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Brood reduction by parents via infanticide is considered rare in passerine birds; however, this behavior may be underreported because of the difficulties observing behaviors at the nest and because researchers tend to attribute partial nestling loss to other causes. Here, we report a confirmed incidence of parental infanticide by Dickcissels ( Spiza americana). While video-recording parental behavior, we documented a 4-day-old nestling being removed by a female Dickcissel. This bird was also observed brooding and feeding, so this event was likely a parental infanticide. We subsequently examined monitoring data from 162 hatched Dickcissel nests across 2 breeding seasons to identify instances of unexplained partial nestling loss, which could potentially be attributable to infanticide. Our data indicate that 9.1-12.7% of hatched nests experienced these events. Infanticide by genetic parents could (1) benefit survival of remaining brood mates by reducing food requirements, disease, or predation risk; (2) represent responses to cuckoldry or intraspecific brood parasitism; (3) represent cases of mistaken chick identity; or (4) be triggered by unusual stressors. We recommend that ecologists monitoring bird nests consider infanticide as a possible explanation for partial nestling loss. Received 3 November 2016. Accepted 27 November 2017. [ABSTRACT FROM AUTHOR]

Offspring are often produced in excess as insurance against stochastic events or unpredictable resources. This strategy may result in high early-life mortality, yet age-specific mortality before offspring independence and its associated costs have rarely been quantified. In this study, we modelled age-specific survival from hatching to fledging using 24 years of data on hatching order (HO), growth and age of mortality of more than 15 000 common tern (Sterna hirundo) chicks. We found that mortality peaked directly after hatching, after which it declined rapidly. Mortality hazard was best described with the Gompertz function, and was higher with later HO, mainly due to differences in baseline mortality hazard, rather than age-dependent mortality. Based on allometric mass-metabolism relationships and detailed growth curves of starving chicks, we estimated that the average metabolizable energy intake of non-fledged chicks was only 8.7% of the metabolizable energy intake of successful chicks during the nestling phase. Although 54% of hatchlings did not fledge, our estimates suggest them to have consumed only 9.3% of the total energy consumption of all hatched chicks in the population before fledging. We suggest that rapid mortality of excess offspring is part of an adaptive brood reduction strategy to the benefit of the parents. [ABSTRACT FROM AUTHOR]

Despite major advances in sex ratio theory, how offspring sex should vary with hatching order remains unclear. We examine nestling sex ratio in the Southern Grey Shrike Lanius meridionalis according to hatching order and clutch size. Southern Grey Shrike nestlings present a different sex ratio with body-mass rank order depending on clutch size. When the clutch size was five eggs (with a very low risk of brood reduction; 13%) the less costly sex (male) was found at the end of the body mass hierarchy. However, when clutch size was six eggs (with a high risk of brood reduction; 42%) the larger sex (female) was found at intermediate positions in the hatching order, possibly to decrease competitive asymmetries. [ABSTRACT FROM AUTHOR]

The breeding biology of Australian Pelicans (Pelecanus conspicillatus) was studied on the coast of South Australia between 2002 and 2004. This study aimed to compare breeding on the coast with previous studies of inland breeding Australian Pelicans and to test whether this species exhibits siblicidal brood reduction. Nests were generally constructed on the ground in spatially and temporally discrete colonies. Nests within colonies were 1.3 ± 0.5 m apart (Range = 0.6-4.5; n = 327), whereas colonies were 329 m apart (Range = 21-1,136; n= 63). Within colonies, 80% of nests were initiated within 10 days of each other. Colonies contained 41.0 ± 37.6 nests (Range = 1-148; n = 63). Most colonies were initiated during the austral winter and spring (June-September) in contrast to opportunistic breeding in inland areas. Clutch size averaged 1.92 ± 0.36 eggs (Range = 1-4; n = 2,297), and incubation lasted 32 days (Range = 28-38; n = 57). Hatching success averaged 41.5 ± 38.6% (Range = 0-99; n = 24 colonies), the number of nestlings that survived to join a creche was 20.3 + 22.4% (Range = 0-66; n = 24 colonies), and fledging success averaged 15.3 ± 18.3% (Range = 0-61; n= 24 colonies). Aggression among nestlings and resulting wounds were observed in 48% of broods (n = 64). Mortality during the 10-day nestling period was high (5.5 ± 2.2%/day; Range = 2.5-10.0; n = 45) and was the major determinant of overall breeding success. These findings suggest that Australian Pelicans exhibit siblicidal brood reduction. [ABSTRACT FROM AUTHOR]

