Reproductive success of monogamous and polygynous pied flycatchers (Ficedula hypoleuca)

One of the most interesting aspects of the mating system of pied flycatchers is the regular occurrence of polygyny. Here we present data on the reproductive success of polygynously paired pied flycatcher males compared to monogamous males based on paternity analyses through DNA fingerprinting. Males paired with two females suffered a higher loss in reproductive output per female compared to monogamous males due to (1) a greater proportion of unhatched eggs in their broods, (2) greater nestling mortality and (3) a greater probability of being cuckolded. Nevertheless, the number of fledglings was significantly greater for polygynous males. Based on the number of nestlings that returned for subsequent breeding seasons, however, the reproductive success of monogamous and polygynous males did not differ significantly. These data raise the question as to why males attempt polygyny. Received: 16 August 1999 / Received in revised form: 15 March 2000 / Accepted: 18 March 2000     

Risk taking during parental care: a test of three hypotheses applied to the pied flycatcher

According to life-history theory, there will often be a conflict between investment in current versus future reproduction. If a predator appears during breeding, parents must make a compromise between ensuring the growth and survival of offspring (nest defence, feeding and brooding of young), and reducing the risk of predation to ensure their own survival. We model three hypotheses for the outcome of this conflict which are particularly relevant for altricial birds. They are not mutually exclusive, but focus on different costs and benefits. (1) Parental investment is determined by the parents’ own risk of predation. This hypothesis predicts that a lone parent should take smaller risks than a parent that has a mate. (2) Parental investment is related to the reproductive value of the offspring: Parents are predicted to take greater risks for larger broods, larger-sized or older offspring. (3) Finally, we present the new hypothesis that parental investment is related to the harm that offspring would suffer during a period of no parental care (incubation, brooding, feeding). This hypothesis predicts that parents should take greater risks for younger offspring, or for offspring in poorer condition, because the marginal benefit of parental care is largest in such cases. Hence, one may also expect that lone parents should take greater risks than two parents because their offspring are more in need of care. We tested these hypotheses on the pied flycatcher (Ficedula hypoleuca) by presenting a stuffed predator of the parents (a sparrowhawk, Accipiter nisus) close to the nest when parents were feeding the young. Risk taking was measured as the time that elapsed until the first visit to the nest. Most support was found for the ‘‘harm to offspring’’ hypothesis. Previous studies have usually measured the intensity of nest defence against typical nest predators, and have found evidence for the ‘‘reproductive value of offspring’’ hypothesis. However, our model predicts that the importance of the reproductive value of the offspring should decrease relative to the harm that offspring would suffer if they were not cared for when the predator type changes from a nest predator to a predator of adults, and when conditions for breeding turn from good to bad. Received: 13 April 1995/Accepted after revision: 11 March 1996     

Experimental evidence of reciprocal altruism in the pied flycatcher

Although human behaviour abounds with reciprocal altruism, few examples exist documenting reciprocal altruism in animals. Recent non-experimental evidence suggests that reciprocal altruism may be more common in nature than previously documented. Here we present experimental evidence of mobbing behaviour, the joint assault on a predator in an attempt to drive it away, as reciprocal altruism in the breeding pied flycatcher (Ficedula hypoleuca). Given a choice, pied flycatchers assisted in mobbing initiated by co-operating neighbours and did not join in mobbing when initiated by conspecific neighbours which had defected from necessary assistance 1 h before. The results suggest the birds followed a ‘tit-for-tat’-like strategy and that mobbing behaviour of breeding birds may be explained in terms of reciprocal altruism.     

Extra-pair paternity and male characteristics in the pied flycatcher

The pied flycatcher (Ficedula hypoleuca) is sexually dichromatic with extreme variation in male plumage coloration. The benefit for males of having black plumage is controversial, and few studies have found evidence for a sexual selection benefit of being black rather than brown. However, blacker males may be better able to achieve extra-pair fertilizations (EPFs), which may be an important component of sexual selection. We studied the role of EPFs in sexual selection in the pied flycatcher by establishing a set-up where two males with different back coloration (blacker vs browner) bred simultaneously near each other. DNA fingerprinting analysis revealed that 11% of offspring resulted from EPFs, and that 22% of broods included extra-pair young (EPY) among 36 nests containing 223 nestlings. We found no evidence that browner males suffered more often from EPFs than blacker males. There was no correlation of male or female morphology or age with EPF frequency. However, breeding pairs with low genetic similarity had EPY in their nests significantly more often. Thus we argue that females paired with genetically dissimilar males may try to avoid the effects of extreme outbreeding by seeking extra-pair copulations (EPCs). Alternatively, incompatibility between genetically dissimilar mates may simply expose females to more extra-pair copulations.     

