Responses by Central‐Place Foragers to Manipulations of Brood Size: Parent Flickers Respond to Proximate Cues but do not Increase Work Rate

Manipulations of brood size measure the willingness or ability of parents to invest in offspring and different reproductive roles may lead to differences in feeding effort between the sexes. Parental investment in birds is usually assessed by quantifying feeding rates, but this provides an incomplete picture of parental effort because it fails to account for how parents collect food on the landscape. We studied northern flickers (Colaptes auratus), a woodpecker in which males provide the majority of parental care and used a repeated measures design and short‐term (24 h) brood enlargements (N = 35) and reductions (N = 27) to assess effects of treatment on feeding rates to nestlings and parental foraging behaviour. Parents of enlarged broods did not significantly increase feeding rate, resulting in a decline in nestling mass. Parents of reduced broods decreased their feeding rates by 84%, but increased per capita feeding rates, resulting in nestling mass gain. The variation in feeding rates to enlarged broods was not influenced by feather corticosterone, body condition, feather re‐growth rate or mass change between the incubation and nestling periods. Foraging pattern on the landscape remained the same during the enlarged treatment for both sexes. We conclude that flickers respond to proximate cues in brood demands, but do not increase feeding rates to enlarged broods, at least in the short term. A literature review suggested that this lack of response is atypical for short‐lived species. We hypothesize that parents in species with large home ranges and long nestling periods face energetic limitations that constrain their ability to respond to enlarged broods. We encourage future studies to assess foraging behaviour on the landscape to document important trade‐offs for parents such as predation risk and energy expenditure while feeding offspring.     

Azure‐winged Magpies Cyanopica cyanus trade off reproductive success and parental care by establishing a size hierarchy among nestlings

It is common in birds that the sizes of nestlings vary greatly when multiple young are produced in one nest. However, the methods used by parents to establish size hierarchy among nestlings and their effect on parental provisioning pattern may differ between species. In the Azure‐winged Magpie Cyanopica cyanus, we explored how and why parents controlled the sizes of nestlings. Asynchronous hatching was the main cause of size hierarchy within the brood, although the laying of larger eggs later in the laying sequence reduced this effect. Parents with asynchronous broods produced more eggs and fledged more nestlings than those with synchronous broods but their brood provisioning rates, food delivery per feeding bout and feeding efficiency did not differ. We performed a cross‐fostering experiment to synchronize some asynchronous broods. Provisioning rates of asynchronous broods were lower than those of synchronized broods, but the daily growth rates and fledging body mass of their nestlings were not different. Our findings indicate that parents of asynchronous broods can achieve higher reproductive success than those of synchronous broods based on the same parental care, and the same reproductive success as those of synchronized broods based on less parental care. It appears that parent birds can better trade off reproductive success and parental care by establishing a size hierarchy among nestlings.     

Condition‐dependent expression of carotenoid‐ and melanin‐based plumage colour of northern flicker nestlings revealed by manipulation of brood size

Carotenoid‐based colouration in feathers is widely accepted to be a reliable signal of the health of an individual, but the condition‐dependence of melanin‐based plumage ornaments has been highly debated. Using broods that were manipulated in size, we tested whether nutritional stress during rearing affected the carotenoid pigmentation in secondary feathers and the size, shape, and symmetry of melanin spots on breast plumage of northern flicker Colaptes auratus nestlings. Two measures of carotenoid colour (chroma and brightness) of secondary flight feathers did not vary according to brood size treatment, but in a larger dataset from the population, carotenoid chroma was positively associated with nestling mass. Nestlings from experimentally enlarged broods had smaller melanin spots than those from reduced broods, which is some of the first experimental evidence that melanin ornament size in growing nestlings is condition‐dependent. However, the shape and symmetry of the melanin breast spots was not associated with nestling mass. Sexual dimorphism was apparent in both types of pigmentation and future studies should investigate whether there are any trade‐offs for nestlings between investing in carotenoid colouration and melanisation and whether trade‐offs differ between the sexes.     

