Asynchronous hatching in the laughing gull: Cutting losses and reducing rivalry

The adaptive significance of hatching asynchrony was investigated in the laughing gull (Larus atricilla). Staggered hatching of the brood is frequently correlated with reduced survival in the later-hatching chicks, and the phenomenon has therefore been the subject of speculation about adaptive function. The relative timing of hatching within the brood was experimentally manipulated in order to compare the effects of synchrony versus asynchrony on parental reproductive success. The average number of young fledging per nest was significantly higher in the asynchronous group. Staggered hatching of the brood is hypothesized to be a parental strategy offering the option of brood reduction under conditions of food shortage. The possibility is discussed that asynchronous hatching benefits parents independently by reducing sibling rivalry over parental investment and minimizing wasteful competition.     

Complex patterns of food allocation in asynchronously hatching broods of crimson rosellas

In birds, asynchronous hatching typically leads to lower growth and survival of last-hatched chicks. However, all crimson rosella Platycercus elegans, chicks grow at the same rate, although first-hatched chicks can be as much as seven times heavier than last-hatched chicks at the end of hatching. We examined the delivery and distribution of food to 18 rosella broods by videotaping feeds and simultaneously recording mass changes in the nestbox using a digital balance. Parents visited the nest infrequently and delivered loads of up to 25% of their body weight during a feeding visit. Male rosellas consistently delivered larger loads and consequently had higher feeding rates (g/h) than females. Parents distributed food between chicks by direct regurgitation in a series of up to 51 food transfers. Overall, chicks of all hatching ranks received equal numbers of transfers, but parents differed in how they distributed food within the brood. Males fed first-hatched chicks more than last-hatched chicks, whereas females distributed food equally to all chicks. Selective feeding of small chicks might be costly to females since they delivered food more slowly than males and spent more time in the nestbox. Thus female rosellas may invest more in current reproduction than males. Parents also distributed food differently to male and female chicks. Large males were fed more than all other nestlings, while female nestlings were fed equally irrespective of size. This study confirms that complex patterns of parental allocation occur in wild populations. Copyright 1999 The Association for the Study of Animal Behaviour.     

Why does the herring gull lay three eggs?

Food supplements placed daily beside the nests of herring gulls, Larus argentatus, for the first 5 days after the first chick hatched produced improved weight gains over this initial period and higher fledging success, particularly in the third chick. The fledging success of the fed group appears to be due to increased weight gain and not to increased parental protection in the supplemented period. Since there is indirect evidence that food is available this suggests that the parents are putting less effort into foraging for their chicks than they are able to, and less than is in the interests of the third chick, in the first days after hatching. On a separate colony we found that having three chicks in the brood for more than 5 days resulted in lower weight gains for the second chick, but not the first. We suggest that fledging three chicks rather than one or two greatly increases the parents' reproductive effort, and consequently interpret the third egg as primarily insurance against the loss of the first or second.     

Food allocation in crimson rosella broods: parents differ in their responses to chick hunger

Food allocation in many asynchronously hatching bird species favours large, competitively superior chicks. In contrast, food is usually distributed equally within broods of crimson rosellas, Platycercus elegans, implying that parents do not simply feed the most competitive chick. We used two temporary removal experiments to manipulate hunger of: (1) individual first- or last-hatched chicks, or (2) the whole brood. When only a single chick was hungry, parents compensated fully and chicks gained the same mass over the day as during controls. Mothers and fathers, however, responded in different ways to chick hunger. Mothers did not strongly alter their food allocation when a single chick was hungry, and controlled the distribution of food by refusing to feed first-hatched chicks when they were hungry and by moving more during feeds. In contrast, fathers allocated more food to hungry last-hatched chicks. When the whole brood was hungry, parents were unable to compensate chicks and all chicks lost mass over the day. In these conditions, mothers preferentially fed first-hatched chicks, while fathers fed all chicks equally. Our results show that both mothers and fathers were able to discriminate and selectively feed chicks, but that parents responded differently to changes in chick hunger within the brood. Fathers responded more strongly to variation in chick hunger within the brood, suggesting they reallocate food based on short-term changes in hunger. Mothers distributed food preferentially to last-hatched chicks except when the whole brood was hungry, when they switched to favouring first-hatched chicks. This pattern is consistent with a strategy of adaptive brood reduction when food is scarce. Copyright 2000 The Association for the Study of Animal Behaviour.     

Overproduction in the Lesser Black‐backed Gull – can marginal chicks overcome the initial handicap of hatching asynchrony?

