Resource allocation varies with parental sex and brood size in the asynchronously hatching green-rumped parrotlet (<Emphasis Type="Italic">Forpus passerinus</Emphasis>)

When eggs hatch asynchronously, offspring arising from last-hatched eggs often exhibit a competitive disadvantage compared with their older, larger nestmates. Strong sibling competition might result in a pattern of resource allocation favoring larger nestlings, but active food allocation towards smaller offspring may compensate for the negative effects of asynchronous hatching. We examined patterns of resource allocation by green-rumped parrotlet parents to small and large broods under control and food-supplemented conditions. There was no difference between parents and among brood sizes in visit rate or number of feeds delivered, although females spent marginally more time in the nest than males. Both male and female parents preferentially fed offspring that had a higher begging effort than the remainder of the brood. Mean begging levels did not differ between small and large broods, but smaller offspring begged more than their older nestmates in large broods. Male parents fed small offspring less often in both brood sizes. Female parents fed offspring evenly in small broods, while in large broods they fed smaller offspring more frequently, with the exception of the very last hatched individual. These data suggest male parrotlets exhibit a feeding preference for larger offspring—possibly arising from the outcome of sibling competition—but that females practice active food allocation, particularly in larger brood sizes. These differential patterns of resource allocation between the sexes are consistent with other studies of parrots and may reflect some level of female compensation for the limitations imposed on smaller offspring by hatching asynchrony.     

Provisioning rules in tree swallows

Conflict between parents and offspring may result in offspring exaggerating their needs and parents devaluing their begging signals. To determine whether this occurs, it is first necessary to establish the link between need, begging and parental response. The purpose of our study was to examine these relationships in tree swallows (Tachycineta bicolor). Parents preferentially fed nestlings that begged sooner, reached higher and were closer to the front of the nestbox (Fig. 1). Begging intensity of both individuals and entire broods increased with relatively long periods between feeding visits. Within broods, parents responded to increased begging intensity by increasing their feeding rate, although this effect was relatively weak. Large and small nestlings did not differ in their begging behavior and all nestlings, regardless of size, were fed at similar rates. Despite the overall equity in feeding, male parents preferentially fed larger nestlings while female parents fed smaller nestlings. Nestlings did not increase their begging intensity in response to begging by nestmates. Our results suggest that begging is related to need in this species and that parents respond to variation in begging intensity. Received: 4 May 1995/Accepted after revision: 17 December 1995     

Parent-absent begging in the Brown-headed Cowbird (<Emphasis Type="Italic">Molothrus ater</Emphasis>): the role of short-term need and nestmate size

Although it is well-established that nestlings of many altricial species beg when parents are away from the nest, we have a poor understanding of parent-absent begging in brood parasites, including the proximate factors that may influence begging frequency and intensity. In this study, I examined how parent-absent begging was influenced by competitive asymmetries between host and Brown-headed Cowbird (Molothrus ater) nestlings under disparate levels of short-term need. Food-deprived cowbird nestlings begged more frequently and for a greater proportion of parent-absent period than when food-supplemented, with similar patterns observed in hosts of different sizes. In contrast, three metrics of cowbird begging intensity varied relative to host size but not due to differences in short-term need. Cowbirds consistently begged more frequently and intensively than host nestlings for a given level of short-term need, providing evidence that cowbird begging displays are more frequent and intense than non-parasitic nestlings during both feeding visits and parent-absent periods. In sum, the frequency of begging by cowbirds was only influenced by short-term need, whereas begging intensity during parent-absent events was only influenced by the host against which cowbirds competed. This study demonstrates that host size and short-term need had differing influences on the frequency and intensity of parent-absent begging in cowbirds, although both factors are likely important in limiting the evolution of parent-absent begging in cowbirds. Because it appears to provide no immediate benefits yet may decrease fitness, parent-absent begging should be included in future theoretical models investigating the evolution of begging displays in nestling birds.     