Parental food allocation in birds has long been a focal point for life history and parent-offspring conflict theories. In asynchronously hatching species, parents are thought to either adjust brood size through death of marginal offspring (brood reduction), or feed the disadvantaged chicks to reduce the competitive hierarchy (parental compensation). Here, we show that parent American coots ( Fulica americana) practice both strategies by switching from brood reduction to compensation across time. Late-hatching chicks suffer higher mortality only for the first few days after hatching. Later, parents begin to exhibit parental aggression towards older chicks and each parent favours a single chick, both of which are typically the youngest of the surviving offspring. The late-hatched survivors can equal or exceed their older siblings in size prior to independence. A mixed allocation strategy allows parents to compensate for the costs of competitive hierarchies while gaining the benefits of hatching asynchrony. [ABSTRACT FROM AUTHOR]

Maternal effects can function as a mechanism of transgenerational plasticity by which the environment experienced by parents is translated into the offspring phenotype and fitness. In birds, parents may affect the competitive ability of their offspring, and hence their fitness, by modifying their hatching pattern and/or egg size. However, little is known about how mothers can modify offspring phenotypes and their fitness in response to a sudden change in environmental conditions during egg-laying. Here, we studied the effect of supplemental food during egg-laying on hatching asynchrony and egg size in the Eurasian roller ( Coracias garrulus), a species with marked hatching asynchrony. We also explored the effects of maternal investment on offspring fitness. Food supplementation did not affect hatching asynchrony. However, females in food-supplemented nests laid eggs that increased in size with laying order except for an ultimate small egg. Meanwhile, size of eggs laid by females in control nests did not change with laying order. Supplemental food positively affected hatchability of the egg laid just before the last one and negatively affected hatchability of the last laid egg, which seemed to be a side effect of egg size. Consequently, food-supplemented nests produced fewer fledglings and had higher probabilities of suffering brood reduction than control nests. We conclude that egg size in rollers is a plastic trait, sensitive to short-term changes in food conditions. Furthermore, our results show that maternal investment in egg size may potentially affect offspring fitness. [ABSTRACT FROM AUTHOR]

In avian families, some offspring are rendered unequal by parental fiat. By imposing phenotypic handicaps (e.g., via asynchronous hatching) upon certain of their offspring and not others, parents structure the sibship into castes of advantaged “core” offspring and disadvantaged “marginal” offspring that results in an asymmetric sibling rivalry. Here, I show how this family structure scales up to population level reproductive consequences. In a 17-year study of red-winged blackbirds ( Agelaius phoeniceus), I show that year-to-year variation in the number of surviving offspring is driven primarily by variation in the number of marginal offspring at hatching and their posthatching survival. Clutch size, core brood at hatching, and fledging varied little from year to year and had little direct effect on year-to-year variation in total brood size at fledging; conversely, variation in the size of the marginal brood at hatching and at fledging was much greater. Marginal but not core brood size at hatching rose with mean clutch size; in years where parents laid larger average clutches they did so by adding marginal progeny. The mean posthatching survival of marginal offspring was always lower than that of core offspring in a given year, and there was no overlap in the distributions. The highest mean survival of marginal offspring across years fell below the lowest mean survival of core offspring; broods were deeply structured. There was an overall female bias among fledglings, and the sex ratio varied across years, with a higher proportion of the smaller female nestlings in years of below average reproductive success. Such variation was especially pronounced in the marginal brood where a higher incidence of brood reduction allowed greater potential for sex-biased nestling mortality. In years of the highest average reproductive success, the sex ratio in the marginal brood approached equality, whereas in years of the lowest average reproductive success, more than two thirds of 8-day-old nestlings were female. Structuring the brood into core and marginal elements allowed parents to modulate both offspring number and sex under ecological uncertainty with direct consequences for population-level reproductive success. They produced fewer and less expensive fledglings in below average years and more and more expensive fledglings in above average years. [ABSTRACT FROM AUTHOR]

Several benefits may be gained from staggered hatching in birds but asynchrony sometimes appears to cause unnecessary starvation of the youngest nestlings. We present a solution to this paradox by suggesting that foraging parents are sometimes constrained by complex interactions between prey size, energy demands and foraging efficiency (the feeding constraint hypothesis). The requirement to provide smaller nestlings with small and soft food items while at the same time satisfying the greater food demands of larger nestlings may cause a conflict that results in the starvation of junior siblings. We examined the hypothesis in a field experiment where pied flycatchers, Ficedula hypoleuca, were video filmed while provisioning synchronous and asynchronous broods. Prey size and load size increased with nestling age suggesting that larger prey were more profitable to bring to the nest than smaller prey. Although small nestlings had difficulty swallowing large prey, parents of asynchronous broods brought large items suitable for the size of the older nestlings. Hence, junior nestlings suffered from reduced growth and increased mortality. [Copyright &y& Elsevier]

