Signalling of Nutritional Need by Magpie Nestlings

The relationship between begging behaviour, chick nutritional state, and parental distribution of food within broods was studied in 4- and 5-chick magpie Pica pica broods under natural conditions. Three components of the begging display (duration, latency, and posture) were highly correlated with each other and also with the emission and duration of begging calls. Begging performance was strongly influenced by the food intake of nestlings during the preceding 1-h interval, indicating that begging may reliably reflect the nutritional need of nestlings. Daily growth during the preceding day, as well as average cumulative food intake by the brood during the preceding 24 h, seemed not to affect begging in a similar way. Begging signals employed by hungrier nestlings involved a higher degree of muscular activity, thus supporting the prediction that nestlings in greater need should employ more costly signals. Overall, those nestlings who begged more tended to obtain more food, but the relationship between feeding success and begging behaviour was weak due to a high variation between broods in the way that parents seemed to respond to variations in begging behaviour. Possible causes for this variation, and its implications for the evolution of reliable begging displays, are discussed.     

Comparison of digestive efficiency in the parasitic great spotted cuckoo and its magpie host nestlings

Altricial nestlings are under strong selection pressures to optimize digestive efficiency because this is one of the main factors affecting nestling growth and survival. Bird species vary in their ability to assimilate different nutrients and current theory predicts that nestlings should also be able to adjust their nutritional physiology to feeding frequency. Variation in parental provisioning to nestlings would select for flexibility in nestling digestive physiology, which would allow maximization of nutrient assimilation. In the present study, by making use of a brood parasite–host study system in which great spotted cuckoo nestlings (Clamator glandarius) are reared by magpie (Pica pica) host foster parents when sharing the nest with host nestlings, we tested several predictions of the adaptive digestive efficiency paradigm. A hand‐feeding experiment was employed in which we fed both great spotted cuckoo and magpie nestlings with exactly the same diet simulating one food abundance period and one food deprivation period. The results obtained show that cuckoo nestlings ingested more food, gained significantly more weight during the abundance period, and assimilated a higher proportion of the ingested food than magpie nestlings. These results demonstrate for the first time that cuckoo nestlings enjoy digestive adaptations that favour a rapid processing of the ingested food, thereby maximizing their intake rate but without decreasing digestive efficiency. © 2013 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 111 , 280–289.     

Begging in Common Redstart Nestlings: Scramble Competition or Signalling of Need?

Begging behaviour by the young affects parental food distribution among nestlings of altricial birds. We present an analysis of two types of begging behaviour (assuming the front nest positions and gaping) based on videotaped natural nestling feeding in European common redstart (Phoenicurus phoenicurus). We test whether these types of begging support the predictions of two mathematical models: scramble competition with competitive asymmetries between nestlings [Anim. Behav. 27 (1979) 1210] or honest signalling model [Nature 352 (1991) 328]. None of the measured variables of nestling or parental behaviour were affected by body weight differences between siblings. In contrast, both gaping and nest positioning were affected by individual differences in nestling hunger. In agreement with the honest signalling model, hungrier nestlings gaped with higher probability and started to gape sooner after the arrival of the parent than did their less hungry nestmates. Those nestlings with the shortest latency to gape also received food more often. Nest positioning was related to nestling hunger in a way unforeseen by the existing models. The intervals between nestling position changes were several times longer than the intervals between parental feeding visits, and parents preferred to feed nestlings in front positions, so nestlings in front positions were always less hungry than nestlings in back. Hence the pattern of movements influenced the feeding decision in favour of the more satiated nestlings and acted against the effect of gaping. Nestling movement seemed to be caused by the less hungry nestlings moving actively from front to rear positions. Low mortality of individual nestlings within broods that survived to fledging and small within‐brood variation in fledging weights indicated low competition among nestmates. We suggest that there are two behavioural mechanisms that contribute to the equalization of fledging weights in common redstart nestlings: the signalling of need through gaping and the regular turnover of nestlings at front positions.     