Foraging Cost of a Long Tail Ornament: An Experiment with Sand Martin Females

The handicap hypothesis assumes that sexual ornaments impose a viability cost upon the bearers. There have been few empirical tests of this assumption. Previous studies show evidence for the cost of a tail ornament in male birds: a negative relationship between an experimentally increased tail ornament (long tail streamers) and efficiency at foraging for nestlings. However, it must be admitted, that the apparent impairing effect of an elongated tail could be a result of a decrease in male parental effort in response to an increase of female parental effort, which might have occurred in response to increased male attractiveness (differential allocation of female parental effort). In this study, the effect of differential parental expenditure was eliminated by lengthening the tail in female, rather than male, sand martins ( Riparia riparia ). Tail-elongated females decreased the rate at which they fed nestlings, and captured more but smaller insects. There was no simultaneous increase of feeding rate in the males that could explain the decrease of feeding rate in the females. These results confirm the existence of a cost of a tail ornament in birds feeding in flight, as is expressed in terms of impaired flight and foraging capacity.     

Male-biased brood sex ratio depresses average phenotypic quality of barn swallow nestlings under experimentally harsh conditions

Sex allocation strategies are believed to evolve in response to variation in fitness costs and benefits arising from the production of either sex and can be influenced by the differential susceptibility of sons and daughters to environmental conditions. We tested the effects of manipulating brood size and the sex ratio of the nestmates and the effect of sex on the phenotypic quality of individual barn swallow (Hirundo rustica) nestlings. Brood enlargement, which results in harsh rearing conditions, negatively affected the morphology and immunity of the nestlings. However, the negative consequences of brood enlargement were more marked among male than female offspring. In enlarged but not reduced broods, high proportions of male nestmates resulted in lowered individual body mass, body condition and feather growth. Thus, the consequences of a harsh environment on individual nestlings differed between the sexes and depended on the sex ratio among the other nestlings in the brood. The evolution of sex allocation strategies may therefore depend on the sex of individual nestlings but also on an interaction between environment and progeny sex ratio.     

Ectoparasites and reproductive trade-offs in the barn swallow (Hirundo rustica)

Parents are predicted to trade offspring number and quality against the costs of reproduction. In altricial birds, parasites can mediate these costs because intensity of parasitism may increase with parental effort. In addition, parasites may mediate a trade-off between offspring number and quality because nestlings in large broods may have reduced anti-parasite immune defence. In this study, we experimentally analysed the effect of brood size on infestation by an ectoparasitic mite in nests of barn swallows (Hirundo rustica). Nests with an enlarged brood had larger prevalence and intensity of infestation than those with a reduced brood. Importantly, each nestling in enlarged broods was exposed to a larger number of mites, even when measured on a per nestling basis, than in reduced broods. Nestlings in enlarged broods had smaller body mass and T-cell-mediated immune response compared to reduced broods. T-cell-mediated immune response and feather growth were negatively correlated with per nestling intensity of infestation in enlarged but not in reduced broods. The results suggest that nestlings in enlarged broods have depressed immunity leading to larger per nestling mite infestation. Hence, exposure to parasites of offspring and parents increases with brood size, and parasitism can thus mediate trade-offs between reproduction and number and quality of the progeny in the barn swallow.     