In response to unpredictability of both food availability and core offspring failure, parents of many avian species initially produce more offspring than they commonly rear (overproduction). When parental investment is insufficient to raise the whole brood the handicap of hatching last means ‘marginal’ chicks are less likely to survive if brood reduction occurs. Conversely, if marginal offspring are required as replacements for failed ‘core’ chicks, or parental investment is sufficient to rear the whole brood, the handicap imposed on marginal chicks must be reversible if overproduction is to be a viable strategy. I investigated the ability of marginal offspring to overcome the handicap imposed by hatching asynchrony using a combination of a field experiment, designed to manipulate both the amount of total competition and the relative competitive ability of chicks within a brood, and data on the growth and survival of unmanipulated, three‐chick broods from three consecutive years. The results indicate that, even when resources are abundant, marginal offspring do not begin to overcome the competitive handicap imposed by hatching asynchrony until the period of growth when energetic requirements reach their peak, and subsequent survival to fledging is almost assured. This is apparently a consequence of parents controlling allocation of early parental investment, so that any brood reduction ‘decisions’ can be left as late as possible. Marginal chicks initially channel resources into maintaining mass, relative to skeletal size, as a buffer against starvation. However this also means competitiveness is reduced, so if conditions are poor marginal chicks are rapidly out‐competed, lose condition and die. Conversely, when food availability is good marginal offspring devote more resources to skeletal growth and quickly close the gap on their core siblings, meaning the handicap is reversible. The benefits of overproduction and hatching asynchrony as reproductive strategies to maximise success in Lesser Black‐backed Gulls are discussed in relation to the reproductive alternatives.     

Family dynamics through time: brood reduction followed by parental compensation with aggression and favouritism

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.     

Brood sex ratio in the Kentish plover

How and why do the mating opportunities of males and femalesdiffer in natural population of animals? Previously we showedthat females have higher mating opportunities than males inthe Kentish plover Charadrius alexandrinus. Both parents incubatethe eggs, and males provide more brood care than females; thusit is not obvious why the females find new mates sooner thanthe males. In this study we investigated whether the sex-biasedmating opportunities stem from biased offspring sex ratios.We determined the sex of newly hatched, precocial chicks usingCHD gene markers. Among fully sexed broods, 0.461 ± 0.024(SE) of chicks (454 chicks in 158 broods) were male, and thissex ratio was not significantly different from unity. The proportionof males at hatching decreased significantly over the breedingseason, which occurred consistently in all 3 years of the study.Large chicks were more likely to be males than females. Neitherparental age nor body size of male and female parents was relatedto brood sex ratio. We also sexed a number of chicks that werecaught after they left their nest (range of estimated ages 0–17days) and found that the proportion of males increased withbrood age. This relationship remained highly significant whencontrolling statistically for hatching date. As brood size decreaseddue to mortality after the chicks left their nest, these resultssuggest that the mortality of daughters was higher than thatof the sons shortly after hatching. Taken together, our resultsshow that the female-biased mating opportunities in the Kentishplover are not due to biased brood sex ratio at hatching but,at least in part, are due to female-biased chick mortality soonafter hatching.     

Asynchronous hatching in the burying beetle,Nicrophorus quadripunctatus,maxmizes parental fitness

Life history theory predicts that natural selection favours parents who balance investment across offspring to maximize fitness. Theoretical studies have shown that the optimal level of parental investment from the offspring's perspective exceeds that of its parents, and the disparity between the two generates evolutionary conflict for the allocation of parental investment. In various species, the offspring hatch asynchronously. The age hierarchy of the offspring usually establishes competitive asymmetries within the brood and determines the allocation of parental investment among offspring. However, it is not clear whether the allocation of parental investment determined by hatching pattern is optimal for parent or offspring. Here, we manipulated the hatching pattern of the burying beetle Nicrophorus quadripunctatus to demonstrate the influence of hatching pattern on the allocation of parental investment. We found that the total weight of a brood was largest in the group that mimicked the natural hatching pattern, with the offspring skewed towards early hatchers. This increases parental fitness. However, hatching patterns with more later hatchers had heavier individual offspring weights, which increases offspring fitness, but this hatching pattern is not observed in the wild. Thus, our study suggests that the natural hatching pattern optimizes parental fitness, rather than offspring fitness.     