Experimental evidence for parental,but not parentally biased,favouritism in relation to offspring size in Blue Tits Cyanistes caeruleus

In species with biparental care, males and females share the benefits of investing in offspring but pay the costs individually. As a result of these evolutionary conflicts of interest between the sexes, it is expected that the two parents should follow different behavioural rules when providing food to the young. Such a discrepancy may be accentuated when parents have to choose between different subsets of offspring (e.g. large and small nestlings). We manipulated the degree of hatching asynchrony in Blue Tits Cyanistes caeruleus and quantified male and female feeding behaviour when nestlings were 7 and 10 days old. First, we tested for a difference in the role of the sexes during the nestling rearing period between experimentally asynchronous and synchronous control broods. We then used experimentally asynchronous broods to assess differences between the sexes in the pattern of food distribution in terms of number of feedings and prey types, between junior and senior siblings. When nestlings in experimental nests were 7 days old, females fed young more often than did males despite facing a trade‐off between brooding the smallest nestlings and bringing food to the nest. At this age, there was also a skew in food delivery in favour of senior siblings, whereas food was more evenly distributed across the brood when nestlings were 10 days old. We found no difference in how male and female Blue Tits distributed feeding visits among junior and senior nestlings. However, females fed the smallest nestlings with more spiders in comparison with their senior siblings. This could be related to their more suitable size relative to other prey types, their high content of essential nutrients, or both, and may represent a more cryptic form of parentally biased favouritism. We compare these findings with previous work on other species and discuss why parents did not feed junior siblings more frequently.     

IBERIAN AZURE-WINGED MAGPIE CYANOPICA (CYANA) COOKI NESTLINGS BEGGING CALLS: CALL CHARACTERIZATION AND HUNGER SIGNALLING

ABSTRACT

Nestling begging behaviour has long been seen as a signal by which nestlings solicit care from parents and most of the existing evidence provides some support for it being an honest signal. Begging is a multicomponent signal in which both sound and vision components are usually important. Although it is known that begging encodes information about nestling hunger the present knowledge about the specific behavioural features that convey the information is still scarce. The aim of this study was to describe begging calls of Iberian Azure-winged Magpie Cyanopica (cyana) cooki nestlings and examine how information on nestling hunger might be encoded in the begging calls. Nestlings were experimentally submitted to different periods of food deprivation and the call variation within individuals was studied. The young were individually tested and stimulated to beg by simulating parental visits. When subject to increasing food deprivation periods, nestlings increased the response level to simulated parental visits. The study also found that for the studied size differences, nestlings did not differ in their response level. Results confirmed that information on nestlings' hunger might be encoded in parameters of the calling behaviour. When the food deprivation periods increased, nestlings tended to start begging earlier, begged more often, extended their calling bout and increased the call duration, changing both at the level of the call and vocal begging bout. Overall the results support the view of begging as an honest signal, namely that begging should reflect nestling hunger and that only some call features might encode information about hunger.     

Acoustic signalling of hunger and thermal state by nestling tree swallows

The begging displays used by altricial nestling birds to solicit care from parents include vigorous movements and loud calling. These begging signals have attracted considerable interest, mainly because their intensity seems excessive for the function of transmitting information about nestling need to parents. However, how information on need is encoded in the various components of the signal, especially its acoustic components, is poorly understood. We examined how begging calls of large and small nestling tree swallows, Tachycineta bicolor, changed during a short period of food deprivation and cooling, as a first step in determining the role that various call characteristics played in advertising nestling need. In contrast to previous studies, we examined several call variables, and related them not only to need for food but also need for warmth. When nestlings were deprived of food, their calls increased in rate and length. Large nestlings also increased the amplitude of their calls. When nestlings were cooled during food deprivation, they decreased the frequency of their calls and their call rate. The latter trend was especially evident in small nestlings. Our results suggest that begging calls carry information not only on the overall hunger level of broods, as emphasized in previous studies, but also on the size, hunger and thermal need of individual nestlings. Further tests are needed to determine whether parents use this information and whether begging calls are optimally designed to convey it. Copyright 2001 The Association for the Study of Animal Behaviour.     