Begging and provisioning in broods of asynchronously-hatched yellow-headed blackbird nestlings

Studies of begging have found a positive relationship between begging level and provisioning level. Studies of unequal nestlings, however, have found that small nestlings generally beg more but are fed less than their larger siblings. We manipulated the begging levels of yellow-headed blackbird (Xanthocephalus xanthocephalus) chicks to investigate how begging benefits individuals in broods of unequal siblings. Food-deprived chicks begged more and were fed more; satiated chicks begged less and were fed less. When we deprived each chick of a brood in turn, large and small chicks generally increased begging and received more provisioning. Small chicks, however, rarely received more food than their larger siblings even when they behged relatively more. Parent yellow-headed blackbirds increase provisioning to hungry begging chicks, but also allocate food based on relative offspring size.     

Sex-related asymmetry in competitive ability of sexually monomorphic barn swallow nestlings

In diverse taxa, offspring solicit parental care using complex displays, which may evolve as reliable signals of condition or as mechanisms to manipulate parental investment. Differential sex allocation may therefore result from adaptive parental decisions or sex-related variation in competitive ability or because of sex-related asymmetries in kin selection. Under normal food provisioning, female barn swallow (Hirundo rustica) nestlings begged more loudly but did not receive more food than male nestlings. After food deprivation, begging call loudness of males but not females increased. Begging loudness positively predicted the number of feedings received by the nestlings, and males gained more mass than females after food deprivation. Male nestlings are more severely affected by chronic food reduction and may therefore accrue a larger benefit compared to females by increasing their food intake under short-term conditions of food scarcity. These results suggest that either females do not increase begging intensity to favour male broodmates which are more vulnerable to prolonged food stress, or that males prevail in scramble competition despite being similar in size to females.     

Do males and females differ in the feeding of large and small siblings? An experiment with the bluethroat

Males and females have been reported to differ in their feeding of large and small siblings in several species of birds. According to recent hypotheses, this phenomenon may be related to a sexual conflict over avian hatching patterns. We designed an experiment to test for the existence of such a sex difference by manipulating nestling size hierarchies of the bluethroat (Luscinia s. svecica) in two directions; half the broods were “asynchronized” to yield large size-differences within broods and the other half were “synchronized” to yield small size-differences. In all broods, nestlings were categorized as being either large or small according to body mass. We recorded male and female food distribution by video early (day 4 after hatching) and late (day 8) in the nestling period. Males and females did not differ in their distribution of food among different-sized nestlings. With large size-differences, both males and females fed large nestlings nearly twice as often as small ones. In contrast, when the size-differences were small, food was more evenly distributed among nestlings. Early in the nestling period, males fed more nestlings during each feeding visit than did females. Our finding that male and female bluethroats do not differ in the feeding of large and small siblings is in contrast to most previous studies. Variation in costs and benefits to males and females from feeding different-sized nestlings, and restrictions to parental choice due to nestling interactions, may explain interspecific variation. Received: 27 June 1997 / Accepted after revision: 26 January 1998     

Begging calls and parental feeding decisions in tree swallows (Tachycineta bicolor)

We conducted playback experiments to examine how parent tree swallows (Tachycineta bicolor) use nestling begging calls to distribute feedings to individuals within broods. In a first study, we used a paired-choice test to determine if parents discriminated between the taped begging calls of nestlings deprived of food and those of nestlings that had been recently fed. Our results showed that parents directed their first feeding attempt towards model nestlings near speakers playing deprived calls significantly more often than to models near speakers playing fed calls. They also made more feeding attempts overall to models with deprived calls. In the second study, we varied call rate and amplitude to examine which call features parents might use to discriminate begging calls. Parents directed significantly more first feeding attempts and more feeding attempts overall towards non-begging nestlings near speakers playing high call rates than to nestlings near speakers playing low call rates. They did not, however, discriminate between calls differing in amplitude. Previous studies have shown that parent birds use begging calls to regulate overall feeding rates to the brood. Our results suggest that parent tree swallows also use begging calls when feeding individual nestlings and, in particular, prefer calls associated with increased levels of nestling hunger. Received: 14 February 2000 / Revised: 6 October 2000 / Accepted: 16 October 2000     

