood reduction, we used a cross-fostering approach to experimentally create asynchronous Eurasian blackbird, Turdus merula , broods. Then, we assessed food allocation by parents in two different situations: when sibling competition was allowed and when competition was prevented by physically separating nestlings with wooden barriers placed within the nest. We found that smaller nestlings experimentally introduced into blackbird nests received less food than their larger nestmates regardless of whether scramble competition among nestlings was allowed or not. Males preferentially fed larger nestlings regardless of the competitive context, while females showed no feeding differences. Our results suggest that nestlings may exert limited control over parental food allocation. • The smallest nestling usually starves in asynchronously hatched broods. • This can be due to nestling control, parental control or both. • Smaller nestlings received less food regardless of the competitive situation. • Parents actively ignore the smallest nestling. [ABSTRACT FROM AUTHOR]

Generalist brood parasites that share nests with host nestlings can optimize resource acquisition from host parents by balancing the benefits that host nest-mates provide, including attracting increased provisions to the nest, against the costs of competing with the same host young over foster parental resources. However, it is unclear how parasitic chicks cope when faced with more nest-mates than are optimal for their survival upon hatching. We suggest that, in the obligate brood parasitic brown-headed cowbird (Molothrus ater), chicks use a niche construction strategy and reduce larger, more competitive host broods to maximize the parasites' survival to fledging. We experimentally altered brood sizes to test for Goldilocks principle patterns (i.e. a 'just right' intermediate brood size) of cowbird survival in nests of prothonotary warbler (Protonotaria citrea) hosts. We found that intermediate brood sizes of two host nestlings maximized cowbird fledging success relative to 0 or 4 host nest-mates at hatching. Specifically, cowbird nestlings lowered host brood sizes towards this optimum when placed in broods with more host nestlings. The results suggest that cowbirds reduce, but do not eliminate, host broods as a niche construction mechanism to improve their own probability of survival. [ABSTRACT FROM AUTHOR]

Hatching asynchrony in birds is considered an adaptation to facilitate brood reduction because under conditions of food scarcity, the smallest nestling usually dies soon after hatching, thereby minimizing parental effort. However, in species with extreme hatching asynchrony, the last hatchlings paradoxically experience a very low probability of survival and death can take so long that it can hardly be considered an adaptation. Here, we propose and experimentally tested a new adaptive hypothesis explaining the brood reduction paradox, namely the “Male Manipulation Hypothesis”. Our hypothesis suggests that by inducing asynchronous hatching, females increase the feeding requirements of the brood, which will induce males to increase provisioning effort. In addition, females may extend the period of male manipulation by feeding the smallest nestling just enough to sustain life. Our study showed that male common blackbirds (Turdus merula) increased their effort (i.e., number of food items per hour) in experimental asynchronous broods compared to synchronous broods, while females reduced their contribution, as predicted by the hypothesis. [ABSTRACT FROM AUTHOR]

The brood reduction hypothesis, which explains asynchronous hatching in birds, as an adaptation that enables selective survival of older nestlings when availability of food is unpredictable. This study was conducted in order to determine whether the brood reduction hypothesis can explain asynchronous hatching in passerines. Infrared cameras were installed inside nest boxes where great tits (Parus major) were attempting to reproduce in order to determine whether the parents practiced selective feeding of older nestlings. According to the results of the study, no significant difference was observed between the hatching order and the average number of feedings per nestling. In addition, when examining the distribution of food according to hatching order over time, every 30 min, beginning at 9 a.m., selective distribution of food to older nestlings was not observed. In conclusion, use of the brood reduction hypothesis, which supports selective provision of beneficial feeding of older and larger nestlings, to explain the asynchronous hatching of passerines is problematic, thus conduct of future studies focusing on other hypotheses in order to explain the asynchronous hatching of this passerine bird will be necessary.

Post-natal growth has long-term consequences for survival and fitness in birds and is subject to strong directional selection. Highly variable patterns of nestling development are influenced by an array of extrinsic and intrinsic factors. This study analysed the impacts of hatching order, nestling subperiod, brood size and weather conditions, all of which could influence the growth rate of Marsh Harrier Circus aeruginosus nestlings. It was conducted over a period of 6 years, during which measurements of 173 nestlings from 66 nests were made at fishpond complexes in eastern Poland. Relative growth rates (RGR) were calculated on the basis of tarsus and wing lengths during two subperiods of nestling development. Linear mixed models showed that the hatching order and nestling subperiod influenced nestling growth, whereas brood size did not affect wing or tarsus relative growth rates. The effect of hatching order differed between the two traits and between subperiods. Marginal nestlings in broods of five exhibited poor wing growth and perished before fledging. In contrast to wing growth, tarsus growth rates were similar in all the siblings in the first subperiod. During the second developmental subperiod, the growth rates of both locomotor traits exhibited the same pattern, i.e., higher values in the later-hatched chicks than in the earlier ones. Precipitation, but not temperature, had a negative impact on growth in the second subperiod. Our results show a variation in growth patterns between nestmates, which was the most evident in the rate of wing growth. The study enhances our understanding of brood reduction and cainism in birds of prey. [ABSTRACT FROM AUTHOR]

The article explores Fisheries managers used the fish toxicant rotenone to eradicate an undesirable brown bullhead (Ameiurus nebulosus) population and all other fish species at Hyatt Reservoir, Oregon, on 12 October 1989. This 4-yr study (1988–1990, 1992) compared effects of that rotenone project on Ospreys (Pandion haliaetus) nesting at Hyatt Reservoir.

