Infanticidal and anti-infanticidal strategies in the swallow Hirundo rustica

Summary Five definite and nine suspected cases of infanticide were documented in the swallow Hirundo rustica. Unmated males visited neighbouring nests and removed entire broods during the first days of the nestling period, preferentially from nests owned by young late-breeding individuals. Infanticide followed the disappearance of the male nest owner in 11 of the 14 cases. The unmated infanticidal male later mated with the female victim although renesting during the same season only took place in 12 of the 14 cases. The fraction of unmated males increased with colony size, and infanticide was relatively more common if unmated males were abundant. Females did not engage in extra-pair copulations in order to avoid infanticide. Intense guarding of the nest and its contents prevented infanticide. Nest guarding intensity was higher in colonial compared with solitarily breeding swallows, and guarding intensity increased with colony size. Experimental removal of male swallows during the early nestling period reduced the nest guarding intensity and increased frequency of visits from unmated males in colonies, but not among solitarily breeding swallows. Colonial nests from which the male was removed suffered from infanticide more often than solitary nests, and nests where infanticide was recorded were guarded significantly less intensely than other nests before the infanticidal incidents.     

Breeding synchrony and paternity in the barn swallow (Hirundo rustica)

The barn swallow (Hirundo rustica) is a socially monogamous passerine which usually breeds in colonies where extra-pair copulations are frequent. Males intensively guard their mates during the female fertile period. Since males are more likely to be available for extra-pair copulations when their mate is not fertile, synchrony in timing of breeding may affect paternity of individual males. In this study, we analysed the change in mate-guarding rate by males in relation to the fertility condition of the female, and the relationships between breeding synchrony and density with paternity in first broods of 52 male barn swallows. Paternity (proportion of nestlings fathered in own brood) was assessed by typing of three highly polymorphic microsatellite loci. Mate guarding by males peaked during the fertile period of their mates. Paternity increased as breeding synchrony in the colony increased. Paternity of barn swallows is positively associated with the degree of exaggeration of male tail ornaments. The relationship between male ornamentation and paternity was partly mediated by an effect of ornament size on breeding synchrony. We suggest that females might delay breeding with low-quality males to enhance their opportunities for being fertilised by high-quality extra-pair males. Received: 16 January 1998 / Accepted after revision: 25 October 1998     

Female preference for apparently symmetrical male sexual ornaments in the barn swallow Hirundo rustica

Summary The outermost tail feathers of barn swallows Hirundo rustica apparently reliably signal the quality of males, because individuals with the longest tails have the lowest degree of fluctuating asymmetry (random deviations from symmetry in the otherwise symmetrical tail trait) despite the size of their secondary sexual character. I experimentally tested whether females preferred males with symmetrical tails without altering the aerodynamic properties of birds by painting the tips of the outermost tail feathers with white or black correction fluid. Unmated males were randomly assigned to one of four treatments: (i) asymmetrical tails, where the outermost 20 mm of one tail feather was painted white and the other black; (ii) symmetrical tails where the outermost 10 mm of both tail feathers was painted white (symmetric I); (iii) symmetrical tails where the outermost 20 mm of both tail feathers was painted white (symmetric II); or (iv) controls where the outermost 20 mm of both tail feathers was painted black. The experimental treatment affected the duration of the premating period since it took longer for asymmetrical males to acquire a mate than for either group of symmetrical males or control males. This gave rise to a delayed start of laying among males with apparently asymmetrical tails. The seasonal production of fledglings therefore decreased from control males through males with either symmetrical treatment to males with the asymmetrical treatment. Females therefore pay direct attention to the level of fluctuating asymmetry in secondary sexual characters even when the asymmetry does not affect the aerodynamic properties of males.     

