Maternal Condition Affects the Offspring Testosterone Concentration in Early Nestling Period: An Experimental Test on Tawny Owls

This study addressed the question of whether maternal condition during egg laying or the rearing environment has a greater effect on offspring testosterone levels. We tested this in field experiments on a population of tawny owls Strix aluco in Duna‐Ipoly National Park, Hungary. In the experiments with females of poor condition, when broods were supplied with extra food none of the nestlings died, whereas in control broods, which were not supplied with extra food, some nestlings did die. Large differences in testosterone levels were correlated positively with hatching order both in experimental and control broods. However, it was only in control broods that the later‐hatched nestlings with low testosterone concentrations, died. In the experiments with females of good condition, the males were removed and females and their broods were supplied with restricted amounts of food. In these broods starving nestlings, whose growth had stopped, were considered as having died and were removed from the nest and hand‐reared. In control broods all nestlings fledged, and both in experimental and control broods testosterone concentrations were more even between siblings. Both types of trials confirmed a maternal influence on offspring testosterone concentration: large between‐sibling differences in concentrations in the broods of females of poor condition, where some nestlings died, could not be reduced with increased food supply, and the more even concentrations in the broods of females in good condition, where all nestlings survived, could also not be increased by restricting the food supply.     

Equivocal Responses of Feeding Parents to Experimental Brood Sizes in a Hawk–Cuckoo Host: Brood Size as a Reference for Parental Provisioning Decisions?

The number of offspring surviving until independence is the fundamental drive in the evolution of parental care. Because of the related costs, parental investment must be balanced with essential resources for parents themselves, among the resources available in the environment under the current parental condition. It is advantageous for parents to adjust their level of investment to the number of offspring; however, there is little evidence that parents employ numerical competence in adjusting their investment level. We investigated how parents respond to experimentally manipulated brood sizes in a passerine species, known as a host of a brood parasitic cuckoo whose chicks presumably deceive their hosts numerically. Parents reduced their provisioning to broods of reduced sizes, whereas parents did not increase provisioning to enlarged broods compared to that in the control condition. These parental responses can be attributed to the response of chicks to the experimental treatments compared to that in the control: chicks lowered begging intensity in the reduced condition, while they did not intensify being in the enlarged condition. Further analyses revealed that eagerness of parents to respond to chick begging intensity differed between the experimental treatments: strong parental response was detected toward begging chicks only in the reduced condition. We propose that the detected equivocality of parental responses might be related to the difference in the number of chicks between the unmanipulated and experimentally manipulated broods, the former reflecting the initial parental decision on the amount of resources to allocate to the brood.     

Maternal investment in a spider with suicidal maternal care, Stegodyphus lineatus (Araneae, Eresidae)

Providing parental care is costly for the parent, but generally beneficial for the young whose survival, growth and reproductive value can be increased. Selection should strongly favour an optimal distribution of parental resources, depending on the relationship between the costs and benefits for parents and their offspring. Parental care is characterized by trade offs in investment, for example between egg size and number of young or providing resources at the egg stage versus the post-hatching stage. Females of the spider Stegodyphus lineatus (Eresidae) produce a single small brood with small eggs and provide the young with regurgitated fluid and later, with their body contents via matriphagy. We asked whether females adjust the investment of resources differentially into eggs, regurgitation feeding and matriphagy, and how maternal investment affects the size of the young at dispersal. We followed the growth of young of broods in the lab and in the field and manipulated brood size in order to determine the pattern of resource allocation. We found that brood size was positively correlated with body mass: larger females had larger broods. Females provided 95% of their body mass to the young, allocating more resources to regurgitation than to matriphagy. Females provided regurgitated food to the young according to the brood size, providing less food when the brood was reduced. Maternal resources had a large influence on offspring mass at dispersal, which is likely to affect their future fitness. The study shows the importance of the female's body mass and her resource allocation decisions for her reproductive outcome.     

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.     

Sex ratio in relation to timing of breeding, and laying sequence in a dimorphic seabird

When the cost of rearing sons and daughters differs and the subsequent survival and reproductive success of one sex is more dependent than the other, on the amount of parental investment, adult females tend to produce more chicks of the more dependent sex if the females are in good condition themselves. One method of varying the total investment in each sex is through modifying the sex ratio of offspring produced. This study shows that in broods of European Shags Phalacrocorax aristotelis , the sex ratio varied with laying date. Presumably in this species, the lifetime reproductive success of males is more dependent on the level of parental investment. Early breeders are in better condition, the brood sex ratio of early broods was male biased (0.63), while that of late broods was female biased (0.36). The overall difference in sex ratio found between early and late nests could be attributed to manipulation of sex in the first laid egg. In early broods, 77% of the first hatched chicks were male but only 30% of the first hatched chicks in late broods were male. The sex combination of the first two chicks in a brood significantly affected growth as measured by asymptotic mass.     