Intensity of mouth coloration in Blackcap Sylvia atricapilla nestlings affects food distribution among siblings but not provisioning of the whole brood

Among various begging stimuli, mouth coloration has received increasing attention in recent years, and previous research has demonstrated that mouths of nestling Canaries Serinus canaria get redder with the extent of food deprivation and that parents preferentially feed nestlings of redder gapes. This study assesses whether the intensity of red mouth colour in nestling Blackcaps Sylvia atricapilla is a signal in parent–offspring communication. This is one of the few species with a naturally red gape in which the function of mouth redness has been tested. Three predictions were experimentally tested: (1) reddening the gape of a single nestling within a brood increases its provisioning in relation to other siblings; (2) reddening the gapes of all nestlings within a brood increases parental feeding rate; and (3) food deprivation increases nestling mouth redness. The effect of nestling quality on mouth redness was also assessed. The intensity of gape coloration affected food distribution, but in a way opposite to that expected: an increase in mouth redness of the nestling caused reduced feeding by parents. However, reddening the gapes of all nestlings had no effect on provisioning of the whole brood, suggesting that Blackcap parents use different cues for provisioning particular nestlings and the whole brood. Intensity of mouth redness in Blackcap nestlings was not affected either by food deprivation or by nestling quality in terms of mass and rank in the nest.     

With a little help from my kin: barn swallow nestlings modulate solicitation of parental care according to nestmates' need

In altricial species, offspring competing for access to limiting parental resources (e.g. food) are selected to achieve an optimal balance between the costs of scrambling for food, the benefits of being fed and the indirect costs of subtracting food to relatives. As the marginal benefits of acquiring additional food decrease with decreasing levels of need, satiated offspring should be prone to favour access to food by their needy kin, thus enhancing their own indirect fitness, while concomitantly reducing costs of harsh competition with hungry broodmates. We tested this prediction in feeding trials of barn swallow (Hirundo rustica) nestlings by comparing begging behaviour and food intake of two similar-sized nestmates, one of which was food-deprived (FD). Non-food-deprived (NFD) offspring modulated begging intensity depending on their nestmate's need: when competing with FD nestmates, NFD nestlings reduced both the intensity and frequency of begging displays compared to themselves in the control trial before food deprivation. Hence, NFD nestlings reduced their competitiveness to the advantage of FD nestmates, which obtained more feedings and showed a threefold larger increase in body mass. Moderation of individual selfishness can therefore be adaptive in the presence of a needier kin, because the indirect fitness benefits of promoting its condition can outweigh the costs of forgoing being fed, and because it limits the cost of begging escalation against a vigorous competitor.     

Barn owl (Tyto alba) siblings vocally negotiate resources

Current theory proposes that nestlings beg to signal hunger level to parents honestly, or that siblings compete by escalating begging to attract the attention of parents. Although begging is assumed to be directed at parents, barn owl (Tyto alba) nestlings vocalize in the presence but also in the absence of the parents. Applying the theory of asymmetrical contests we experimentally tested three predictions of the novel hypothesis that in the absence of the parents siblings vocally settle contests over prey items to be delivered next by a parent. This 'sibling negotiation hypothesis' proposes that offspring use each others' begging vocalization as a source of information about their relative willingness to contest the next prey item delivered. In line with the hypothesis we found that (i) a nestling barn owl refrains from vocalization when a rival is more hungry, but (ii) escalates once the rival has been fed by a parent, and (iii) nestlings refrain from and escalate vocalization in experimentally enlarged and reduced broods, respectively. Thus, when parents are not at the nest a nestling vocally refrains when the value of the next delivered prey item will be higher for its nest-mates. These findings are the exact opposite of what current models predict for begging calls produced in the presence of the parents.     

Begging in the absence of parents by nestling tree swallows

Begging by nestling passerine birds has become a model systemfor studies in animal communication. Although most beggingoccurs when parents arrive at the nest to feed (here called"primary begging"), it also occurs between feeding visits andimmediately after parents leave the nest. Begging in thesecontexts (here called "secondary begging") may have relativelylittle influence on the probability of receiving food, but could increase the overall cost of the signal and thus influence nestlingbegging strategies. The purpose of our study was to determinehow often tree swallow (Tachycineta bicolor) nestlings begin contexts other than to parents with food and to examinewhat factors influence the frequency of this begging. Secondarybegging ranged from 7% of measured begging responses at day2 to 30% by day 8 and was more frequent when the interval betweenparental feeding visits was relatively long and when the timeto respond to the arrival of parents with food was short. Increasesin both age and intervisit interval were associated with decreasesin nestling response times, suggesting that secondary beggingmay be related to the speed with which nestlings respond to stimuli. We discuss possible functions of secondary beggingand raise the possibility that it may, in fact, be an error.     