Mixed reproductive strategy and mate guarding in a semi-colonial passerine,the swallow Hirundo rustica

Summary Both male and female swallows Hirundo rustica have a mixed reproductive strategy (parental care for offspring and extra-pair couplations). Mate guarding protects females from male harassment and male swallows from being cuckolded. Females hide their fertile period by copulating successfully with their mates for an extended period during first clutches. Males guard in the pre-fertile period, when many unpaired males are present. Early breeding swallows guard more than late breeders since more sexual chases of females by non-mate males take place in the early period. Solitarily breeding females experience few chases by strange males; copulation frequency, length of copulation period and mate guarding is adjusted to a lower level than among colonial birds. Male guarding activity is more intense in the fertile than in the pre-fertile period. Guarding reduces success of extra-pair copulation attempts.Three female swallows each paired and copulated with a single male and later changed to a new male before starting to breed. Extra-pair copulations most often take place between neighbouring swallows in the fertile period of the female. Many old, early breeding males and many young, late breeding females participate in extra-pair couplations. Successful extra-pair copulations peak in the fertile period contrary to success of pair copulations which does not change during the copulation period.     

Sexual selection, feather breakage and parasites: the importance of white spots in the tail of the barn swallow (Hirundo rustica)

The long outermost tail feathers of barn swallows Hirundo rustica have white spots that are larger in males than in females and in adults than in juveniles. Spot size increases with age among adults and is positively correlated with tail length. We tested the functional significance of these white spots by randomly assigning males to either of three groups during spring: (1) subjected to a considerable reduction in the size of all spots using a black permanent pen; (2) subjected to a small reduction in the size of all spots; or (3) a control group with no reduction. Experimental treatment significantly affected the total number of offspring produced per season: breeding tended to be delayed and second broods were less frequent after a considerable reduction in the size of the males' tail feather spots. White parts of feathers lacking melanin were more often found to be the site of feather breakage than the melanised parts of similar width. Furthermore, white tail spots were the preferred feeding site of feather-eating Mallophaga and, since long-tailed males have fewer Mallophaga than short-tailed males, long-tailed males may pay a lower cost for their large white tail spots than short-tailed males. Hence, the cost of increased risk of feather breakage at white tail spots and the non-random distribution of feather lice among individuals will render tail spots a reliable signal of phenotypic quality. Received: 25 May 1998 / Accepted after revision: 4 January 1998     

Variation in sperm morphometry and sperm competition among barn swallow (Hirundo rustica) populations

Spermatozoa vary greatly in size and shape among species across the animal kingdom. Postcopulatory sexual selection is thought to be the major evolutionary force driving this diversity. In contrast, less is known about how sperm size varies among populations of the same species. Here, we investigate geographic variation in sperm size in barn swallows Hirundo rustica, a socially monogamous passerine with a wide Holarctic breeding distribution. We included samples from seven populations and three subspecies: five populations of ssp. rustica in Europe (Czech, Italy, Norway, Spain, and Ukraine), one population of ssp. transitiva in Israel, and one population of ssp. erythrogaster in Canada. All sperm traits (head length, midpiece length, tail length, and total length) varied significantly among populations. The variation among the European rustica populations was much lower than the differences among subspecies, indicating that sperm traits reflect phylogenetic distance. We also performed a test of the relationship between the coefficient of between-male variation in total sperm length and extrapair paternity levels across different populations within a species. Recent studies have found a strong negative relationship between sperm size variation and extrapair paternity among species. Here, we show a similar negative relationship among six barn swallow populations, which suggests that the variance in male sperm length in a population is shaped by the strength of stabilizing postcopulatory sexual selection.     

Male tail streamer length predicts fertilization success in the North American barn swallow (Hirundo rustica erythrogaster)

In socially monogamous species, extra-pair paternity has the potential to increase the variance in male reproductive success, thereby affecting the opportunity for sexual selection on male extravagant ornamentation. In the European barn swallow (Hirundo rustica rustica), the tail streamer length is a sexually selected male ornament and an honest indicator of viability. The North American barn swallow (Hirundo rustica erythrogaster) also shows sexual dimorphism in tail streamer length, but whether this trait holds the same signalling function in this subspecies is a controversial issue, and the available literature is presently scarce. Here, we present data on paternity in the North American barn swallow, including a complete sampling of extra-pair sires in four colonies. We analysed how extra-pair paternity affected the variance in male fertilization success and examined whether male tail streamer (i.e. the outermost tail feather) length correlated with fertilization success (n=86 males). Extra-pair paternity constituted 31% of all offspring and significantly increased the variance in male fertilization success. The number of offspring sired by extra-pair males accounted for almost half of the total variance in male fertilization success. Males with naturally long tail streamers had a higher fertilization success than males with shorter tail streamers, and this pattern was mainly caused by a higher extra-pair success for males with long tail streamers. Males with long tail streamers also paired with early breeding females in prime body condition. These results are consistent with the idea that there is directional sexual selection on male tail streamer length, possibly mediated through male extra-pair mating success or the timing of breeding onset.     

