An experimental study of competition for food between male and female nestlings of the red-winged blackbird

Summary Ten experimental broods of red-winged black-bird (Agelaius phoeniceus) nestlings, experimentally manipulated to contain two males and two females of similar age, were observed with the aid of video cameras to determine (a) which nestling characteristics were most important in influencing its chances of being fed and (b) if males and females differed with respect to these characteristics. Nestlings that could reach highest while begging were most successful at obtaining food from parents during individual feeding bouts. However, while there was a tendency for nestlings that begged earlier to be offered food first, a nestling's position in the nest during these bouts did not influence its feeding success. Males begged more than often females and were offered more food by parents. Males were also more likely to be fed when they begged. This was most likely because males, being larger than females, were able to reach higher while begging. Neither sex was more likely to beg sooner or occupy certain positions in the nest. However, while males that had been transferred from other nests received less food than natal males, there were no differences in food acquisition between transferred and natal females. Differences in the relative success of males and females under different conditions may help explain differences in fledgling sex ratios observed in many dimorphic species.     

Begging affects parental effort in the pied flycatcher, Ficedula hypoleuca

It has been suggested that nestlings use begging to increase their share of parental resources at the expense of current or future siblings. There is ample evidence that siblings compete over food with nestmates by begging, but only short-term effects of begging on parental provisioning rates have been shown. In this study, we use a new experimental design to demonstrate that pied flycatcher (Ficedula hypoleuca) nestlings that beg more are able to increase parental provisioning rates over the major part of the nestling period, thus potentially competing with future siblings. Parents were marked with microchips so that additional begging sounds could be played back when one of the parents visited the nest. By playing back begging sounds consistently at either male or female visits, a sex difference in provisioning rate that lasted for the major part of the nestling period was induced. If each parent independently adjusts its effort to the begging intensity of nestlings, begging may also be the proximate control mechanism for the sexual division of labour. Received: 24 January 1997 / Accepted after revision: 30 August 1997     

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.     

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.     

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.     

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     

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.     

Parental care of nestlings by male red-winged blackbirds

Summary Nestling feeding by males is less common among birds with polygynous mating systems than in monogamous species, because of the pronounced trade-off between parental behavior and the attraction of additional mates. In this study, however, we found that male red-winged blackbirds (Agelaius phoeniceus) commonly assisted females in feeding nestlings in several Ontario marshes. Male parental care was additional to that provided by females, and it significantly enhanced the fledging success of nests (Table 2). Male redwings did not help to feed all nests on their territories: primary and secondary nests were much more likely to receive male parental care than tertiary and later nests. Contrary to expectation, male parental care was not restricted to the nests of primary females: a greater proportion of secondary than primary nests were assisted (Tables 4a and b). The presence of new females settling on the territory at the same time that a resident female had nestlings, resulted in males deferring parental care until a later brood. This suggests that males trade off the recruitment of females against parental care to an existing brood. Although the number of nestlings fledging from a male's territory was strongly influenced by the number of females in the harem, males could additionally increase their reproductive success by feeding nestlings in one or more nests on their territories (Fig. 2). The reproductive success of females was significantly enhanced by male assistance in feeding nestlings (Table 3). However, those females not receiving male assistance on territories of feeding males did not suffer a significantly reduced reproductive success in comparison to females on territories of non-feeding males (Table 2). Males varied considerably in the quality of brood care given. We therefore suggest that the quality of male parental care may be a factor considered by females in choosing a breeding situation.     

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.     

Food abundance and parental care in yellow warblers (Dendroica petechia)

Emlen and Oring (1977) suggested that monogamy in birds is maintained because of the need for strict biparental care. A corollary of their suggestion is that paternal care should decrease under conditions of high food abundance. An alternative is that paternal care would increase if males take advantage of the higher food abundance by trying to reduce the length of the nestling feeding period. We tested these two ideas using yellow warblers (Dendroica petechia) by providing some pairs with supplemental food, thereby reducing the importance of biparental care. However, the extra food did not decrease paternal effort, nor did it increase it (Fig. 2). Early in the nestling period experimental females brooded more but visited their nestlings less than did control females, but later, when brooding times decreased, experimental females fed their nestlings more than did control females (Fig. 3). There were no significant differences in nestling survival (Fig. 5), but nestlings in the control treatment were larger and heavier up to 6 days old (Fig. 6). The main effect of supplemental food was on maternal, not paternal behaviour. Models of biparental care assume interdependence between the parental effort of both parents. In this species, however, males and females provide for their brood independently from each other.     