Brood Reduction in White Storks Mediated through Asymmetries in Plasma Testosterone Concentrations in Chicks

We hypothesized that increasing chick plasma testosterone concentrations, transmitted from the mothers via their eggs, enhances survival of their offspring and that the fitness of the young, depending on the maternal hormones, is influenced by parental quality. To test our hypotheses we distinguished the broods of white storks Ciconia ciconia L. where chicks died and those where all chicks survived. We analysed the plasma testosterone concentrations in the chicks, the ability of the chicks to be first to receive food and the mass of chicks before fledging in relation to their hatching order and recorded the body mass of parents and food mass delivered by them.
Female storks used the asymmetries in testosterone concentrations within a brood to control brood size and adjusted the number of young hatched to match the parental ability to rear offspring. Females of poor condition altered the testosterone concentrations to produce large differences between the chicks: The first-hatched chicks, which had high plasma testosterone levels, responded faster to the feeding parent and received more food than did their younger siblings. One or two later-hatched chicks, which had lower testosterone levels, died in these broods. Females in good condition produced small differences in testosterone concentrations between the chicks and all chicks survived in their brood. Chicks that were raised by the females of poor condition in reduced broods were heavier than chicks that were raised by females of good condition in broods where all chicks survived.
We suggest that the control of brood size by testosterone concentration, transmitted by the mother to the chicks, is a hormonal means of condition-dependent reproductive strategy in the white stork.     

Brood division in robins

Broods of fledgling robins Erithacus rubecula are sometimes divided between their parents so that each parent feeds only some of the chicks. These associations between a parent and certain of its young (‘family units’) are shown to be stable over periods of days. Brood division is most frequent in broods that are not followed by another nesting attempt. Experimental manipulation of the food supply suggests that brood division is relaxed when food is readily available. An analysis of the interactions between parents and chicks demonstrates that both adults and fledglings play a role in brood division. The possible functions of brood division are discussed with reference to ‘other types of care’, ‘parental efficiency’, ‘cheat countermeasure’ and ‘two types of chick’ hypotheses. Male parents feed chicks with shorter winglengths than do females: since females tend to have shorter winglengths than males, this suggests that chicks are cared for by parents of the opposite sex. Possible causes and consequences of this observation are discussed.     

Hatching Asynchrony Decreases the Magnitude of Parental Care in Domesticated Zebra Finches: Empirical Support for the Peak Load Reduction Hypothesis

Parent–offspring conflict over the supply of parental care results in offspring attempting to exert control using begging behaviours and parents attempting to exert control by manipulating brood sizes and hatching patterns. The peak load reduction hypothesis proposes that parents can exert control via hatching asynchrony, as the level of competition amongst siblings is determined by their age differences and not by their growth rates. Theoretically, this benefits the parents by reducing both the peak load of the offspring's demand and their overall demand for food and benefits the offspring by reducing the amplification of their competition. However, the peak load reduction hypothesis has only received mixed support. Here, we describe an experiment where we manipulated the hatching patterns of domesticated zebra finch Taeniopygia guttata broods and quantified patterns of nestling begging and parental feeding effort. There was no difference in the begging intensity of nestlings raised in asynchronous or experimentally synchronous broods, yet parental feeding effort was lower when provisioning asynchronous broods and particularly so when levels of nestling begging were low. Further, both parents acted in unison, as there was no evidence of parentally biased favouritism in relation to hatching pattern. Therefore, our study provided empirical support for the prediction that hatching asynchrony reduces the feeding effort of parents, thereby providing empirical support for the peak load reduction hypothesis.     

Darwin's Finch Begging Intensity Does Not Honestly Signal Need in Parasitised Nests

Parental care should be selected to respond to honest cues that increase offspring survival. When offspring are parasitised, the parental food compensation hypothesis predicts that parents can provision extra food to compensate for energy loss due to parasitism. Chick begging behaviour is a possible mechanism to solicit increased feeding from attending parents. We experimentally manipulated parasite intensity from Philornis downsi in nests of Darwin's small ground finch (Geospiza fuliginosa) to test its effects on chick begging intensity and parental food provisioning. We used in‐nest video recordings of individually marked chicks to quantify nocturnal parasite feeding on chicks, subsequent diurnal chick begging intensity and parental feeding care. Our video analysis showed that one chick per brood had the highest parasite intensity during the night (supporting the tasty chick hypothesis) and weakest begging intensity during the day, which correlated with low parental care and rapid death. We observed sequential chick death on different days rather than total brood loss on a given day. Our within‐nest video images showed that (1) high nocturnal larval feeding correlated with low diurnal begging intensity and (2) parent birds ignored weakly begging chicks and provisioned strongly begging chicks. Excluding predation, all parasite‐free chicks survived (100% survival) and all parasitised chicks died in the nest (100% mortality). Weak begging intensity in parasitised chicks, which honestly signalled recent parasite attack, was not used as a cue for parental provisioning. Parents consistently responded to the strongest chick in both parasitised and parasite‐free nests.     