Age-dependent reproductive costs and the role of breeding skills in the Collared flycatcher

This study addressed whether there are any age‐related differences in reproductive costs. Of especial interest was whether young individuals increased their reproductive effort, and thereby their reproductive cost, as much as older birds when brood size was enlarged. To address these questions, a brood‐size manipulation experiment with reciprocal cross‐fostering of nestlings of young and middle‐aged female Collared flycatchers, Ficedula albicollis, was performed on the Swedish island of Gotland. Nestlings’ body mass, tarsus length and survival were recorded to estimate the parental ability and parental effort of the experimental female birds. Female survival and clutch size were recorded in the following years to estimate reproductive costs. We found that middle‐aged female flycatchers coped better with enlarged broods than younger females or invested more in reproduction. In the following year, young female birds that had raised enlarged broods laid smaller clutches than the females from all the other experimental groups. This result shows that the young female birds pay higher reproductive costs than the middle‐aged females. Both young and middle‐aged female flycatchers seemed to increase their reproductive effort when brood size was increased. However, such an increase resulted in higher reproductive costs for the young females. The difference in reproductive costs between birds of different ages is most likely a result of insufficient breeding skills of the young individuals.     

Insights into life history theory: a brood size manipulation on a southern hemisphere species,Tachycineta leucorrhoa,reveals a fast pace of life

Life history traits exhibit substantial geographical variation associated with the pace of life. Species with a slow pace are expected to invest more in their future/residual reproductive value and are more common at tropical latitudes, whereas species from high latitudes, with a faster pace, are expected to prioritize the current reproductive effort. Most evidence supporting this pattern comes from studies conducted in tropical and north temperate species; very little is known about patterns in southern South American species. Here, we describe the life history of a southern swallow Tachycineta leucorrhoa and use an experimental approach to test their breeding strategy over four breeding seasons. We manipulated brood size for 105 nests of white‐rumped swallows to measure whether costs of reproduction were borne by adults or nestlings as alternative selection strategies towards maintaining residual or current reproductive value. Adults increased their feeding effort in enlarged broods, at least enough to maintain nestlings’ development/growth. In addition, adults decreased the number of visits to the nest (without having a negative effect on nestlings) in reduced broods. We did not detect differences in fledging success among treatments, suggesting there were no differences in nestlings’ survival. However, enlarged broods more frequently incurred in complete nest failure, suggesting only some adults were able to cope with increased costs of reproduction. We conclude this species is characterized by a fast pace of life similar to their northern congeners and less like its tropical ones. This is one of the first studies to use an experimental approach to test a life history hypothesis of pace of life using data from a southern South American species. We encourage researches to include southern species when evaluating latitudinal variations as we still do not have enough evidence to assume all southern subtropical species are indeed similar to tropical ones.     

No evidence for adjustment of sex allocation in relation to paternal ornamentation and paternity in barn swallows

Sex allocation theory predicts that parents should adjust investment in sons and daughters according to relative fitness of differently sexed offspring. In species with female preference for highly ornamented males, one advantage potentially accruing to parents from investing more in sons of the most ornamented males is that male offspring will inherit characters ensuring sexual attractiveness or high-quality genes, if ornaments honestly reveal male genetic quality. Furthermore, in species where extra-pair fertilizations occur, offspring sired by an extra-pair male are expected to more frequently be male than those of the legitimate male if the latter is of lower quality than the extra-pair male. We investigated adjustment of sex ratio of offspring in relation to ornamentation of the extra-pair and the social mate of females by direct manipulation of tails of male barn swallows Hirundo rustica . Molecular sexing of the offspring was performed using the W chromosome-linked avian chromo-helicase-DNA-binding protein (CHD) gene while paternity assessment was conducted by typing of hypervariable microsatellite loci. Extra-pair offspring sex ratio was not affected by ornamentation of their biological fathers relative to the experimental ornamentation of the parental male. Experimental ornamentation of the parental males did not affect the sex ratio of nestlings in their broods. Female barn swallows might be unable to bias offspring sex ratio at hatching according to the quality of the biological father. Alternatively, fitness benefits in terms of sexual attractiveness of sons might be balanced by the cost of compensating for little parental care provided by highly ornamented parental males, if sons are more costly to rear than daughters, or the advantage of producing more daughters, if males with large ornaments contribute differentially more to the viability of daughters than sons.     