Pair bond duration influences paternal provisioning and the primary sex ratio of brown thornbill broods

Parents should vary their level of investment in sons and daughters in response to the fitness costs and benefits accrued through male and female offspring. I investigated brood sex ratio biases and parental provisioning behaviour in the brown thornbill, Acanthiza pusilla, a sexually dimorphic Australian passserine. Parents delivered more food to male-biased than female-biased broods. However, factors determining parental provisioning rates differed between the sexes. Female provisioning rates were related to brood sex ratio in both natural and experimental broods with manipulated sex ratios. In contrast, male provisioning rates were not affected by brood sex ratio in either natural or experimental broods. However, males in established pairs provisioned at a higher rate than males in new pairs. Data on the sex ratio of 109 broods suggest that female brown thornbills adjust their primary sex ratio in response to pair bond duration. Females in new pairs produced broods with significantly fewer sons than females in established pairs. This pattern would be beneficial to females if the costs of rearing sons were higher for females in new than established pairs. This may be the case since females in new pairs provisioned experimental all-male broods at elevated rates. The condition of nestlings also tended to decline more in these all-male broods than in other experimental broods. This will have additional fitness consequences because nestling mass influences recruitment in thornbills. Female thornbills may therefore obtain significant fitness benefits from adjusting their brood sex ratio in response to the status of their pair bond. Copyright 2002 The Association for the Study of Animal Behaviour. Published by Elsevier Science Ltd. All rights reserved.     

Brood reduction in the Red-necked Grebe Podiceps grisegena

Brood reduction in Red-necked Grebes Podiceps grisegena breeding on fish ponds in south-eastern Poland occurred either through the desertion of the last-laid eggs after partial hatching of the clutch and/or the selective starvation of the smallest chicks. Abandonment of unhatched eggs was not influenced by the number of young already hatched or by the breeding date, but it was more likely in larger clutches and in families suffering chick starvation. Chicks from the largest broods had a higher probability of survival until fledging than those from single-chick broods. Larger chicks obtained food more successfully through better positioning during food delivery. In families that did not suffer brood reduction, chicks were better provisioned with food than in reduced broods. Although allocation of food among chicks in reduced broods was more skewed to the disadvantage of the younger siblings, dominant chicks obtained less food prior to brood reduction than dominant siblings in unreduced broods. Sibling aggression did not differ between unreduced and reduced broods before death of the weakest chicks. Post-laying adjustment of the number of offspring to prevailing feeding conditions occurred at two stages: by parental manipulation of the number of hatched eggs at the time when parents and chicks leave the nest and by competition between chicks. It is suggested that late egg desertion may be an adaptive mechanism of brood-size adjustment, when elimination of the weakest chicks through sibling competition is not very efficient.     

No effect of parental quality or extrapair paternity on brood sex ratio in the blue tit (Parus caeruleus)

Sex allocation theory predicts that parents should manipulatebrood sex ratio in order to maximise the combined reproductivevalue of their progeny. Females mating with high quality malesshould, therefore, be expected to produce brood sex ratiosbiased towards sons, as male offspring would receive a relativelygreater advantage from inheritance of their father's characteristicsthan would their female siblings. Furthermore, it has been suggested that sex allocation in chicks fathered through extrapair fertilizations should also be biased towards sons. Contraryto these predictions, we found no evidence that the distributionof sex ratios in a sample of 1483 chicks from 154 broods ofblue tits (Parus caeruleus) deviated significantly from thatof a binomial distribution around an even sex ratio. In addition,we found no significant effect on brood sex ratio of the individualquality of either parent as indicated by their biometrics, feather mite loads, time of breeding, or parental survival. This suggeststhat females in our population were either unable to manipulateoffspring sex allocation or did not do so because selectionpressures were not strong enough to produce a significant shiftaway from random sex allocation. The paternity of 986 chicks from 103 broods was determined using DNA microsatellite typing.Extrapair males sired 115 chicks (11.7%) from 41 broods (39.8%).There was no significant effect of paternity (within-pair versusextrapair) on the sex of individual offspring. We suggest that,in addition to the weakness of selection pressures, the possiblemechanisms responsible for the allocation of sex may not besufficiently accurate to control offspring sex at the levelof the individual egg.     