Mate retention and male polyterritoriality in the pied flycatcher Ficedula hypoleuca

Summary We studied pair bond maintenance in the pied flycatcher in an experiment on nest predation at the incubation stage. Males defending one or more additional nestboxes (i.e. polyterritorial) on the day of nest predation were significantly more successful in retaining their mate than were the monoterritorial males (54% and 13% succeed, respectively). We manipulated the number of nest sites available for renesting, and found that the females remained preferentially with those males that had access to a new site, regardless of the previous territorial status of the male. Thus, the success of the polyterritorial males in mate retention was related primarily to the resource defended (a new nest site) rather than to some quality of the particular males per se. When the females could choose between alternative nestboxes for renesting, they selected the one located at the greatest distance from the initial site (Fig. 1). Male polyterritorial behaviour has previously been interpreted as a means of achieving polygyny. Our results show that this behaviour may be advantageous even in the absence of polygyny, especially at high levels of nest predation.     

Female nutritional state affects the rate of male incubation feeding in the pied flycatcher Ficedula hypoleuca

Summary Male pied flycatches Ficedula hypoleuca regularly feed their mates during incubation. By experimentally supplying some females with extra food we studied how the female's nutritional state affected her incubation schedule and the rate at which her mate fed her. Females that received extra food spent more time on the nest and shorter periods away from it, compared with control females. This suggests that nest attentiveness is governed by the amount of energy available to the female. When females reccived extra food, males decreased their rate of incubation feeding. They also did so in response to increasing ambient temperatures, whereas incubation schedules were unaffected. We, therefore, conclude that our results support the female nutrition hypothesis, i.e., that the food provided by the male constitutes a significant nutritional contribution to the incubating female.     

Costs of overlapping reproduction and moult in passerine birds: an experiment with the pied flycatcher

The process of moult in birds requires the investment of substantial amounts of energy and nutrients in feather production and attendant processes. Flight performance may be reduced during the moult of the wing feathers, and moulting birds may suffer from an increased predation risk. These factors may explain why the moult is usually separated in time from other energetically demanding processes, such as reproduction. In this study, we investigated the importance of the temporal separation of moult from breeding activities in the pied flycatcher Ficedula hypoleuca. We induced a moult-breeding overlap by removing the two innermost primaries on both wings, thus imitating the natural loss of these feathers during the initial stage of the moult. The experiment probably did not stimulate feather regrowth, but may have reduced flight performance. Just before fledging of the young, manipulated males and females, as well as the fledglings, had a lower body condition than control birds. The return rate of adult males in the year following the manipulation was significantly lower for males in the manipulated group than for control males. The return rate of females was lower than that of males, but there was no difference between the two female groups. The number of nestlings recruited in the year after the manipulation was significantly lower for the group with an induced moult-breeding overlap than for the control group. The reproductive performance in recruited and returning old females was independent of the manipulation, and in returning adults and recruits the feather quality in terms of number of barbs/mm was independent of the manipulation. These results suggest that a moult-breeding overlap induces fitness costs in terms of reduced recruitment rate and adult survival in a long-distance migrant. Received: 17 September 1997 / Accepted after revision: 23 February 1998     

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.     