Effects of parental effort on blood stress protein HSP60 and immunoglobulins in female blue tits: a brood size manipulation experiment

1. Physiological stress in animals may impose a limit for investment in current reproduction in the wild. A brood manipulation experiment was conducted in a population of blue tits Cyanistes caeruleus to study the effect of parental effort on changes in two types of proteins related with stress: the blood stress protein HSP60 and the plasma immunoglobulins. 2. Levels of HSP60 were reduced across the experiment for females attending reduced broods, and females attending enlarged broods experienced a reduction of immunoglobulin levels. Moreover, the overall changes in the levels of both proteins were positively related. 3. By controlling for the change in immunoglobulin levels we found an increase in HSP60 for females in the enlarged treatment, presumably to offset deleterious effects derived from increased effort. 4. Maternal effort was able to partially compensate for the effect of treatment as nestlings did not differ in mass and levels of immunoglobulins and HSP60 among treatments. 5. Physiological stress as reflected in stress and immunoglobulin proteins may limit maternal effort in breeding blue tits.     

Intraseasonal effects of clutch manipulation on parental provisioning and residual reproductive value of eastern phoebes (Sayornis phoebe)

Summary First clutches of double-brooded eastern phoebes Sayornis phoebe were manipulated (up two eggs, down 2 eggs or no change) to test for intraseasonal reproductive tradeoffs and to test whether size of first brood influenced food delivery rates to nestlings and nestling quality in second broods.Considering all nests from both broods, rate of feeding nestlings increased linearly with brood size but nestling mass per nest decreased with increasing brood size. High nestling weights in small broods may have resulted from parents delivering better quality food, but we did not test this.Among treatment groups in first broods, nestlings from decreased broods weighed more than those in control or increased broods. Treatment did not influence the likelihood that second nests would be attempted after successful first nests nor did it alter the interval between nests. Nestlings of parents that renested weighed more than those of parents that did not, regardless of treatment, suggesting that post-fledging care may preclude renesting. Mass of individual females did not change between broods, regardless of brood size. Clutch sizes of second attempts were not affected by manipulations of first broods but increasing first broods reduced the number of nestlings parents were able to raise to day 11 in their second broods. However, manipulation of first broods did not affect mean nestling mass per nest of nestlings that survived to day 11.In phoebes, parents of small first broods are able to raise nestlings in better condition. We predict that in harsh years, parents of small first broods would be more likely to renest. Parents of enlarged first broods sacrificed quality of offspring in second broods, which seems a reasonable strategy if nestlings from second broods have lower reproductive value.     

Features of begging calls reveal general condition and need of food of barn swallow (Hirundo rustica) nestlings

Altricial offspring of birds solicit food provisioning by complexbegging displays, implying acoustic and visual signals. Differentcomponents of begging behavior may function as reliable signalsof offspring state and thus reproductive value, on which parentsbase optimal parental decisions about allocation of criticalresources (e.g., food). We experimentally manipulated componentsof general condition of nestling barn swallows (Hirundo rustica)by (1) altering brood size by cross-fostering an unbalanced number of nestlings between pairs of synchronous broods andthus manipulating the level of within-brood competition forfood, (2) injecting some nestlings with a harmless immunogen,simulating an infection, and (3) preventing part of the nestlingsfrom receiving food for a short period while establishing controlgroups. We recorded rate of begging response by individual nestlings as parents visited the nest and recorded begging calls usinga DAT recorder to analyze six sonagraphic features of vocalizations.Our factorial experiment revealed that nestlings deprived offood begged more frequently when parents visited the nest comparedto their non—food-deprived nest mates. Food deprivationincreased duration of syllables forming begging calls, whereas brood size enlargement resulted in increased latency of responseto parental calls. Heavy nestlings in good body condition vocalizedat a relatively low peak frequency. To our knowledge, thisis the first study in which begging rate and sonagraphic structureof begging calls are shown to reliably reveal a diverse setof components of offspring general state, on which parental decisions may be based.     