1. The UK population of yellowhammers has declined since the mid-1980s. Concurrent increases in the use of pesticides are believed to have reduced the availability of food resources for farmland birds, including yellowhammers. To mitigate the consequences of insecticide applications on yellowhammer productivity, the relationships between insecticide application, arthropod food abundance and breeding success require quantification. 2. We studied nesting yellowhammers on a lowland arable farm in North Yorkshire between 2001 and 2003, to examine the effects of food abundance on breeding success and the effects of insecticide on food abundance. Arthropod abundances around individual nests were sampled and the timing and location of insecticide applications were recorded. 3. Nestling condition and mass on day 6 after hatching were positively correlated with the abundance of arthropods important in the diet of nestling yellowhammers. Greater mean body mass and condition corresponded with a lower incidence of brood reduction. 4. The abundance of arthropods important in the diet of nestling yellowhammers increased between mid-May and the end of July. However, arthropod samples collected within 20 days of an insecticide application did not show this seasonal increase in abundance and were depressed at levels likely to affect yellowhammer breeding performance adversely. 5. Synthesis and applications. We have demonstrated how insecticide applications can depress yellowhammer breeding productivity. We provide the requisite data for a framework that enables predictions to be made about the probable population effects of particular pesticide products. If the risk of indirect effects can be predicted accurately then appropriate mitigation and compensation measures could be incorporated into pesticide regulatory procedures and/or agri-environment schemes. [ABSTRACT FROM AUTHOR]

1. We examined the influence of helpers on reproductive success in the apostlebird using data gathered over three breeding seasons (116 nests, 34 breeding groups). Initially, annual productivity and individual components of reproductive success were examined in relation to group size (two to 17 birds). The effect of helping per se was then examined in a reduced data set by comparing (a) the predictive power of group size and actual helper numbers, and/or (b) the productivity of groups that changed in size between nests or seasons. 2. Group size was positively and linearly related to annual fledgling production and the number of offspring that survived to the following season. This was attributed to the presence of helpers, as the number of birds that fed nestlings, but not group size, was significantly related to annual fledging success. Neither group size nor helper number influenced fledgling survival over the subsequent winter. 3. The probability of re-nesting after a successful first brood increased with group size. Among double-brooded groups, the interval between nests was inversely related to group size. 4. Clutch size increased linearly with group size among groups with two to five birds, but was not related to group size among larger groups. A matched comparison of groups that changed size between years further suggests that clutch size was facultatively adjusted to group size, a phenomenon rarely reported in species with non-breeding helpers. 5. Hatching success was not related to group size or the number of birds participating in incubation. 6. The number of birds provisioning the brood was a better predictor of feeding rate per chick than group size, and was significantly and inversely related to the incidence of nestling starvation. However, there was no relationship between group size and fledgling body mass. 7. Fledging success per brood increased linearly among groups with three to five members. The only case of pair-breeding was unsuccessful, suggesting that this species is one of only a few birds that may be obligately cooperative. 8. We found no evidence that helpers influenced the likelihood of nest predation, or the probability of group members surviving to the following season. [ABSTRACT FROM AUTHOR]

Female birds of several species have control over the production of daughters and sons. However, most studies failed to find a relationship between egg size and sex. This is intriguing as adjustment of egg size would constitute a powerful tool for the female to meet different resource demands of the sexes, particularly in size dimorphic species. Our results show that, within clutches of black-headed gulls ( Larus ridibundus) the proportion of males was positively associated with egg mass. This applied for all three laying positions, independently of the absolute egg mass. There was a significant relationship between the distribution of the sexes over the laying sequence and the egg mass change. When egg mass decreased over the sequence, first-laid eggs were male biased and last-laid eggs female biased, and vice versa. The potential adaptive value of this allocation strategy is evaluated with regard to male sensitivity to egg quality and competitive differences between the sexes. [ABSTRACT FROM AUTHOR]

Lesser kestrels (Falco naumanni) lay clutches which appear excessiveas only 3% of them yield as many young as eggs laid. Four hypothesesmay explain the adaptive value of producing surplus eggs: (1) the bet-hedging hypothesis assumes that the environment varies unpredictably and surplus eggs serve to track uncertain resources; (2) the ice-box hypothesis suggests that surplus offspring serve as a reserve food during a period of shortage; (3) the progeny choice hypothesis says that parents produce surplus offspring in order to choose these with higher fitness; and (4) the insurance-egg hypothesis proposes that extra eggs are an insurance against the failure of any egg. To test the significance of this strategy in the lesser kestrel, an experiment manipu-lating brood size at hatching was carried out over 2 years, withgood and bad feeding conditions. The experiment consisted of adding a chick to experimental broods where one egg failed to hatch or removing a randomly selected chick from experimental broods where all eggshad hatched. Independently of annual food availability, pairs with brood sizes reduced by one chick fledged more nestlings than pairs with brood size equalling their clutch sizes. Body condition of young was also better in the former group, but only in 1993 (a high-food year). Independently of year, mean local survival of parents with complete broods at hatching was lower than for parents raising reduced broods. These results supported only the insurance-egg hypothesis which says that surplus eggs may be an insurance against the failure of any egg, but parents may suffer reproductive costs when all eggs hatch. [ABSTRACT FROM AUTHOR]