Long-lasting effects of yolk androgens on phenotype in the pied flycatcher (Ficedula hypoleuca)

The hormonal environment during early development, such as maternally derived androgens in bird eggs, shapes the development of the offspring in ways that may have important long-term consequences for phenotype and behavior and, ultimately, fitness. We studied the long-term effects of yolk androgens on several phenotypic and physiological traits in male and female pied flycatchers (Ficedula hypoleuca) by experimentally elevating yolk androgen levels and rearing birds in common-garden environment in captivity. We found that high yolk androgen levels increased the basal metabolic rates in both females and males in adulthood. High yolk androgen levels did not affect male melanin coloration or plumage ornaments, or timing or speed of moult in either sex. No effect of androgen treatment on cell-mediated or humoral immune response was found in either sex. Covariation among the measured phenotypic traits was further not altered by androgen treatment. Our results suggest that exposure to high androgen levels can have long-lasting effects on some offspring traits, but do not seem to lead to different phenotypes. Furthermore, the role of yolk androgens affecting sexually selected male traits in our study species seems to be minor. The fitness consequences of yolk androgen-induced higher metabolic rates remain to be studied.     

Maternal energy expenditure does not change with flight costs or food availability in the pied flycatcher (Ficedula hypoleuca): costs and benefits for nestlings

We manipulated parental work load without changing brood size in a population of pied flycatchers Ficedula hypoleuca by removing two primaries (7 and 9) from each wing of females, thus reducing wing area and increasing flight costs. At other nests, we offered supplementary food in the form of live mealworms (10–20 g daily from hatching) to reduce brood demand and thus parental foraging costs. Other nests were left as controls. The daily energy expenditure of females feeding 12-day-old nestlings was measured with doubly labelled water D₂ ¹⁸O. Females in both treatments expended the same amount of energy, fed at the same rate and had similar body masses to birds in the control group. No effect of treatment on male mass and feeding effort was detected. More nestlings, however, died in nests of handicapped females. Nestlings of handicapped females had significantly lower body mass and haematocrit values than nestlings in food-supplemented nests, with nestlings in control nests occupying an intermediate position. The effects of both treatments on nestling mass, haematocrit values and mortality rates were only noticeable in nests infested with mites. Maternal energy expenditure is apparently constrained and offspring pay the costs imposed by reduced provisioning rate or increased demand caused by ectoparasites, while receiving benefits when food supply improves. The presumption that avian reproductive costs derive from changes in a flexible energy output may not be met in many cases. Received: 24 October 1998 / Received in revised form: 15 March 1999 / Accepted: 26 April 1999     

Female reactions to male absence after pairing in the pied flycatcher

Mating with an already mated pied flycatcher (Ficedula hypoleuca) male is costly for a female. Two hypotheses explain why some females still mate with already mated males. The deception hypothesis suggests that some females mate with already mated males since it is difficult to assess perfectly the mating status of a male with separate territories (polyterritoriality). The search cost hypothesis states that females are aware of male mating status but the costs of searching for an unmated male exceed costs associated with the status of secondary female. One potential cue that could disclose a male's mating status is the existence of brief visits to the primary territory by polyterritorial males. To mimic such visits I removed the male from the territory for 60 min soon after female settlement. Only few females abandoned their mates as a consequence of male removal even though they had available unmated males close by. This result is most consistent with the deception hypothesis. Females did not use occasional male absence as a cue to avoid presumably polyterritorial males and in this respect they were not perfect in assessing males.     

The effect of pup vocalisations on food allocation in a cooperative mammal, the meerkat (Suricata suricatta)

In the meerkat (Suricata suricatta), a cooperative mongoose, pups follow potential feeders while the group is foraging and emit incessant calls when soliciting food from them. In contrast to a ’stationary’ brood of chicks, in which nestlings are fed at a fixed location, meerkat pups are ’mobile’ and become spread out. The question arises whether meerkat pups that experience different constraints to those facing chicks have evolved similar begging strategies. This paper describes the vocalisations that meerkat pups emit in the context of begging and investigates the influence of these calls on food allocation by older group members and on the behaviour of littermates. Meerkat pups use two types of calls when soliciting food from a potential feeder. The most common is a ’repeat’ call, which pups emit continuously when following an older forager over several hours a day. In addition, when a potential feeder finds a prey item, the pups next to it emit a bout of calls with increased calling rate, amplitude and fundamental frequency, termed ’high-pitched’ calls. Observations, together with playback experiments, showed that more prey was allocated to pups that called longer and more intensely. The pup closest to a feeder was almost always fed. The probability of emitting high-pitched calls did not depend on the time since a pup had received food, and the change from repeat to high-pitched calls occurred suddenly. The main function of the high-pitched call, therefore, does not appear to be to signal a pup’s hunger state. More likely, the two calls, in the context of begging, may be an adaptation to energetic constraints in a mobile feeding system. Pups, which are dispersed during foraging, may emit repeat calls over long periods to prevent potential feeders from eating all the prey themselves. At the moment a potential feeder finds prey, pups may give the more intense high-pitched calls to direct feeders to bring the food item to them and not to a littermate. Therefore, unlike the stationary feeding system where chicks emit one type of begging call when the feeder approaches the nest, meerkats, with a mobile feeding system, have evolved two discrete types of vocalisations in the context of begging. Received: 22 November 1999 / Revised: 1 July 2000 / Accepted: 17 July 2000     