Sibling cannibalism is relatively common in nature, but its evolution in birds and certain other vertebrates with extended parental care had been discarded. Here, however, we demonstrate its regular occurrence in two European populations of the Eurasian hoopoe (Upupa epops) and explore possible adaptive and non-adaptive explanations. Results showed that sibling cannibalism was more frequently detected in Spain (51.7%) than in Austria (5.9%). In these two populations, the hoopoes laid similar clutch sizes, resulting in similar fledging production, but hatching failures were more frequent in the northern population. Consequently, having more nestlings condemned to die in the southern population may explain the higher incidence of sibling cannibalism. In accordance with this interpretation, hatching span and failure, but not breeding date, explained the probability of sibling cannibalism in the Spanish hoopoes, while all three variables predicted brood reduction intensity. Furthermore, experimental food supply reduced the probability of sibling cannibalism, but not the intensity of brood reduction. Finally, females allocated fewer resources to the smallest nestlings when they were going to starve, but not necessarily when they were going to be used as food for their siblings. These results suggest that hoopoes produce extra eggs that, in the case of reduced hatching failure and food scarcity, produce nestlings that are used to feed older siblings. These findings provide the first evidence that sibling cannibalism occurs regularly in a bird species, thus expanding our evolutionary understanding of clutch size, hatching asynchrony, parent-offspring conflict, infanticide, and sibling cannibalism in the animal kingdom. [ABSTRACT FROM AUTHOR]

The article presents the discussion on sibling aggression leading to siblicide documented in a wide variety of bird species. Topics include facultative siblicide resulting from competition for limited food resources; and showing asynchronous hatching, altricial development, sexual size dimorphism, spatial and temporal concentration of food, and small brood size.

Aggression represents the backbone of dominance acquisition in several animal societies, where the decision to interact is dictated by its relative cost. Among siblings, such costs are weighted in the light of inclusive fitness, but how this translates to aggression patterns in response to changing external and internal conditions remains unclear. Using a null-model-based approach, we investigate how day-to-day changes in food provisioning affect aggression networks and food allocation in growing red kite ( Milvus milvus ) nestlings, whose dominance rank is largely dictated by age. We show that older siblings, irrespective of age, change from targeting only close-aged peers (close-competitor pattern) when food provisioning is low, to uniformly attacking all other peers (downward heuristic pattern) as food conditions improve. While food allocation was generally skewed towards the older siblings, the youngest sibling in the nest increased its probability of accessing food as more was provisioned and as downward heuristic patterns became more prominent, suggesting that different aggression patterns allow for catch-up growth after periods of low food. Our results indicate that dynamic aggression patterns within the nest modulate environmental effects on juvenile development by influencing the process of dominance acquisition and potentially impacting the fledging body condition, with far-reaching fitness consequences., Competing Interests: We declare we have no competing interests., (© 2023 The Authors.)

Altricial birds often display biased preferences in providing parental care for their dependent offspring, especially during food shortages. During this process, such inflexible rules may result in provisioning errors. To demonstrate how parents optimize their provisioning strategies, we proposed a "diagnosis model" of parental care to posit that parents will undergo a diagnosis procedure to test whether selecting against some particular offspring based on phenotype is an optimal strategy. We tested this model in an asynchronous hatching bird, the Azure-winged Magpie Cyanopica cyanus , based on 10 years of data about demography and parental provisioning behaviors. Given their higher daily survival rates, core offspring (those hatched on the first day) merits an investment priority compared with their marginal brood mates (those hatched on later days). However, a marginal offspring also merited a priority if it displayed greater weight gain than the expected value at the early post-hatching days. Parents could detect such a marginal offspring via a diagnosis strategy, in which they provisioned the brood at the diagnosis stage by delivering food to every nestling that begged, then biased food toward high-value nestlings at the subsequent decision stage by making a negative response to the begging of low-value nestlings. In this provisioning strategy, the growth performance of a nestling became a more reliable indicator of its investment value than its hatching order or competitive ability. Our findings provide evidence for this "diagnosis model of parental care" wherein parents use a diagnosis method to optimize their provisioning strategy in brood reduction., Competing Interests: The authors have no conflict of interest to any other organizations or individuals with regarding to the publication of this manuscript., (© The Author(s) 2022. Published by Oxford University Press on behalf of Editorial Office, Current Zoology.)

Sibling cannibalism is relatively common in nature, but its evolution in birds and certain other vertebrates with extended parental care had been discarded. Here, however, we demonstrate its regular occurrence in two European populations of the Eurasian hoopoe ( Upupa epops ) and explore possible adaptive and non-adaptive explanations. Results showed that sibling cannibalism was more frequently detected in Spain (51.7%) than in Austria (5.9%). In these two populations, the hoopoes laid similar clutch sizes, resulting in similar fledging production, but hatching failures were more frequent in the northern population. Consequently, having more nestlings condemned to die in the southern population may explain the higher incidence of sibling cannibalism. In accordance with this interpretation, hatching span and failure, but not breeding date, explained the probability of sibling cannibalism in the Spanish hoopoes, while all three variables predicted brood reduction intensity. Furthermore, experimental food supply reduced the probability of sibling cannibalism, but not the intensity of brood reduction. Finally, females allocated fewer resources to the smallest nestlings when they were going to starve, but not necessarily when they were going to be used as food for their siblings. These results suggest that hoopoes produce extra eggs that, in the case of reduced hatching failure and food scarcity, produce nestlings that are used to feed older siblings. These findings provide the first evidence that sibling cannibalism occurs regularly in a bird species, thus expanding our evolutionary understanding of clutch size, hatching asynchrony, parent-offspring conflict, infanticide, and sibling cannibalism in the animal kingdom.