Experimental mate-removal increases the stress response of female house sparrows: the effects of offspring value?

Vertebrates secrete elevated levels of glucocorticoids in response to various stressors, which mobilize energetic reserves but concurrently interfere with reproduction. In accordance with life-history theory, recent evidence suggests that the corticosterone response to stress is modulated according to the value of the brood. Since brood value is positively related to parental care, the stress response modulation may be either the consequence of offspring value (e.g. large broods have high fitness potential - the brood value hypothesis) or the consequence of parental workload (e.g. large broods are energetically demanding for the parents - the workload hypothesis). In this experiment, we aimed at experimentally separating the effects of brood value and workload and to confront the latter two hypotheses. To do so, we captured the male parents from breeding pairs of house sparrows (Passer domesticus) and took them in captivity for 48 h. During the absence of males, mate-removed females made more food deliveries than controls (increased workload) but were unable to fully compensate the lack of their mate, thus their chicks were in worse condition (reduced brood value) than control chicks. After the experimental period, mate-removed females responded more strongly to the standardized stressor than controls. In both groups, the corticosterone response to stress was negatively related to the nestlings' mass gain. These results provide experimental support for the brood value hypothesis, i.e. that individuals may actively modulate their stress response (either down- or upwards) with respect to the value of their current reproduction.     

EVOLUTION OF OBLIGATE SIBLICIDE IN BOOBIES. 2: FOOD LIMITATION AND PARENT–OFFSPRING CONFLICT

Proximate limitation on parental food delivery has long been invoked to explain the evolution of single-chick broods of pelagic seabirds such as masked boobies (Sula dactylatra). A second possible proximate limit on brood size is siblicide driven by genetic parent–offspring conflict (POC) over brood size, if siblicidal offspring can reduce brood size to one even if the parents' optimal brood size is greater than one. I tested these two hypotheses by experimentally suppressing obligate siblicide in masked booby broods and comparing breeding parameters of these broods with unmanipulated single-chick control broods. Per capita mortality rate of experimental nestlings was higher than that of controls, but this deficit was more than made up by larger brood size. Parents of experimental broods brought more food to offspring, had higher fledging success, and apparently incurred no additional major short-term cost of reproduction, relative to parents of control broods, thus refuting the food limitation hypothesis. Estimates of inclusive fitness of chicks in experimental broods were higher than were those of control nestlings, a result inconsistent with the POC hypothesis that the siblicidal offspring's optimal brood size is one while the parents' optimum is greater than one. This discrepency between natural brood size and apparent brood size optima might be resolved in several ways: experimental artifacts may give misleading estimates of optimal brood size; experimental and control offspring may have different reproductive values at the time of fledging; nestling masked boobies may face a special frequency-dependent case of POC in which the high risk of sharing a nest with a siblicidal sibling makes invasion of other behavioral genotypes difficult even when offspring and parent inclusive fitnesses are higher from a nonsiblicidal brood of two than from a brood of one.     

Nestling immunocompetence and testosterone covary with brood size in a songbird

The social and ecological conditions that individuals experience during early development have marked effects on their developmental trajectory. In songbirds, brood size is a key environmental factor affecting development, and experimental increases in brood size have been shown to have negative effects on growth, condition and fitness. Possible causes of decreased growth in chicks from enlarged broods are nutritional stress, crowding and increased social competition, i.e. environmental factors known to affect adult steroid levels (especially of testosterone and corticosteroids) in mammals and birds. Little, however, is known about environmental effects on steroid synthesis in nestlings. We addressed this question by following the development of zebra finch (Taeniopygia guttata) chicks that were cross-fostered and raised in different brood sizes. In line with previous findings, nestling growth and cell-mediated immunocompetence were negatively affected by brood size. Moreover, nestling testosterone levels covaried with treatment: plasma testosterone increased with experimental brood size. This result provides experimental evidence that levels of circulating testosterone in nestlings can be influenced by their physiological response to environmental conditions.     