Examining the Cost of Experimentally Imitated Ornaments

Long forked tail ornament in male barn swallows (Hirundo rustica) were suggested to impose a condition‐dependent viability cost ( Møller 1989 ; reviewed in Møller 1994 ; Møller & de Lope 1994 ; Møller et al. 1995 ): long tails impair male flight and foraging ability in terms of mean size of prey captured. In a recent study, we ( Matyjasiak et al. 1999 ) showed such a foraging cost of an experimentally imitated tail ornament in the sand martin (Riparia riparia), which have no tail ornament. We lengthened the tail in females, instead of in males, and thus controlled experimentally for the possible effect of differential allocation of female parental expenditure (differential‐allocation hypothesis, Burley 1986 ). Cuervo (2000) raises important issues in his comments about our paper. He questions the method used in our study ( Matyjasiak et al. 1999 ). He points out that ‘in order to test the cost of flight of an ornament, the trait to be experimentally manipulated has to be an ornament’, and that we should have not only elongated but also shortened tail feathers. Secondly, he suggests that we did not provide, in our article, the exclusive evidence for the handicap model of sexual selection. Thirdly, he disagrees with our suggestion that the results from the barn swallow experiments could be confounded by the differential allocation of female parental expenditure. Here, we outline the areas of agreement and disagreement between Cuervo's critique and our paper. Firstly, we agree with Cuervo regarding the importance of tail shortening in studies of fully developed tail ornaments, which has already been pointed out by other authors ( Thomas & Rowe 1997 ; Evans & Thomas 1997 ). However, shortening of an ornament that is at equilibrium would always produce a decrease in costs. Therefore it will only confirm that an ornament is costly. However, if shortening is done to individuals of various, known condition, it may result in crucial information for distinguishing between hypotheses of sexual selection. We agree that for a character that is not an ornament, as in our experiments, shortening would not give new insights apart from another way of confirming the measures used in the barn swallow studies. However, we disagree with Cuervo's claims of irrelevance of our experiment to the issues of the evolution of signals. We have chosen sand martins as a model species because the shape of its tail resembles that in ancestors of modern tail‐ornamented swallows, from which tail feather elongation under sexual selection may have started ( Matyjasiak et al. 2000 ). Possible scenarios of the early evolution of a tail ornament may be examined by experimentally adding such an ornament, which requires manipulating original, non‐ornamental traits (e.g. Goötmark 1994, 1996 ). We disagree with Cuervo that the existence of traits that may reduce the costs of ornaments (e.g. Møller 1996 ) eliminates the validity of tail manipulation studies in species without ornaments. We believe that such cost‐reducing traits may not have existed during the early stages of evolution of tail ornaments (as well as other sexual traits) but may have developed later, and our experiment may represent such a situation very well. Hence, by imitating the initial development of a forked tail ornament in sand martins, we performed a biologically relevant manipulation. Secondly, we do not state in our paper that we have tested or proved the handicap principle. We only mention in the last paragraph of the Discussion, that our results are consistent with this principle. However, we do admit that mentioning only the handicap hypothesis in the Introduction and Discussion only, and omitting other processes by which costly tail ornaments might arise, was unwarranted, as Cuervo properly argued; this might have left a false impression that our goal was to test the handicap hypothesis. We do believe that we have properly measured the costs of tail elongation, and this was our goal, as stated in the Abstract and in the last paragraph of the Introduction. Nowhere in the paper did we state that we actually aimed to test the differential costs of ornaments that are crucial to the handicap hypothesis. This issue was the objective of a different paper ( Matyjasiak et al. 2000 ) in which we consider conditions