The vigilance components of begging and sibling competition

The conflict between siblings over how parental resources are divided has promoted the evolution of specific behaviour to outcompete each other. Young animals look out for parents’ arrival in order to start begging as quickly as possible, since a rapid begging reaction increases the likelihood of being fed before nestmates. If the young can physically intercept the parents, selection might be operating on the offspring ability to monitor parent arrival (vigilance towards parents) and any sudden modifications in siblings’ behaviour (vigilance towards siblings). To investigate the adaptive value of nestling vigilance in the context of family interactions, we recorded which direction barn owl Tyto alba siblings were facing in 89 two‐chick broods before the first parental feeding visit of the night. Nestlings were more vigilant towards nest entrance than expected by chance suggesting that vigilance towards parents is an important component of sibling competition. When positioned near the nest‐box entrance where parents predictably deliver food, the younger individual (i.e. junior) looked more towards the entrance than its older sibling. Thus, when the likelihood of obtaining a food item is relatively high, juniors are more vigilant than seniors to detect the incoming parent. When positioned at the back of the nest, the senior looked relatively more frequently towards its sibling than the junior did in the same situation. This suggests that when the likelihood of obtaining a food item is relatively low, seniors are more vigilant than juniors to observe their sibling. Because vigilance was not related to hunger level and prey obtaining, we propose the hypothesis that vigilance towards parents and siblings only indirectly influences the outcome of sibling competition.     

Coadaptation of Offspring Begging and Parental Provisioning - An Evolutionary Ecological Perspective on Avian Family Life

Offspring begging and parental provisioning are the two central social behaviours expressed during the period of parental care. Both behaviours influence each other and it is, therefore, hypothesized that they should ultimately become (genetically) correlated, stabilized by fitness costs to parents and/or offspring. By reciprocally exchanging entire clutches in canaries (Serinus canaria), we tested (1) whether there is covariation between these behaviours and (2) whether a mismatch - as introduced by cross-fostering - entails costs. Begging was scored in a standardized begging test and parental provisioning was measured via (a) the actual feeding rate and (b) using the growth rate of the foster nestlings as a proxy. Costs were established in terms of future reproductive investment in subsequent clutches and offspring growth. We found a positive and significant phenotypic covariation between offspring begging and parental feeding when using the growth rate as a proxy and, to a lesser extent, in case of the parental feeding rate. Female parents suffered no future reproductive costs when feeding foster nestlings that were more demanding than their own nestlings. Neither growth measured amongst all offspring nor the reproductive investment measured amongst the female offspring as adults was influenced by their begging behaviour. However, the reproductive investment of female offspring tended to depend on the parental qualities of their foster parents. Thus, offspring may only be able to extract resources within the limit of generosity of their foster parents. This suggests parental control of feeding, which is also supported by the positive covariation between offspring begging and parental feeding.     

杂色山雀亲代喂食与子代乞食间的行为

在育雏期,晚成鸟的子代一般都是由双亲共同来抚育,子代为了更好地存活,会用自己的方式竞争获得更多的食物和更好的生存空间,同时亲代也会根据子代的乞食信号来分配食物。2011年3～7月采用针孔摄像技术录制了杂色山雀(Parus varius)育雏期巢内亲代与子代间的行为,统计了亲鸟站位、雏鸟站位、雏鸟乞食强度及亲鸟的喂食情况等数据。分析结果表明:(1)雌雄亲鸟在巢中的站位各有特点,雄鸟在整个育雏期都喜欢站在距离巢口较近的位置;雌鸟站位不太固定,前期离巢口相对较远,中期和后期离巢口相对较近;(2)雏鸟离亲鸟越近,乞食强度越大,获得食物的机会就越多;离亲鸟越远的雏鸟越不爱乞食,所以站位对雏鸟的食物获得影响最大;(3)雌鸟承担主要的育雏任务,喂食频率远大于雄鸟;(4)育雏期的不同阶段雏鸟乞食强度、亲鸟喂食频率变化很大:中期雏鸟乞食强度最大,亲鸟喂食频率最高,后期雏鸟乞食强度最弱;(5)整个育雏期雌性亲本没有表现出明显的偏爱行为,但雄性亲本在中、后期更偏爱体型大的雏鸟。可见杂色山雀子代的行为和体型大小影响着亲代的食物分配,亲代也会根据雏鸟日龄调整站位和喂食行为。     