Need and nestmates affect begging in tree swallows

We conducted an experiment on nestling tree swallows (Tachycineta bicolor) to examine predictions from signalling models for the evolution of conspicuous begging behaviour. Specifically, we examined the relationship between (1) nestling begging intensity and hunger, (2) begging intensity and parental provisioning and (3) begging intensity and nestmate condition. Forty broods of 9-day-old nestlings were removed from their nests for 1 h and assigned to one of the following three treatments: (1) all nestlings in the brood deprived of food (n = 13), (2) all nestlings in the brood fed (n = 11) or (3) half the nestlings in the brood deprived and half fed (n = 16). Videotapes before and after the treatments showed that begging intensity increased in broods in which all of the nestlings had been deprived and decreased in broods in which all of the nestlings had been fed. Deprived nestlings in the half-and-half treatment did not change their begging intensity in response to treatment, while fed nestlings in this treatment group showed a decrease in begging intensity. Parent tree swallows increased their feeding rate to deprived broods and decreased their rate to fed broods. Within broods, parents decreased their feeding rate to fed nestlings, but showed no significant change in feeding to deprived nestlings. Our results suggest that begging intensity is influenced by hunger and that parents appear to respond to variation in begging intensity. The begging of nestmates also appears to influence begging independently of need. These results are consistent with predictions derived from signalling models of begging. Received: 20 June 1997 / Accepted after revision: 19 January 1998     

Parental feeding rate in relation to begging behavior in asynchronously hatched broods of the great tit Parus major

Summary Variations in the begging behaviour of the nestlings of altricial birds can provide the parents with information about the nestlings' nutritional needs and thus influence the parental feeding rate. In a series of four experiments, the stimulus situation encountered by great tits on their feeding visits to the brood was manipulated to explore its effect on feeding rate.A higher feeding rate was observed under the following conditions: (1) after a period of food deprivation, as compared with both normal conditions and satiation through artificial feeding; (2) in periods when recorded begging calls were played during feeding visits, as compared with control periods; (3) after temporary removal from the nest of heavier, as compared with lighter, siblings. The lighter nestlings in the brood benefitted more —in terms of gain in weight — from the increase in parental feeding rate following the playing of begging calls than did the heavier nestlings. Differences in weight spread within broods did not affect the amount of food the parents brought.We conclude that parental feeding rate is affected not simply by the begging of the hungriest nestling, but rather by the behaviour of all the nestlings in the brood, which makes possible an adjustment of the feeding rate to the average hunger level of the brood. The effects of hatching asynchrony and sibling competition on parental feeding rate are discussed.     

Provisioning rules and chick competition in asynchronously hatching common terns (Sterna hirundo)

Interactions between nestling birds and their parents are models for examining parent–offspring communication and sibling competition. Most studies have focused on species where young are restricted to a nest. However, offspring of many species are mobile and fed by parents for an extended period post-hatch. These chicks mobility may provide an opportunity to examine the role of signalling and physical competition on parental feeding decisions. We examined parental provisioning rules in relation to offspring behaviour and hatching order (i.e., competitive ability) in a species with mobile young, the common tern. We determined that about 95% of feedings were directed to the first chick to reach the parent when it landed with food. We developed a probabilistic model to predict the likelihood of a chick reaching the parent first, and thus receiving food. Our model showed that begging intensity, feeding history, and the interaction between begging intensity and relative proximity to the parent best predicted which chick would arrive first. Increased begging was associated with arriving first significantly more when a chick was relatively further from the parent than when it was closer than its siblings. Independently of these factors, larger, earlier-hatched chicks were more likely to be fed than smaller, later-hatched chicks. Additional analyses showed that parents landed closer to more intensively begging chicks, however, increased begging did not explain the advantage of earlier-hatched chicks because begging intensity did not vary with hatching order. Instead, earlier-hatched chicks were more likely to outrun later-hatched siblings and reach the parent first.     