important for the early evolution of tail ornaments, with special emphasis on the handicap principle. Thirdly, we are more cautious at interpreting the barn swallow studies and we do not exclude a possibility that differential allocation of parental expenditure can affect the size of prey brought by males. Differential allocation does exist in this species ( Møller 1992 ; de Lope & Møller 1993 ), as shown by the higher feeding rate by females responding to male higher attractiveness (longer tails). We believe that even though simple logical, intuitive reasoning may suggest that prey size should be unaffected by a differential allocation mechanism, it is not just reasoning but empirical proof that is needed here. Because there was no proof for a lack of the effect of differential allocation on the size of prey, we thought of an independent way of illustrating the costs of tail elongation in swallows, using a species without possible differential allocation effects. We thought that if we obtained results confirming the barn swallow studies by Møller and collaborators, we could help in validating the measures of tail elongation costs used in those studies. This was possible using the sand martin females for reasons discussed in the paper, all of which point to the lack of any differential allocation effects in this species. From this point of view, experimentally coupling tail elongation with tail shortening in our study appears unnecessary. Even though, as argued by Cuervo (2000) , it seems likely that the prey size in male swallows is unaffected by a change in feeding rate of females in response to male attractiveness (an assumption inherent in the barn swallow studies), another scenario is also possible. Imagine a female that has increased the amount of food for nestlings, in response to increased sexual attractiveness of her male partner due to experimental elongation of his tail. In such a situation the male has been relieved from a considerable duty of providing the young with a large amount of food. Hence, it is possible that such a male shifts from maximising the energy brought to nestlings per unit time (i.e. maximising foraging rate) to the criterion of obtaining the daily energetic needs at the least expense (i.e. minimising foraging costs). Such shifts may lead to including some non‐preferred, small insects that can be captured quickly and inexpensively in terms of energy, because it does not require the use of expensive flapping flight, which is required to capture larger, preferred insects ( Waugh 1978 ; Bryant & Turner 1982 ; Turner 1980, 1982 ). By including more small insects in their catch at the expense of a reduced foraging rate, attractive males could save energy for future use. Barn swallows appear to compromise between maximising their foraging efficiency (maximising foraging gains per costs) and maximising energy intake per unit time (see fig. 5.7 in Turner 1980 and table IV in Turner 1982 ). Hence, if sexually attractive males aim at minimising foraging costs rather than maximising foraging rate for nestlings, then we cannot exclude the possibility that this may result in smaller insects being caught, on average, by males with experimentally longer tails. By a similar reasoning, it is theoretically possible that differential allocation effects could lead to larger mean prey size in males with experimentally shortened tails ‐ an effect actually shown in the barn swallow studies ( de Lope & Møller 1993 ; Møller & de Lope 1994 ; Møller et al. 1995 ). Hence, whether shortening, elongating or performing both experimental manipulations, in our view we cannot be entirely sure whether the results are affected by differential allocation. This was sufficient motivation for us to investigate, independently, the usefulness of the change in prey size as an indicator of foraging costs due to elongated tails. In contrast to Cuervo's opinion, we believe that our results are relevant to the issues important for the evolution of forked tail ornaments. We have measured the costs of a character that imitates the hypothetical early stages of the evolution of sexual ornaments, and we may use these results to discuss the early evolution of sexual ornaments (see Matyjasiak et al. 2000 ). We also confirmed the validity of measures of tail elongation costs used in the barn swallow studies.     