Causes of couspecific nest parasitism in the northern masked weaver

Summary Conspecific nest parasitism is a common reproductive strategy in the northern masked weaver (Ploceus taeniopterus). Parasites appear to be females with nests of their own who lay an additional egg in another female's nest as a way to enhance their reproductive success. Brood size in the northern masked weaver is practically constrained to three: starvation in four-chick broods is very common. The constraint on brood size is probably imposed by the extreme hatching asynchrony characteristic of this species: the last egg in a four-egg clutch hatches 72 h after the first egg, and when a chick starves, it is almost always the youngest chick. Late-hatching chicks also grow more slowly than do early-hatching chicks. If a female were to lay a fourth egg in her own nest, there is little chance that it would succeed. However; if she places it in another female's nest before that host lays her third egg, then it may have a greater chance of success.     

Tests of landscape influence: nest predation and brood parasitism in fragmented ecosystems

The effects of landscape fragmentation on nest predation and brood parasitism, the two primary causes of avian reproductive failure, have been difficult to generalize across landscapes, yet few studies have clearly considered the context and spatial scale of fragmentation. Working in two river systems fragmented by agricultural and rural-housing development, we tracked nesting success and brood parasitism in > 2500 bird nests in 38 patches of deciduous riparian woodland. Patches on both river systems were embedded in one of two local contexts (buffered from agriculture by coniferous forest, or adjacent to agriculture), but the abundance of agriculture and human habitation within 1 km of each patch was highly variable. We examined evidence for three models of landscape effects on nest predation based on (1) the relative importance of generalist agricultural nest predators, (2) predators associated with the natural habitats typically removed by agricultural development, or (3) an additive combination of these two predator communities. We found strong support for an additive predation model in which landscape features affect nest predation differently at different spatial scales. Riparian habitat with forest buffers had higher nest predation rates than sites adjacent to agriculture, but nest predation also increased with increasing agriculture in the larger landscape surrounding each site. These results suggest that predators living in remnant woodland buffers, as well as generalist nest predators associated with agriculture, affect nest predation rates, but they appear to respond at different spatial scales. Brood parasitism, in contrast, was unrelated to agricultural abundance on the landscape, but showed a strong nonlinear relationship with farm and house density, indicating a critical point at which increased human habitat causes increased brood parasitism. Accurate predictions regarding landscape effects on nest predation and brood parasitism will require an increased appreciation of the multiple scales at which landscape components influence predator and parasite behavior.     

Intraspecific brood parasitism in the moorhen: parentage and parasite-host relationships determined by DNA fingerprinting

Parasitic female moorhens (Gallinula chloropus) lay from one to six eggs in the nests of conspecific neighbours. DNA fingerprinting was used to show that parasitic eggs could be correctly identified when they appeared in addition to or outside the host’s laying sequence. Moorhen hosts accept all parasitic eggs laid after the 2nd day of their laying period. To understand why moorhen hosts tolerate parasitic eggs, we tested two hypotheses. (1) The quasi-parasitism hypothesis: females lay their eggs in the evening when the host males are normally in attendance at the nest, so host males may allow parasitic females to lay in their nests in exchange for fertilizing their eggs. However, DNA fingerprinting showed that all the parasitic eggs were sired by the parasites’ mates. Parasitic moorhens frequently continue laying a clutch in their own nest, without a break in the laying sequence after a parasitic laying bout. The eggs laid by brood parasites in their own nests were also sired by their own mates. Therefore this hypothesis was rejected. (2) The kin selection hypothesis: if one or both members of the host pair are close relatives of the parasite, the costs of rearing parasitic chicks will be to some degree offset by inclusive fitness benefits. We examined the genetic relationships between parasites and their hosts using DNA fingerprinting and genealogical data. Natal philopatry by both sexes was relatively common in this population, and the probability that a neighbour of either sex was a first-order relative (parent-offspring) was calculated as 0.18. Although first-order relatives were not preferentially chosen as hosts over individuals that were not first-order relatives, even through random host selection there is almost a one-in-five chance that brood parasites in this population are closely related to their hosts. This may facilitate host tolerance of parasitic eggs. Other hypotheses are also discussed. Received: 3 February 1995/Accepted after revision: 27 August 1995     