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     

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.     

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     

Experimental increase in food supply influences the outcome of within-family conflicts in Tengmalm’s owl

Within a family there are conflicts of interest between parents and offspring, and between male and female parents, over the supply of parental care. The observed pattern of parental care is the outcome of negotiations within the family, and may be influenced by environmental factors such as food abundance. We experimentally increased food supply to ten Tengmalm’s owl (Aegolius funereus) nests from hatching to fledging, mimicking natural cached prey. Ten un-supplemented nests served as controls. Parents and offspring were fitted with radio-tags. Food provisioning by parents was measured both in the (1) mid- and (2) late nestling stage and in the (3) early and (4) late post-fledging stage. In response to food supplementation, both males and females reduced food provisioning, but the effect was more pronounced in females. Females generally contributed much less to food provisioning than males, and food supplementation increased the difference between the sexes. Mass loss during the brooding stage was substantially lower for supplemented than for control females. Food supplementation did not improve offspring survival, and had no effect on body measurements of nestlings. In conclusion, parents of both sexes used the increased food supply to reduce the costs of caring for their current offspring, but females responded more strongly than males.     

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 behavior and food acquisition by brown-headed cowbird nestlings

Understanding the selective forces that limit the exaggeration of begging signals is a critical issue in understanding the evolution of begging behavior. I studied the begging behavior of nestlings of the brown-headed cowbird (Molothrus ater), a brood parasite. In the nests of indigo buntings (Passerina cyanea), brown-headed cowbird nestlings received approximately twice as much food per hour than their host nestmates. I tested three hypotheses for the mechanism by which cowbirds acquired more food than their bunting nestmates: the size advantage hypothesis, the signal exaggeration hypothesis, and the novel begging behavior hypothesis. I found support for the hypotheses that cowbirds acquire more food as a result of their larger body size, and due to the exaggeration of begging signals that are not dependent on body size. I did not find support for the role of novel begging behaviors in cowbird food acquisition. These results suggest that food acquisition by host chicks in unparasitized nests could be increased by the exaggeration of begging signals. Recent work suggests that such exaggeration may be limited by the risk of nest predation, but further studies are needed. Received: 12 December 1997 / Accepted after revision: 29 December 1997     

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     

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     

Solicitation displays reliably reflect oxidative damage in barn swallow nestlings

Reproduction entails parental decisions over allocation of limiting resources. ‘Begging’ solicitation displays by the offspring have been hypothesised to evolve as reliable indicators of quality, allowing parents to adopt optimal allocation strategies. To function as such, however, begging signals must covary with offspring traits that affect viability. Here we tested whether oxidative damage of nestling barn swallows predicts begging bout duration, call amplitude and intensity of postural displays both under normal food provisioning and under food shortage. Oxidative damage as determined by normal metabolism, pathological conditions and functioning of the immune system can serve as a comprehensive indicator of offspring condition because it can profoundly affect viability, senescence and reproduction. Begging bout duration was negatively predicted by oxidative damage but only after food deprivation. Postural begging negatively covaried with oxidative damage, at least when nestlings were confronted with related nest mates. Plasma antioxidant capacity did not predict begging behaviour. Thus, we show for the first time that behavioural begging covaries with a comprehensive index of chick condition, lending support to reliability of such begging traits also as signals of quality rather than need or hunger only. Future studies are candidate to decouple the effect of hunger state and individual condition on behavioural begging.     

Learning during competitive positioning in the nest: do nestlings use ideal free ‘foraging’ tactics?

Begging behaviour of nestling birds may involve more than a simple, honest source of information for parents to use in provisioning. Many aspects of begging behaviour relate instead to sibling competition for food items within the nest, and we might expect evidence of adaptive learning and behavioural adjustment in response to experience of the competitive environment. In this study, we consider begging in different locations within the nest as analogous to foraging in different patches, varying in food availability. Using hand-feeding trials, we created zones of differing profitability within an artificial nest by adjusting either the prey size or number of items delivered, and allowed only indirect competition between pairs of southern grey shrike (Lanius meridionalis) nestlings. Nestlings demonstrated the ability to detect differences in zone profitability and position themselves accordingly. By the end of both the prey size and delivery rate trials nestlings had increased the amount of time spent in the high quality zone. Such movement in response to differences in load quality, as well as frequency, demonstrates the ability of nestlings to learn about their environment and to facultatively adjust their begging in order to maximise energetic rewards.