Parasite-mediated growth patterns and nutritional constraints in a cavity-nesting bird

1. Trade-offs between growth and immunity of nestling birds can be influenced by parasites, but the magnitude of these effects may depend on availability of critical dietary nutrients. Owing to their importance for both immune system function and growth, dietary carotenoids have the potential to mediate parasite-induced developmental strategies of avian hosts. 2. The effects of ectoparasitic blow flies Protocalliphora spp. and dietary carotenoids (lutein and zeaxanthin) on immune function and patterns of growth in nestling mountain bluebirds Sialia currucoides were investigated by combining parasite removal and carotenoid supplementation treatments in a 2 x 2 design. 3. Supplemental carotenoids enhanced nestlings' T-cell-mediated immune response following intradermal injection of phytohaemagglutinin. 4. The effect of carotenoid supplementation on rate of mass gain depended on whether broods were exposed to parasites: among parasitized broods, those receiving supplemental carotenoids gained mass more rapidly than nonsupplemented broods, whereas there was no effect of supplemental carotenoids on growth of mass in broods that had parasites removed. This suggests that additional dietary carotenoids allowed nestlings to compensate for the otherwise detrimental effects of parasites on mass gain. For length of the eighth primary feather at fledging, early and late broods differed in their response to parasitism: early broods showed an increase in feather length when parasites were removed, while nestlings in late broods had shorter feathers in the absence of parasites. We suggest that this may reflect within-season variation in parasite-mediated growth strategies of nestlings. 5. Maternal condition was positively associated with mass, condition and rate of feather growth of offspring under all conditions, and also influenced nestling immunocompetence, but only in the absence of parasites. 6. We conclude that dietary carotenoids alleviate some of the detrimental effects of parasites on nestling birds; however, parasites also appear to specifically influence other growth and resource allocation strategies, and possibly constrain maternal or genetic effects on offspring phenotype, irrespective of dietary carotenoid availability.     

Cell-mediated immunity predicts the probability of local recruitment in nestling blue tits

We investigated whether the variation in T-cell-mediated immune function of blue tit nestlings affected their fledgling success and the probability of local survival. We studied the relationship between immune function and survival under two rearing conditions: control, unmanipulated, and experimentally enlarged broods. Brood enlargement had negative effects on nestling immune response. Immune response was positively related to fledgling success and it predicted the probability of local recruitment. However, the relationship between immune response and the probability of recruitment was significantly positive only among control broods and nonsignificant among enlarged broods. The effect of immune response on the recruitment probability was not affected by variation in body mass. Our study suggests that selection for immune responsiveness seems to be weak or even absent under unfavourable rearing conditions as simulated by brood size enlargement. Therefore, year-to-year environmental variation and environmental heterogeneity may constrain evolution towards higher immune responsiveness.     

Cost of reproduction: parental survival and production of recruits in the Willow Tit Parus montanus

Summary Brood sizes of the Willow Tit were altered experimentally by subtracting or adding two nestlings in 1986 and 1987 in the vicinity of Oulu, northern Finland. The manipulated broods were within the normal range observed in natural conditions. Unaltered broods were used as controls. Data from natural broods from 1978–1985 were available for comparison. When the nestlings were 13 days old they were ringed and weighed and their tarsus, wing, and tail lengths were measured. On the same day the parents were caught, weighed, and measured. In 1986 there were no differences in nestling mortality between the reduced, control, or enlarged broods; i.e. parents were able to fledge the two extra young. In 1987 starvation was most pronounced in the enlarged broods. This resulted in the number of fledglings being practically the same in each manipulation category. Especially the body weight, but also the other indices of body size, decreased as a function of the brood size category, suggesting that there may be quality differences between the young reared in different experimental groups. In 1986 there was a non-significant trend towards lower body weight of the parents attending reduced, control, or enlarged broods, in that order. In 1987 the differences were much smaller. These results were not due to size differences between the groups, so possibly the increased reproductive effort of raising extra young was responsible for the trend observed in 1986. There were no significant differences in parental survival associated with the manipulation category, although the trend in the females was consistent with the hypothesis of reproductive cost. It is possible that environmental conditions in 1986 were so favourable that the tits were not unduly stressed even when attending two extra young. Correlative data from 1978–1985 did not support the cost hypothesis either. A non-significant trend towards reduced post-fledging survival and recruitment of the young was observed with increased brood size. The average fitness value of parents, incorporating parental survival and number of recruits, showed that the success of the adults raising enlarged broods may be lower than that of others. It seems that the reproductive cost, if it exists, decreases individual fitness value by reducing the chances of recruiting descendants into the next generation. The reproductive stress may be insufficient to reduce the subsequent survival of parents. More data are however needed to confirm these results.     