Barn swallow (<Emphasis Type="Italic">Hirundo rustica</Emphasis>) parents use past information to decide on food provisioning to the brood,but not to decide on allocation within the brood

The begging behavior of birds is thought to influence the allocation of food within the brood (allocation function) or to increase the total amount of food delivered to the brood (delivery function). Considering the nature of the feeding activity of passerines, in which hundreds of feeding events occur in a single day, parents may not necessarily decide the amount of resources allocated to each nestling/brood during a single feeding event. To examine this possibility, the relationship between begging intensity and parental responses to allocation and delivery functions was tested and modeled at multi-temporal scales in the barn swallow (Hirundo rustica). Comparison of models revealed that barn swallow parents differed in the temporal scales at which they respond to nestlings with respect to these two functions. While barn swallow parents decide which nestling to feed at a focal feeding event according to chick begging of the focal feeding event, they integrated past information on total begging effort to determine delivery rates to their offspring during the 14 and 6 min prior to the focal feeding event in males and females, respectively. These findings highlight that it is important to investigate parental response to begging at appropriate temporal scales when analyzing begging functions.     

Parental Sex Differences in Food Allocation to Junior Brood Members as Mediated by Prey Size

Individual offspring within a brood may receive different amounts of provisioning from the male and female parents. Some hypotheses suggest that this bias is the result of an active and adaptive choice by parents. An alternative hypothesis is that feeding biases arise as a result of a constraint of fitting large prey items into small gapes. In an experiment with pied flycatchers, Ficedula hypoleuca , we tested for sex-biased allocation to junior nestlings in asynchronous broods and whether this could be explained by active parental choice or by passive allocation according to prey size and gape size. In both control broods and broods with experimentally increased degree of asynchrony, prey types did not differ between parents but females brought smaller prey than males at younger but not older nestling stages. At younger but not older nestling stages, the majority of feeds to junior nestlings were from females, and the smaller nestlings consumed smaller prey than older siblings. However, there was no evidence of active preference of small nestlings by females as parents did not differ in the tendency to bypass a begging senior nestling in order to feed a junior nestling. Provisioning rates by females were lower than those by males when nestlings were young and we suggest that foraging time constraints caused by the need to brood offspring result in females bringing smaller prey than males. In turn, the larger prey brought by males was more often transferred to larger offspring after the smaller ones failed to swallow it. In such cases, 'preferential' feeding of small nestlings by females may simply be a passive side effect of foraging constraints and gape-size limitations.     

Begging coordination between siblings in Black-headed Gulls

Communication behaviours are now considered from a signallers–receivers network perspective. This concept seems well suited to the study of interactions between parents and offspring in birds, so far mainly treated as a dyadic signalling system involving the brood or a single chick as a signaller and the parent as a receiver. Family conflicts over resource allocation drive parent–offspring and sib–sib communication. In the Black-headed Gull Larus ridibundus, parents respond to the whole-brood begging intensity and siblings often synchronize their begging signalling thus limiting individual effort. By monitoring five nests of two-chick broods during the whole rearing period in the nest, we show how an intra-brood simultaneity of begging emerges from successive phases of solitary begging of junior and senior nestlings. Although this result remains preliminary due to the sample size, it underlines a dynamical aspect of chicks’ behaviour. Because they always favour coordinated begging and because they elevate their response threshold across the rearing period, parents may play a major role in the plasticity of begging 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.     