Nest intrusions, infanticide, and parental care in the burying beetle, Nicrophorus orbicollis (Coleoptera: Silphidae)

Duration of paternal care in the burying beetle Nicrophorus orbicollis Say is highly variable. Both parents bury and defend mouse-sized vertebrate carcasses as food resources for their offspring, but males abandon their broods several days before females. Nests defended by single female parents were taken over by aggressive conspecifics in live of nine cases, whereas only six of 16 nests defended by both parents were taken over. In the event of a takeover, the intruding beetle replaced the resident beetle of the same sex, destroyed any eggs that were present, and paired with the remaining resident to produce a new clutch. Broods raised by usurpers following takeovers were less successful than broods raised by initial residents on unused carcasses. The majority of takeovers occurred 35 days after carcass burial. The occurrence of nest intrusions by conspecifics did not significantly influence duration of male parental care; when conspecific intruders were excluded from nests males remained with their broods (± S.E.) 11·2 ± 0·8 days ( n = 15), and when intruders were added to nests males remained with their broods 12·2 ± 0·6 days ( n = 8). Conflict for carcasses intensified in response to larger brood mass, but duration of male care was unaffected by brood mass. Overall. brood mass and the presence or absence of intruders explained only 5% of the variance associated with brood abandonment by males.     

Delayed benefits of paternal care in the burying beetle Nicrophorus vespilloides

Burying beetles, Nicrophorus spp., inter the carcasses of small vertebrates as a food source for their offspring. Females can bury a carcass and rear a brood on it alone, but are frequently assisted by a male whose presence reduces the risk of the carcass being taken over by other beetles. However, the male often stays for longer than the carcass is vulnerable to take-over, and he cares for the brood without conferring any further benefits on it. In a laboratory experiment using N. vespilloides, we found that, in the absence of competitors, male assistance conferred no advantages on the brood for which he was caring, but significantly increased the subsequent reproductive success of his mate, in terms of the mass and rate of development of a second brood, reared alone. We suggest that this is due to a reduced parental effort of assisted females, who spent less time feeding offspring and more time resting than unassisted females whilst rearing their first broods. In the field, a female is unlikely to pair with the same male for consecutive broods, so we discuss the possible benefits a male may accrue from increasing his mate's reproductive success. We also discuss the relevance of these results to our understanding of the evolution of biparental care in birds. Copyright 2000 The Association for the Study of Animal Behaviour.     

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.     

Ecological constraints on breeding system evolution: the influence of habitat on brood desertion in Kentish plover

1. One of the fundamental insights of behavioural ecology is that resources influence breeding systems. For instance, when food resources are plenty, one parent is able to care for the young on its own, so that the other parent can desert and became polygamous. We investigated this hypothesis in the context of classical polyandry when females may have several mates within a single breeding season, and parental duties are carried out largely by the male. 2. We studied a precocial wader, the Kentish plover Charadrius alexandrinus, that exhibits variable brood care such that the chicks may be raised by both parents, only by the female or, more often, only by the male. The timing of female desertion varies: some females desert their brood at hatching of the eggs and lay a clutch for a new mate, whereas other females stay with their brood until the chicks fledge. Kentish plovers are excellent organisms with which to study breeding system evolution, as some of their close relatives exhibit classical polyandry (Eurasian dotterel Eudromias morinellus, mountain plover Charadrius montanus), whereas others are polygynous (northern lapwing Vanellus vanellus). 3. Kentish plovers raised their broods in two habitats in our study site in southern Turkey: saltmarsh and lakeshore. Food intake was higher on the lakeshore than in the saltmarsh as judged from feeding behaviour of chicks and adults. As the season proceeded and the saltmarsh dried out, the broods moved toward the lakeshore. 4. As the density of plovers increased on lakeshore, the parents spent more time defending their young, and female parents stayed with their brood longer on the lakeshore. 5. We conclude that the influence of food abundance on breeding systems is more complex than currently anticipated. Abundant food resources appear to have profound implications on spatial distribution of broods, and the social interactions between broods constrain female desertion and polyandry.     

Cross‐fostering reveals that among‐brood differences in ornamental mouth coloration mostly reflect rearing conditions in nestling house sparrows

Dependent offspring use specialized traits to attract parental care. In birds, this includes morphological ornaments (e.g. colourful plumage or mouthparts) that are associated with nestling condition and shape the allocation of parental care. Ornament expression often differs among broods, even after differences in individual condition are accounted for statistically. Understanding how this variation arises is important for understanding the information content of these signals, their functional importance, and their evolution. The present study used a cross‐fostering experiment to assess the relative contributions of parental effects to among‐brood differences in the mouth coloration of nestling house sparrows, specifically the carotenoid‐richness, overall brightness, and ultraviolet (UV) coloration of rictal flanges. The expression of carotenoid‐based coloration was explained by synchronous breeding, nest‐of‐rearing and nest‐of‐origin. Brightness and relative UV intensity, however, were explained only by synchronous breeding, and there was substantial unexplained variation in all three colour parameters. Among‐brood variation in mouth coloration, then, may primarily contain information about the environment in which offspring are reared. At the individual level, ontogenetic changes in the carotenoid‐richness and brightness of flanges positively reflected mass gain (a proxy for food intake). Larger and yellower chicks gained more mass, consistent with parental preferences for these traits. © 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, 106 , 169–179.     