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

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

Variable sexual ornaments in scarlet-tufted malachite sunbirds (Nectarinia johnstoni) on Mount Kenya

In order to be elaborated by sexual selection, sexual ornaments must vary perceptibly and genetically among individuals in natural populations. Rather little is known about ornament variation in monogamous species, in which sexual selection should act more weakly than in polygynous species. We report phenotypic variation in feather ornament size (elongated tails and pectoral tufts) and body size in the scarlet-tufted malachite sunbird Nectarinia johnstoni , a monogamous, sexually dimorphic nectarivore of East African alpine zones. Fully-expressed male ornaments are highly significantly more variable (CVs = 12–29%) than are skeletal and wing measures primarily affected by natural selection (CVs = 2 4%). Female sunbirds have pectoral tufts which are significantly (22–25%) smaller than those of adult males, but more variable (CV_s= 21–22%, CV_s= 12–15%), and more variable than body size. Among males with fully-grown ornaments, those with longer tails tend to have longer wings and wider tufts. The high variation in fully-grown ornaments in malachite sunbirds is consistent with the view that the ornaments are condition-dependent sexual signals. Finally, we review studies of feather ornament variation to date, and show that ornaments are much more variable in monogamous than non-monogamous species, apparently due to the relatively weak pressure of sexual selection.     

The Process and Causes of Fledging in a Cavity-Nesting Passerine Bird, the House Wren (Troglodytes aedon)

Little is known about the process or causes of fledging or nest‐leaving in passerine birds because researchers can rarely predict when fledging will occur in a given nest. We used continuous videotaping of nests to both document the process of fledging in the house wren, Troglodytes aedon, a small, cavity‐nesting songbird, and test hypotheses as to what might cause fledging to begin. Fledging began any time from 14 to 19 d after hatching commenced. Slower‐developing broods fledged later than faster‐developing broods. Fledging typically began within 5 h of sunrise and over 80% of all nestlings fledged before noon. All nestlings fledged on the same day at 65% of nests and over two consecutive days in most other nests. We found no evidence that fledging was triggered by changes in parental behaviour. Parental rate of food delivery to nestlings did not decline during a 3‐h period leading up to the first fledging, nor was the rate of feeding just prior to the first fledging lower than the rate at the same time the day before. Moreover, parents did not slow the rate of food delivery to nests after part of the brood had fledged. Hatching is asynchronous in our study population which creates a marked age/size hierarchy within broods. At most nests, the first nestling to fledge was the most well‐developed nestling in the brood or nearly so (as measured by feather length). This suggests that fledging typically begins when the most well‐developed nestlings in the brood reach some threshold size. However, at about one‐fifth of nests, the first nestling to fledge was only moderate in size. At these nests, severe competition for food may have caused smaller, less competitive nestlings to fledge first to increase their access to food. We found no strong support for the suggestion that the oldest nestlings delay fledging until their least‐developed nestmate reaches some minimum size, although further experimental work on this question is warranted.     

The sexual selection paradigm: have we overlooked other mechanisms in the evolution of male ornaments?

Extravagant male ornaments expressed during reproduction are almost invariably assumed to be sexually selected and evolve through competition for mating opportunities. Yet in species where male reproductive success depends on the defence of offspring, male ornaments could also evolve through social competition for offspring survival. However, in contrast to female ornaments, this possibility has received little attention in males. We show that a male ornament that is traditionally assumed to be sexually selected—the red nuptial coloration of the three-spined stickleback—is under stronger selection for offspring survival than for mating success. Males express most coloration during parenting, when they no longer attract females, and the colour correlates with nest retention and hatching success but not with attractiveness to females. This contradicts earlier assumptions and suggests that social selection for offspring survival rather than for sexual selection for mating success is the main mechanism maintaining the ornament in the population. These results suggest that we should consider other forms of social selection beyond sexual selection when seeking to explain the function and evolution of male ornaments. An incorrect assignment of selection pressures could hamper our understanding of evolution.