Past reproductive effort affects parental behaviour in a cichlid fish,Cichlasoma nigrofasciatum: a comparison of inexperienced and experienced breeders with normal and experimentally reduced broods

Defensive and parental care behaviour of convict cichlids that differed in past effort was compared. Before testing, some fish were bred three times while others were not bred. Age was held constant; all individuals in this study were approximately 20 months old (±2 months) at test time. Furthermore, half of the pairs in this study had their broods experimentally reduced by 50%. Results indicated that past effort across breeding attempts affects investment in the current brood. Experienced pairs were more aggressive toward a model predator than inexperienced parents. However, no major differences were observed in depreciable care (i.e. fanning). Contrary to previous studies, brood size had minor effects on parental care. This discrepancy could be due to the age of the parents; individuals in this study were significantly older than fish tested in previous studies. The results support parental investment theory and suggest that past effort is not only important within breeding episodes but also within an animal's lifetime.     

Determinants of parental effort: a behavioural study in the Eurasian kestrel,Falco tinnunculus

We recorded behaviour of kestrels (Falco tinnunculus) in western Finland during the courtship (1988–1992), incubation (1989–1991), early nestling (age of young 1–2 weeks, 1989–1992) and late nestling stages (3–4 weeks, 1989–1991) to examine determinants of their parental effort (PE). In males, PE was estimated as the hunting effort (the proportion of budget time spent in flight-hunting) and in females as the food provisioning rate (number of prey items delivered to the nest per hour). The following predictions derived from the parental investment theory were examined. (1) Parents rearing large clutches and broods should invest more in breeding than do parents rearing small clutches and broods. The hunting effort of parents did not increase with clutch or brood size, but males tending large broods had a higher prey delivery rate than males tending small broods (Figs 1–2). (2) PE of parents should increase in the course of the breeding season. In males, this was true only between the incubation and early nestling phases (Fig. 3). (3) The early pairs should invest more in breeding than late ones. This tended to be true during the early (for males) and late nestling phases (for females) (Fig. 4). (4) There should be a negative correlation between PE of mates within pairs, but no evidence for such adjustment was found (Fig. 5). (5) Females mated with bright-coloured attractive males should show higher PE than females mated with dull-coloured males but our results were inconsistent with this prediction. We conclude that PE decisions of kestrels are mainly based on cost-benefit estimates of residual reproductive value, rather than on current investment indicators, like clutch or brood size. This might be beneficial in environments with highly variable survival prospects of offspring caused by pronounced among-year variation in abundance of the main food (microtine rodents). The results also show that hypotheses explaining variation in PE in the short term are not necessarily valid for long-term PE, e.g. tending clutches or broods, which also reflects the demands of female and young.     

Regulation of parental effort in a long-lived seabird an experimental manipulation of the cost of reproduction in the antarctic petrel,Thalassoica antarctica

Summary The fitness of a parent in an altricial bird species is likely to be a function of the proportion of resources allocated to offspring production in relation to the amount spent on its own survival. Here we report an experiment on the Antarctic petrel in which we manipulated the costs of rearing an offspring by placing small lead loads on the legs of one parent. The bird could then either decrease its own body reserves or reduce the food load to the chick. The manipulated birds decreased their food load and increased the feeding interval, compared with unmanipulated birds. Consequently, the rate of chick loss increased. No significant difference was found between the body weights of experimental and control birds during the experiment.Publication no. 113 from the Norwegian Antarctic Research Expedition (NARE) 1989/90 Correspondence to: B.-E. Sæther     

Sibling aggression, hatching asynchrony, and nestling mortality in the black kite (Milvus migrans)