Vitamin E supplementation enhances growth and condition of nestling barn swallows (Hirundo rustica)

To optimize fitness, organisms may have to trade the number and quality of individual offspring against their own condition and survival. Limiting micronutrients such as antioxidants may be crucial to this trade-off. We investigated whether vitamin E, a major antioxidant in the diet of vertebrates, is limiting to barn swallow (Hirundo rustica) nestlings. We manipulated brood size to alter the intensity of sib–sib competition and supplemented nestlings with two different physiological doses of vitamin E while establishing a control group. Treatment effects were measured on body mass and size, feather growth, T cell-mediated immune response and hematocrit. Supplementation with vitamin E at intermediate physiological doses improved nestling mass and condition and feather growth, whereas higher physiological doses did not enhance offspring quality compared to a control treatment. The positive effects of vitamin E on body mass and condition were only detectable from days 6 to 10 when maximum growth rate is attained. Experimental enlargement of broods reduced body mass and size and T cell-mediated immune response only during the late nestling period. The effect of vitamin E supplementation did not depend on brood size manipulation, as revealed by the nonsignificant statistical interaction. This result contradicts the hypothesis that availability of vitamin E depends on intrabrood competition and instead suggests that it depends on concentration of vitamin E in the insect prey of swallows. Thus, antioxidants may be available in limited amounts to barn swallow nestlings and such limitation affects growth. In addition, present results confirm that barn swallow parents trade progeny number against growth and immunity of individual offspring.     

Settlement patterns of female barn swallows Hirundo rustica across different group sizes: access to colorful males or favored nests?

In most avian species, individuals are faced with two critical decisions at the start of a breeding season: choosing a breeding site and a mate. An analysis of these decisions in light of population-level patterns, such as group size variation in social breeders, can illuminate the causes and patterns of habitat selection behavior. Group sizes are variable in barn swallows; however, no clear and consistent benefits of group breeding have been found in this species, and it is puzzling as to why individuals breed socially. Previous analyses demonstrated that individuals aggregate to gain access to nests that were constructed during previous seasons; however extra-pair matings are also prevalent in this species, raising questions about the mate-selection strategies of females across different group sizes. In this paper, I address the question of how females make their first site-selection decisions in terms of two features known to be causally related to seasonal reproductive success: (1) colorful males or (2) old nests. Using experimental and observational data, I tested, but found no support for, the hypotheses that propose female settlement decisions are a function of (1) the prevalence of colorful males or (2) the increased opportunity for extra-pair matings at group sites. Instead, it is apparent that female settlement patterns are strongly tied to the availability of old nests at a site. Extra-pair fertilizations are equally common across all group sizes in this population, suggesting that females do not face a trade-off between old nests and the possibility of extra-pair mating decisions when making settlement decisions.     

Conspecific brood parasitism in an upland-nesting bird: cues parasites use to select a nest

Behavioral Ecology and Sociobiology - In the animal kingdom, conspicuous colors are often used for inter- and intra-sexual communication. Even though primates are the most colorful mammalian taxon,...     