Nestling American robins compete with siblings by begging

Summary The evolution of intense begging by dependent nestling birds has recently been the subject of several theoretical papers. The interesting problem here is that nestlings should be able to communicate their nutritional status to parents in ways that are less costly energetically and less likely to attract predators. Thus, conspicuous begging behaviour is thought to have evolved as a result of either competition among nestmates or the manipulation of their parents to provide more food than would otherwise be favoured by selection. We studied sibling competition for parental feedings in the American robin (Turdus migratorius). We demonstrate that the probability that an individual nestling received food was related to several indices of begging. When we experimentally prevented parents from feeding part of their brood, both the intensity of begging and the number of feedings subsequently received by food-deprived nestlings increased. Furthermore, the begging intensity of those nestlings that were not food-deprived also increased in response to the begging of their hungrier siblings.Offprint requests to: R. Montgomerie     

Begging signals more than just short-term need: cryptic effects of brood size in the pied flycatcher (Ficedula hypoleuca)

The begging of nestling birds is known to reliably signal short-term nutritional need, which is used by parents to adjust rates of food delivery and patterns of food distribution within broods. To test whether begging signals reflect more than just short-term feeding history, we experimentally created 18 "small" (4-nestling) and 18 "large" (8-nestling) broods in the pied flycatcher (Ficedula hypoleuca). Compared to small broods, large broods were provisioned by parents at a greater rate, but at a lower visit rate per nestling and with no obvious differences in load mass per visit. However, lower rates of food mass delivery per nestling in large broods did not result in any measurable reduction in nestling growth (i.e. "long-term need") or in any increase in the begging effort per individual nestling whilst in the nest. Mid-way through the nestling period we also used hand-feeding laboratory trials to assess in more detail individual begging behaviour and digestive performance of the three mid-ranking nestlings from each brood. More food items were required at the start of each trial to satiate nestlings from large broods, but despite this initial control for "short-term need", nestlings from large broods went on to beg at consistently higher rates and at different acoustic frequencies. Large brood nestlings also produced smaller faecal sacs, which were quantitatively different in content but did not differ in frequency. We suggest that different nutritional histories can produce cryptic changes in nestling digestive function, and that these can lead to important differences in begging signals despite controlling for short term need.     

Effects of nest site concealment on hatching success,reproductive success,and paternal behavior of the threespine stickleback,Gasterosteus aculeatus

Summary Male sticklebacks (Gasterosteus aculeatus) actively competed for a limited number of clay flowerpots as nest sites in circular wading pools. Males nesting in pots spawned earlier and more often, had higher mean and lower variance for hatching success (number of fry per gram of estimated clutch weight), and suffered fewer stolen fertilizations, nest raids, and territorial encounters than did males nesting outside pots, and in pools without pots (Table 2). Males nesting in pots had the more even temporal distribution of fanning bouts and interfanning intervals; the durations of fanning bouts and interfanning intervals were less variable for these males (Table 2).In replicate aquaria, one male nested on each side of a glass partition and both males spawned approximately simultaneously. Flowerpots were provided to one, both, or neither male. Males with pots had higher mean hatching success, fewer territorial encounters, and the more even temporal distribution of fanning bouts and interfanning intervals (Table 4).Thus it appears that nest site concealment, in the form of clay flowerpots, affected male hatching success, reproductive success, and parental behavior, even when physical contact between males was prevented. These data suggest that a causal relationship may exist between the paternal fanning regime and the subsequent hatching success. Furthermore, these data suggest that males which nest in high concealment increase their reproductive success, and that females which spawn with these males reduce the variance of their hatching success as well as increase their mean hatching success.     