In siblicidal species, hatching asynchrony could act to reduce sibling rivalry or promote the death of last-hatched chicks. The pattern of hatching asynchrony was experimentally altered in the black kite Milvus migrans. Hatching asynchrony in control broods was intermediate between those of experimentally synchronised and asynchronised broods. Sibling aggression and wounds on the chicks were more commonly observed early in the nestling period and in synchronous nests. Serious injuries were observed on last-hatched chicks in asynchronous nests, as were observations of intimidated or crushed chicks. Sibling aggression was related to food abundance, but some chicks died at an early age in nests with abundant food (cainism). Cainism was more commonly found in asynchronous nests. For species with facultative siblicide, moderate hatching asynchrony could be a compromise between reducing sibling rivalry and avoiding large size differences between sibs that would result in cainism. Female black kites preferentially fed the smallest chicks and exhibited behaviours to reduce sibling aggression, contrary to observations in other siblicidal species. In a highly opportunistic forager such as the black kite, a strategy may exist to protract the life of all the chicks in the brood, waiting for unpredictable situations of food overabundance. This would induce the appearance of a parent-offspring conflict over brood reduction, reflected in the existence of a possible anticipated response by some of the chicks (cainism) and in the appearance of special behaviours by the parents to selectively feed smaller chicks or reduce sibling aggression. In this facultatively siblicidal species, cainism does not seem to be the final stage of an evolutionary trend favouring the raising of high-quality chicks, but a manifestation of a parent-offspring conflict over brood size. Received: 9 March 1998 / Accepted after revision: 8 August 1998     

Adjustment of parental effort in the puffin; the roles of adult body condition and chick size

 We examined the adjustment of parental effort of puffins by switching 20-day-old chicks randomly between parents of known body condition. Among unmanipulated birds mass gain (5–20 days) and mass of 20-day-old chicks was positively correlated with the body condition of parents at day 6. During the first 5 days after chick switching 28% (n = 55) of the parents deserted their foster chick. Parents which deserted their foster chick originally had a chick of their own that was smaller than that of those which did not desert their foster chick. Whether parents deserted their foster chick was also negatively related to the size of the foster chick. The mass of the foster chick was more important than the size of the parents' own chick in determining the desertion rate of chicks. The mass gain of the foster chick during the first 5 days after switching was positively related to the body condition of foster parents and also positively related to the mass of the foster parents' own chick, but negatively related to the size of the foster chick. The results suggest that puffins adjust their parental effort according to both their own body condition and the size of the chick. The latter may indicate the chick's prospect of survival and recruitment to the population. Received: 20 January 1996 / Accepted after revision: 27 October 1996     

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     

Adjustment of parental effort to manipulated foraging ability in a pelagic seabird,the thin-billed prion Pachyptila belcheri

An experiment was designed to examine in a long-lived seabird, the thin-billed prion (Pachyptila belcheri), how adults adjust their food provisioning strategy when their foraging abilities are reduced and when the chick's needs are increased. To reduce the foraging abilities of adults we impaired their flying ability by removing some flight feathers (handicapped), and to increase the food needs of the chick one parent was retained (single). Birds made either short foraging trips lasting 1–3 days, or long trips lasting 5–9 days. Control birds alternated long and short trips whereas single birds or handicapped birds made several successive short trips and thereafter a long trip. In each treatment, food loads tended to be heavier after long trips than after short trips, and single birds tended to bring heavier loads than control or handicapped birds. Birds in the three treatments lost similar amounts of mass after short trips and gained similar amounts of mass after long trips. However, the mass of handicapped birds declined through the experiment, while that of control and single birds remained stable. Although the proportion of chicks that died during the experiment was similar among the three treatments, the chicks fledged by a single bird were lighter than those in control nests. The results of the experiment suggest that thin-billed prions adjust their breeding effort differently to decreased flying ability or increased food demand by the chick. Single birds increase foraging effort without allowing their condition to deteriorate. Conversely, handicapped birds are unable to maintain their body condition while sustaining the chick at the same rate as control birds. It is suggested that in this long-lived seabird, adults probably adjust their breeding effort so that they do not incur the risk of an increased mortality, this risk being monitored by the body condition.