Brood parasitism disproportionately increases nest provisioning and helper recruitment in a cooperatively breeding bird

Obligate avian brood parasites lay their eggs in nests of other species (hosts), which raise parasitic young. Parasitic nestlings are likely to influence host’s parental behaviours as they typically beg for food more vigorously than young host for a given hunger level. However, few studies have tested this idea, with conflicting results. These prior studies were largely limited to biparental hosts, but little is known about the effect of brood parasitism on parental behaviours in hosts that breed cooperatively. We followed a multimodel approach to examine the effect of brood parasitism on nest provisioning and helper recruitment in the baywing (Agelaioides badius), a cooperative breeder parasitised by screaming (Molothrus rufoaxillaris) and shiny (Molothrus bonariensis) cowbirds. Multimodel inference results indicated that feeding visits increased with nestling age, cooperative group size and number of cowbird nestlings in the brood. Brood size had little influence on feeding visits, which further suggests that baywings adjusted their provisioning effort in response to cowbird parasitism. In addition, nests parasitised artificially with shiny cowbird eggs or hatchlings recruited more helpers than unmanipulated nests having only host or screaming cowbird young. Our results provide novel evidence that brood parasitism and cooperative breeding interact in determining the levels of nest provisioning.     

Foraging costs of a tail ornament: Experimental evidence from two populations of barn swallows Hirundo rustica with different degrees of sexual size dimorphism

Secondary sexual characters are assumed to be costly to produce or maintain. A test of this assumption was performed using the sexually exaggerated outermost tail feathers of male barn swallows Hirundo rustica, a trait currently subject of a directional female mate preference. A possible cost of sexual signalling in male barn swallows arises from increased flight cost during foraging in this aerially insectivorous species. A longer tail may impose a greater drag during flight and thereby affect foraging ability. This was tested by determining the relationship between experimentally modified male tail lengths and number and size of prey delivered to offspring in Spain, where sexual size dimorphism in tail length is small, compared to Denmark, where dimorphism is large. Food boluses contained significantly fewer small insects in Spain than in Denmark. Males with elongated tails captured more and smaller insects while males with shortened tails captured fewer and larger prey items at both sites. Males with naturally long tails were less affected by experimental treatment in terms of effects on the number and the size of prey delivered to their offspring, a finding consistent with a long tail being a condition-dependent viability indicator. The effect of a given degree of tail manipulation on prey size and number of prey per bolus was larger in Spain than in Denmark. These results demonstrate that (1) tail length in male barn swallows affects foraging, and (2) larger sexual size dimorphism occurs where the foraging cost of an increment in ornament size is smallest.Communicated by M. Zuk     

Intraspecific parasitism and split sex ratios in a monogynous and monandrous ant (Leptothorax nylanderi)

Sex ratios were bimodally distributed in a population of the monogynous and monandrous ant Leptothorax nylanderi during each of 3 study years. The population-wide investment ratios suggested worker control of sex allocation. Nest-level variation in the proportional investment in virgin queens was not affected by the presence or absence of a queen and only slightly by collecting year, but was correlated with nest size, total sexual investment and, unexpectedly, with differences in nestmate relatedness: small, low-investment nests and nests with several worker lineages produced male-biased sex ratios. Colonies containing several worker lineages arise from usurpation of mature colonies by unrelated founding queens and the fusion of unrelated colonies under strong nest site limitation. In contrast to facultatively polygynous and polyandrous species of social insects, where workers can maximize their inclusive fitness by adjusting sex ratios according to the degree of relatedness asymmetry, workers in mixed colonies of L. nylanderi do not benefit from manipulating sex allocation, as here relatedness asymmetries appear to be the same as in homogeneous colonies. Received: 7 December 1999 / Received in revised form: 29 February 2000 / Accepted: 13 March 2000     

Mate guarding in the swallowHirundo rustica

Summary The importance of mate guarding by males in the monogamous swallowHirundo rustica was studied by temporarily detaining the males. Mate guarding reduced the frequency of extra-pair copulations and of sexual chases involving female mates. Males participated in sexual chases more frequently if they had a non-fertile female. Neighbouring males of ‘widowed’ females increased their own mate guarding presumably in response to the experimentally increased rate of sexual chases. Neighbouring males with a fertile female increased their mate guarding more than did males with a non-fertile female. Addition of eggs to swallow nests in the post-fledging period of the first brood induced mate guarding by male nest owners. These males also copulated more frequently with their mates than did control males. Neighbouring male swallows responded to the increased mate guarding by showing sexual interest in the guarded females. removal of eggs from swallow nests during the laying period, leaving only one egg in the nest, resulted in reduced nest attendance by females. Male mates responded by increasing their mate guarding intensity as compared to controls, and neighbouring males showed an increased sexual interest in these females.     