Replacement males in the western bluebird: opportunity for paternity, chick-feeding rules, and fitness consequences of male parental care

In western bluebirds (Sialia mexicana), most pairs remain together for life and share equally in post-hatching parental care. We removed resident males of socially monogamous pairs during laying and after clutch completion to examine chick-feeding rules used by replacement males and current and future fitness consequences of paternal care. Replacement males were not infanticidal and 7 (47%) fed nestlings. Feeding replacement males and the females they joined fed at rates similar to controls. Females without a feeding replacement male compensated by feeding more themselves so that overall feeding rates were not compromised, but they reduced their brooding time. Unlike assisted females, unassisted females exhibited reduced nesting success and their 14-day-old chicks weighed less than controls. Field metabolic rates of unassisted females were 17% higher than those of control females, but the difference was not statistically significant. Older females were better able to raise young without the male's help than were yearling females. Female condition was not affected by male provisioning and unassisted females were as likely to survive to the next breeding season as assisted females. We found no future benefits of provisioning by replacement males; those that fed were no more likely to breed with the female on her subsequent attempt than were males that did not feed, and subsequent clutch sizes were not reduced for females rearing young without the male's help. These experiments suggest that male parental care increases nesting success in western bluebirds and that replacement males use an all-or-none rule to determine whether or not to feed chicks: if they are present during the fertile period they feed at typical rates; if they are not, they usually do not feed at all. Because chick-feeding by males is tied to opportunity for paternity, influences success in the current nest, and does not affect the male's future breeding success, it appears to be parental rather than mating effort. Received: 8 May 1998 / Accepted after revision: 23 July 1998     

Honest begging in the blue-footed booby: signaling food deprivation and body condition

To determine whether solicitation by blue-footed booby chicks accurately encodes their need for food, we independently manipulated the body condition and recent food ingestion of singleton chicks and recorded three measures of begging. Variations in the three measures of begging covaried only partially, but in general, chicks begged more intensely when they were in poor body condition and also when suffering recent food deprivation. Effects of body condition and recent food deprivation on begging were broadly additive, although deprived chicks in poor condition failed to beg more intensely than those in good condition. Protracted short-term deprivation may create such a high level of need that the body condition component of need becomes temporarily unimportant. Parents more frequently fed chicks that begged more intensely, chicks in poor condition, and chicks suffering food deprivation. Deprived chicks in poor condition received more food than deprived chicks in normal condition even though they did not beg more intensely, possibly because parents responded not only to current begging but also to begging earlier in the day, or to other cues to body condition. These results support the hypothesis that begging of boobies represents honest signaling of need.     

Begging and short-term need in cowbird nestlings: how different are brood parasites?

Because brood parasitic nestlings are usually unrelated to their nestmates and to the provisioning adult, they are free from indirect costs of begging. Consequently, they are predicted to beg more intensely than host nestlings, and some models predict they will beg at an invariantly high level, regardless of short-term need. Previous work has shown intense begging by parasitic cowbirds, but short-term need was not controlled. In this study, we manipulated short-term need and measured begging intensity in two species pairs of host and parasitic nestlings: shiny cowbirds (Molothrus bonariensis) parasitizing larger hosts than themselves (rufous-bellied thrushes, Turdus rufiventris), and brown-headed cowbirds (M. ater) parasitizing smaller hosts than themselves (yellow warblers, Dendroica petechia). All four species increased their begging intensity with short-term need, though the change was much less pronounced between food-deprived and control treatments than between control and hand-feeding treatments. Shiny cowbirds begged more intensely than rufous-bellied thrushes following each treatment. In contrast, brown-headed cowbirds did not beg significantly more intensely than yellow warblers under any of the treatments. Intense begging by both species of parasites was as effective as host begging in stimulating the adults to make provisioning visits, but shiny cowbirds were less successful at acquiring food from adult thrushes. A wide array of factors may underlie the apparent differences in the begging behavior and success of brown-headed and shiny cowbirds, including relative size, experience, and local risk of nest predation. Our experiments clearly demonstrate, however, that these two species of parasitic cowbirds adjust their begging intensity based on short-term need.Communicated by H. Kokko     

Parent-absent signalling of need and its consequences for sibling competition in the barn swallow