Females that experience threat are better teachers

Superb fairy-wren (Malurus cyaneus) females use an incubation call to teach their embryos a vocal password to solicit parental feeding care after hatching. We previously showed that high call rate by the female was correlated with high call similarity in fairy-wren chicks, but not in cuckoo chicks, and that parent birds more often fed chicks with high call similarity. Hosts should be selected to increase their defence behaviour when the risk of brood parasitism is highest, such as when cuckoos are present in the area. Therefore, we experimentally test whether hosts increase call rate to embryos in the presence of a singing Horsfield''s bronze-cuckoo (Chalcites basalis). Female fairy-wrens increased incubation call rate when we experimentally broadcast cuckoo song near the nest. Embryos had higher call similarity when females had higher incubation call rate. We interpret the findings of increased call rate as increased teaching effort in response to a signal of threat.     

Prolactin stress response does not predict brood desertion in a polyandrous shorebird

One of the fundamental principles of the life-history theory is that parents need to balance their resources between current and future offspring. Deserting the dependent young is a radical life-history decision that saves resources for future reproduction but that may cause the current brood to fail. Despite the importance of desertion for reproductive success, and thus fitness, the neuroendocrine mechanisms of brood desertion are largely unknown. We investigated two candidate hormones that may influence brood desertion in the Kentish plover Charadrius alexandrinus: prolactin ('parental hormone') and corticosterone ('stress hormone'). Kentish plovers exhibit an unusually diverse mating and parental care system: brood desertion occurs naturally since either parent (the male or the female) may desert the brood after the chicks hatch and mate with a new partner shortly after. We measured the hormone levels of parents at hatching using the standard capture and restraint protocol. We subsequently followed the broods to determine whether a parent deserted the chicks. We found no evidence that either baseline or stress-induced prolactin levels of male or female parents predicted brood desertion. Although stress-induced corticosterone levels were generally higher in females compared with males, individual corticosterone levels did not explain the probability of brood desertion. We suggest that, in this species, low prolactin levels do not trigger brood desertion. In general, we propose that the prolactin stress response does not reflect overall parental investment in a species where different parts of the breeding cycle are characterized by contrasting individual investment strategies.     

Food Provisioning in Red-Necked Grebes (Podiceps Grisegena) at Common Carp (Cyprinus Carpio) Ponds

Parental feeding patterns were studied in red-necked grebe (Podiceps grisegena) broods throughout the entire period of parental care in a common carp (Cyprinus carpio) fish-pond area in SE Poland in 1993–2002. Fish formed a substantial part of prey provided to the flightless young from their second week of life. Although the numbers of large invertebrates and tadpoles, the alternative prey to fish, did not decrease during the chick rearing period, grebe parents gradually shifted from delivering predominately invertebrates to delivering fish, and the average size of fish fed to chicks increased with brood age. Broods with relatively high fledging success (at least two chicks fledged) had a larger proportion of fish in their diet than broods seriously reduced because of undernourishment. The dive duration of foraging grebe parents did not differ between carp, wild fish and non-fish prey, but carp prey required significantly more time for handling. The percentage of prey rejected by chicks increased over the prefledging period from 2 to 24%. Of the prey rejected, 82% were fish apparently too large for the young to swallow. Fish prevalence in the diet of red-necked grebe chicks at carp ponds contradicts the results of other studies on the feeding habits of the nominative subspecies during breeding season. However, the red-necked grebe is a gape-limited predator and the piscivory of the chicks is limited to small-bodied fish.     

Providing chicks with extra food lowers male but not female provisioning in the House Sparrow Passer domesticus

We assessed whether adult House Sparrows Passer domesticus adjusted their provisioning in response to an experimental increase in the nutritional condition of their nestlings. When we supplemented chicks directly with additional food, male parents, but not female parents, reduced their provisioning. The results for males, but not females, run contrary to a previous experiment in this species. In addition, female provisioning was positively associated with both brood size and the age of the brood. In contrast, whereas male provisioning was positively associated with brood size, males did not increase provisioning as their chicks grew older. Males, but not females, exhibited repeatability in their provisioning. Food supplementation had a larger positive effect upon nestling survival in smaller broods than in larger broods. Overall, there appear to be fundamental differences between males and females in how decisions regarding the level of parental investment in the current brood are made.     