Intraspecific brood parasitism as a conditional reproductive tactic in the treehopper<Emphasis Type="Italic"> Publilia concava</Emphasis>

In species exhibiting egg guarding as well as communal egg laying, females may adopt the strategy of laying eggs in the nests of conspecifics and leaving without providing care (termed intraspecific brood parasitism). This study is the first to describe such a behavior in the insect Publilia concava (Hemiptera: Membracidae) through field studies that followed 849 marked females across 1,828 brood associations. While brood parasitism increased the total number of eggs in a host brood, it did not reduce the overall hatching success of host broods. Solitary females exhibited a range of guarding durations while parasitic females rarely remained to guard eggs. Females exhibiting the parasitic tactic increased their lifetime number of clutches without decreasing the number of solitary clutches that they were able to initiate. Estimates of egg number for these individual broods revealed that females adopting the parasitic tactic (in addition to solitary breeding) had higher lifetime fecundity relative to females that did not parasitize. In females that exhibited both tactics (solitary and parasitic), the parasitic tactic yielded a higher rate of oviposition. The major component of oviposition rate was the time to find hosts and this time decreased with increasing host availability across 222 replicate groups. Females exhibited a shift toward the parasitic tactic when host broods were more abundant (i.e. in larger groups and later in the season). However, the time to find hosts increased as the frequency of the parasitic tactic increased, suggesting that this tactic may be maintained through negative frequency dependence. The results of this study suggest that brood parasitism may be the preferred tactic, as part of a conditional strategy, when hosts are readily available with solitary breeding being the preferred tactic when hosts are in short supply.     

Avoiding parasitism by breeding indoors: cuckoo parasitism of hirundines and rejection of eggs

Brood parasitism is costly to hosts, and, therefore, a number of anti-parasite defenses have evolved. Surprisingly, several high-quality hosts such as martins and swallows are rarely parasitized, raising the question why that is the case. We hypothesize that martins and swallows may avoid parasitism by breeding in close association with humans, and by building nests that are inaccessible for common cuckoos Cuculus canorus and other brood parasites. Here we show using egg rejection experiments that red-rumped swallows Hirundo daurica, house martins Delichon urbica, and barn swallows Hirundo rustica in Europe do not reject foreign eggs placed in their nests, while barn swallows in China often reject foreign eggs. The frequency of parasitism of barn swallows in Europe was significantly higher than in house martins relative to the expectation based on the abundance of the two species. Barn swallows in Europe that were parasitized by cuckoos more often placed their nests outdoors than expected by chance, suggesting that avoidance of cuckoo parasitism can be achieved by breeding indoors. These findings suggest that barn swallows in China have gained egg rejection behavior because they cannot avoid parasitism when breeding outdoors.     

Intraspecific brood adoption in convict cichlids : a mutual benefit

Summary Convict cichlids (Cichlasoma nigrofasciatum) are biparental, substrate brooding cichlid fish which have extended care of their eggs, larvae (wrigglers) and free-swimming young (fry). Field observations indicate that intraspecific brood adoption of fry occurs frequently under natural conditions. Of 232 broods 29% contained foreign fry and of 232 broods 15% were identified as fry donors. Foreign fry were usually of similar size to or smaller than the host brood fry. Experimental fry transfers showed that parents accept foreign fry smaller than their own but immediately reject or eat foreign fry larger than their own. A predation experiment showed that within a mixed brood, smaller fry are eaten more often than larger fry. Thus, accepting foreign fry may benefit parents through the dilution effect and through a differential predation effect. In comparison with intact families, broods from which the male parent was removed were less likely to reach independence, had fewer fry surviving to independence, and had a greater probability of having some fry transferred to neighbouring broods. We propose that intraspecific brood adoption in convict cichlids is mutually beneficial to the adult donors and recipients. Correspondence to: B.D. Wisenden