In species with parental care, competition among siblings for access to limited parental resources is common. Sibling competition can be mediated by begging behaviour, a suite of different visual and acoustic displays by which offspring solicit parental care. These are mostly addressed to the parents upon food provisioning, but can also be performed in the absence of the attending parents. This so-called parent-absent begging (PAB) may function as an intrabrood communication signal and potentially affect intrabrood competition dynamics for access to food. We investigated the role of PAB in moulding sibling interactions and its effect on food intake among altricial barn swallow (Hirundo rustica) nestlings, both under normal and experimentally reduced food intake. Frequency of PAB increased after food deprivation. Nestlings that had performed PAB increased their begging intensity upon the subsequent parental feeding visit, while siblings reduced their own begging level, but only when they had not been food-deprived. As a consequence, nestlings which had performed PAB before parental arrival had larger chances of receiving food. However, nestlings did not benefit from displaying PAB when competing with food-deprived siblings. Our findings show that PAB reliably reflects need of food, indicating that a nestling will vigorously compete for the subsequent food item. By eavesdropping siblings' PAB displays, nestlings may optimally balance the costs of scrambling competition, the direct fitness gains of being fed and the indirect fitness costs of subtracting food to needy kin. However, large asymmetries in satiation between competitors may lead individual offspring to monopolize parental resources, irrespective of PAB displays.     

Parental investment in two solitary wasps Ancistrocerus adiabatus and Euodynerus foraminatus (Eumenidae: Hymenoptera)

Summary Females of the solitary eumenid wasps Ancistrocerus adiabatus and Euodynerus foraminatus control the adult size of their offspring by the amount of food provded to the larvae. For both species, larger females provision more offspring and collect more food than smaller females. Males of E. foraminatus, upon emergence as adults, fight for control of the nest area. The winning brother remains at the nest and mates with his sisters as they emerge several days later. Males of A. adiabatus are not as agonistic as males of E. foraminatus so that a number of males may remain near the natal nest. When females nest near each other, male-male interactions increase, and the likelihood of sibling mating decreases. Under conditions of clumped nesting, females make larger offspring and invest more in males than when nesting in isolation.     

Testosterone-induced depression of male parental behavior in the barn swallow: female compensation and effects on seasonal fitness

In birds, many aspects of male socio-sexual and parental behavior are influenced by androgens, most notably testosterone (T). We report the effects of subcutaneous T-implants in male barn swallows (Hirundo rustica) on male and female parental behavior and on seasonal reproductive success. Males were assigned to one of three experimental groups: (i) implanted with a T-filled Silastic tube; (ii) implanted with an empty Silastic tube; and (iii) not implanted. T-implanted males provided a smaller proportion of feedings (number of feedings by the male/total number of feedings by both parents) and fed nestlings less frequently (number of feedings/h) than males of the other two groups. Females paired to T-implanted males fed nestlings significantly more often than females paired with unimplanted males. Females almost fully compensated for their mates' shortfall, and this resulted in similar combined feeding efforts among treatments. Reproductive success in their first broods or during the entire breeding season was unaffected by T- treatment. These results confirm earlier reports of the suppressive effects of T on male parental behavior. However, they are inconsistent with current ESS models that predict partial compensation as the optimal response by one individual to reduction of parental effort by its mate in monogamous, biparental systems.     

An experimental study of paternal behavior in red-winged blackbirds

Summary The effect of brood size and female nesting status on male parental behavior was investigated in red-winged blackbirds Agelaius phoeniceus using brood size manipulation experiments. Male redwings allocated parental effort on the basis of brood size and nestling age. Males began assisting females only at nests with at least three offspring older than three days. Female nesting status had no singificant influence on male parental care. When females were unable to meet a brood's demand for food, males assisted females with nestling feeding. Females did not reduce the amount of food delivered to nestlings when males assisted. The amount of food brought to nestlings by the male was additional to the amount of food provided by the female. Male assistance increased fledgling success. When female provisioning was sufficient to meet a brood's demand for food males did not assist. The value of male parental care varied inversely with the ability of the female to meet nestling food demands. The ability of unassisted females to provide sufficient food and to raise a brood of nestlings successfully appeared to be influenced by resource abundance.