Nest mate size, but not short-term need, influences begging behavior of a generalist brood parasite

Hosts of generalist brood parasites often vary with regardsto their life-history traits, and these differences have thepotential to influence the competitive environment experiencedby brood-parasitic nestlings. Although begging by brood parasitesis more exaggerated than their hosts, it is unclear if generalistbrood parasites modulate their begging behavior relative tohost size. I examined the begging behavior of brown-headed cowbird(Molothrus ater) nestlings when competing against nest matesthat differ in size and under different levels of short-termneed. Cowbird nestlings begged on nearly all feeding visits,responded to adults as fast as (or faster than) their nest mates,and typically begged more intensively than their nest mates.Latency to beg, time spent begging, and maximum begging postureof cowbirds were similar during supplementation and deprivationtreatments, indicating begging intensity was not influencedby short-term need. Time spent begging by cowbirds varied amonghosts of 3 different sizes when short-term need was standardized,suggesting that nest mate size strongly influenced begging behavior.Cowbirds obtained more food when competing against an intermediate-sizedhost due to lower provisioning rates of small hosts or becauseof increased competitive ability of large host nestlings. Overall,cowbirds obtained the greatest volume of food per unit timespent begging when competing against intermediate hosts, butthis value approached that of the small host when adjusted formodal brood size. These results demonstrate that cowbirds adjusttheir begging relative to the size of the hosts against whichthey compete but not to levels of short-term need.     

Corticosterone Promotes Scramble Competition Over Sibling Negotiation in Barn Owl Nestlings (Tyto alba)

In species with parental care, siblings compete for access to food resources. Typically, they vocally signal their level of need to each other and to parents, and jostle for the position in the nest where parents deliver food. Although food shortage and social interactions are stressful, little is known about the effect of stress on the way siblings resolve the conflict over how food is shared among them. Because glucocorticoid hormones mediate physiological and behavioral responses to stressors, we tested whether corticosterone, the main glucocorticoid in birds, modulates physical and vocal signaling used by barn owl siblings (Tyto alba) to compete for food. Although corticosterone-implanted (cort-) nestlings and placebo-nestlings were similarly successful to monopolize food, they employed different behavioral strategies. Compared to placebo-nestlings, cort-individuals reduced the rate of vocally communicating with their siblings (but not with their parents) but were positioned closer to the nest-box entrance where parents predictably deliver food. Therefore, corticosterone induced nestlings to increase their effort in physical competition for the best nest position at the expense of investment in sib?Csib communication without modifying vocal begging signals directed to parents. This suggests that in the barn owl stress alters nestlings?? behavior and corticosterone could mediate the trade-off between scramble competition and vocal sib?Csib communication. We conclude that stressful environments may prevent the evolution of sib?Csib communication as a way to resolve family conflicts peacefully.     

Begging, food provisioning, and nestling competition in great tit broods infested with ectoparasites

Ectoparasites are a ubiquitous environmental component of breedingbirds, and it has repeatedly been shown that hematoph-agousectoparasites such as fleas and mites reduce the quality andnumber of offspring of bird hosts, thereby lowering the valueof a current brood. Selection acting on the hosts will favorphysiological and behavioral responses that will reduce theparasites' impact. However, the results of the few bird studiesthat addressed the question of whether parasitism leads to ahigher rate of food provisioning are equivocal, and the beggingresponse to infestation has rarely been quantified. A changein begging activity and parental rate of food provisioning couldbe predicted in either direction: parents could reduce theirinvestment in the brood in order to invest more in future broods,or they could increase their investment in order to compensatefor the parasites' effect on the current brood. Since the nestlingsare weakened by the ectoparasites they may beg less, but onthe other hand they may beg more in order to obtain more food.In this study we show experimentally that (1) hen fleas (Ceratophyllusgallinae) reduce the body mass and size of great tit (Parusmajor) nestlings, (2) nestlings of parasitized broods more thandouble their begging rate, (3) the male parents increase thefrequency of feeding trips by over 50%, (4) the females do notadjust feeding rate to the lowered nutritional state of nestlings,and (5) food competition among siblings of parasitized broodsis increased. Ultimately the difference in the parental feedingresponse may be understood as the result of a sex-related differencein the trade-off of i0vesting in current versus future broods.