Parental and chick responses of Laughing Gulls (Leucophaeus atricilla) to increase of flight costs and brood size

One main line of thought in life history theory is that investment in offspring must be balanced to minimize negative impacts on adult survival and future breeding. Seabirds have been regarded as fixed investors, although they exhibit a whole gradient of life history traits. We studied the consequences of brood size (one and three chicks) and of increased flight costs to one mate of a pair (3- and 5-cm trimming of the edge of the primary feathers) on parental response and on survival and body condition of chicks of Laughing Gulls (Leucophaeus atricilla). Parent gulls modified their nest attendance when their mate was handicapped, in a pattern dependent on the sex of the latter. Trimming of males affected chicks more severely than that of females. On its part, brood size affected amount of feeding sessions. Both chick body condition and survival were negatively affected by larger broods and by increased flight costs of one of their parents, especially when it was the male. Overall, parental inversion exhibited adjustments depending on the requirements of the brood and the fact that males compensated better the increased flying costs of their mates than vice versa. Despite a certain capacity by the males to compensate for the increased flight costs of the females, compensation was insufficient, and much less so in females, especially in larger broods, affecting chicks’ body condition and survival.     

Differential maternal testosterone allocation among siblings benefits both mother and offspring in the zebra finch Taeniopygia guttata

Parents are selected to preferentially invest in the offspring with highest reproductive value. One mechanism for achieving this is the modification of competitive asymmetries between siblings by maternal hormones. In many organisms, offspring value varies according to birth position in the brood, which determines survival chances and competitive advantage over access to resources. In birds, variation in yolk androgen allocation over the laying sequence is thought to modulate dominance of senior chicks over junior brood mates. We tested this hypothesis in zebra finches, which show a naturally decreasing pattern of within-clutch testosterone allocation. We abolished these within-clutch differences by experimentally elevating yolk testosterone levels in eggs 2-6 to the level of egg 1, and we assessed fitness measures for junior offspring (eggs 2-6), senior offspring (egg 1), and their mothers. Testosterone-injected eggs hatched later than control eggs. Junior, but not senior, chicks in testosterone-treated broods attained poorer phenotypic quality compared to control broods, which was not compensated for by positive effects on seniors. Mothers were generally unaffected by clutch treatment. Thus, naturally decreasing within-clutch yolk testosterone allocation appears to benefit all family members and does not generally enhance brood reduction by favoring senior chicks, in contrast to the widely held assumption.     

Paternal influence on growth and survival of dark-eyed junco young: do parental males benefit?

Recent studies of typically monogamous passerine birds have suggested that the fitness benefits males derive by caring for their young may not be as great as was previously thought. This study was conducted to determine whether parental care by male dark-eyed juncos, Junco hyemalis, serves to increase either the quantity or quality of young that they produce. Over a 4-year period, males were caught at the time their eggs hatched, and the subsequent growth and survival of the young of unaided females and control pairs were compared. Broods raised by unaided females gained body mass more slowly and fledged at slightly lower mass than those raised by two parents. However, fledging mass was not correlated with survival to independence. There were no differences in tarsus growth between the two treatment groups. Entire brood loss to predators occurred as often among females without male help as it did among those with male help. However, partial brood loss was more common among female-only broods than among controls; this difference was largely attributable to higher rates of starvation and exposure in female-only broods. There appeared to be an interaction between growth and predation. Female-only broods that were below the median mass of combined treatment groups at 5 days of age were more likely than all other broods to experience partial or complete predation. Male impact on offspring survival varied with age of the offspring. When years were combined, males tended to increase survival during the first half of the nestling period, but their impact at the time of nest-leaving was minimal. In all years, from nest-leaving to independence (ca. 2 weeks), broods without male help survived only about half as well as did those with male help. Independent young raised by one parent were as likely to return the following spring as were young raised by two parents. Thus, paternal care benefits males by improving the survivorship of their fledglings, and may also act as a buffer against poor female parental quality and inclement weather. However, the magnitude of these benefits is such that bigamous males might achieve greater reproductive success than monogamous males. Various possible male strategies are discussed.     

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.