Parental provisioning behaviour in a flock-living passerine, the Vinous-throated Parrotbill Paradoxornis webbianus

The amount of food delivered by parents to their chicks is affected by various life history traits as well as environmental and social factors, and this investment ultimately determines the current and future fitness of parents and their offspring. We studied parental provisioning behaviour in the Vinous-throated Parrotbill Paradoxornis webbianus, a species with an unusual social system that is characterised by flock-living, weak territoriality and variable nesting dispersion. Parental provisioning rate had a positive influence on chick mass gain, suggesting that provisioning rate is an effective measure of parental investment in this species. Males and females fed nestlings at approximately the same rate, and no other carers were observed at nests. Parents coordinated provisioning rates so that they mostly fed chicks synchronously. However, the extent to which parents coordinated provisioning was associated with their social environment, synchrony being positively related to local breeding density and negatively to nearest neighbour distance. The rate at which parents provisioned nestlings showed the same relationships with social measures, being greatest at higher density and when neighbours were closer. Visit rate was also related to chick age, but not to brood size, brood sex ratio, extra pair paternity, laying date, temperature, parents’ body characters, time of day or year. We conclude that a breeding pairs’ social environment plays an important role in determining parental investment, probably through its effects on the opportunities that parents have for foraging with conspecifics.     

Juvenile helping in the moorhen,Gallinula chloropus

In a 3-year study of the moorhen, Gallinula chloropus, some chicks from first broods stayed on their natal territory once they had reached independence, and helped to rear their younger, second brood, siblings. Juvenile dispersal was constrained by habitat saturation, and first brood young were forced to stay on their natal territory. Juveniles that hatched early in the year were forced to stay longer, and helped more than those that hatched late. The total feeding rates to broods with and without juvenile helpers were the same, but parents with helpers reduced their feeding rates relative to parents without helpers. Pairs with helpers (=pairs attempting second broods) reared more chicks per nesting, attempt than pairs rearing chicks at the same time of year without helpers (=pairs attempting first clutch renests). This was true both for all clutches, and for hatched clutches only, even when controlling for parental quality, territory size and scasonal effects.     

Food availability and the male's role in parental care in double-brooded Treecreepers Certhia familiaris

The aim of this work was to examine differences in paternal and maternal care in a double-brooded, monogamous species, the Treecreeper Certhia familiaris, in relation to food availability. As a measure of parental care, we recorded the hourly feeding activity of parents when the nestlings from their first and second breeding attempts were 7 and 12 days old. Feeding frequency of the first brood increased with the age of the nestlings and also with the brood size when 12 days old. While the feeding activities of the females were similar with respect to the first and second broods, the males were less active and failed to provide any food to their nestlings in 15 cases out of 28 second broods. In spite of this, the fledglings from the second broods were heavier than those in the first. Such a pattern of male behaviour was possible without being a disadvantage to the chicks because the food supply increased during the breeding season and the female could provide food for the young alone. Thus paternal care was particularly important in times of poor food supply, i.e. during the first brood, where the extent of these males' activity in feeding the 7-day-old nestlings was positively correlated with the average mass of the nestlings. Our results support the idea that the male of monogamous, altricial bird species often makes important contributions to raising the young, especially during periods when it is difficult for the female to do so alone. Males show flexibility in their pattern of parental care, and male Treecreepers change their contribution to the first and second broods within the same season.     

Brood desertion by female shorebirds: a test of the differential parental capacity hypothesis on Kentish plovers.

The aim of this study was to examine whether the energetic costs of reproduction explain offspring desertion by female shorebirds, as is suggested by the differential parental capacity hypothesis. A prediction of the hypothesis is that, in species with biparental incubation in which females desert from brood care after hatching, the body condition of females should decline after laying to a point at which their body reserves are too low for continuing parental care. We tested this prediction on Kentish plovers (Charadrius alexandrinus) in which both sexes incubate but the females desert from brood care before the chicks fledge. We found no changes in either the body masses or body compositions of both individual male and female plovers from early incubation and throughout early chick rearing. Furthermore, the timing of brood desertion by females was not affected by their body condition. Neither did we find gender differences in the energetic costs of incubation. There were no differences in the timing of brood desertion between experimental and control females in an experiment in which we lengthened or shortened the duration of incubation by one week. These results indicate that energetic costs do not explain offspring desertion by female Kentish plovers and that the needs of chicks for parental care rather than cumulative investment by females is what determines the timing of brood desertion.