Common Cuckoos Cuculus canorus change their nest‐searching strategy according to the number of available host nests

In recent decades, numerous studies have examined factors affecting risk of host nest parasitism in well‐known avian host–parasite systems; however, little attention has been paid to the role of host nest availability. In accordance with other studies, we found that nest visibility, reed density and timing of breeding predicted brood parasitism of Great Reed Warblers Acrocephalus arundinaceus by the Common Cuckoo Cuculus canorus. More interestingly, hosts had a greater chance of escaping brood parasitism if nesting was synchronized. Cuckoo nest searching was governed primarily by nest visibility at high host‐nest density. However, even well‐concealed nests were likely to be parasitized during periods when just a few hosts were laying eggs, suggesting that Cuckoos adjust their nest‐searching strategy in relation to the availability of host nests. Our results demonstrate that host vulnerability to brood parasitism varies temporally and that Cuckoo females are able to optimize their nest‐searching strategy. Moreover, our study indicated that Cuckoos always manage to find at least some nests to parasitize. Thus, in this case, the co‐evolutionary arms race should take place mainly in the form of parasitic egg rejection rather than via frontline pre‐parasitism defence.     

Effects of nest-site characteristics and parental activity on cowbird parasitism and nest predation in Brown-and-yellow Marshbirds

ABSTRACT Nest‐site selection and nest defense are strategies for reducing the costs of brood parasitism and nest predation, two selective forces that can influence avian nesting success and fitness. During 2001–2002, we analyzed the effect of nest‐site characteristics, nesting pattern, and parental activity on nest predation and brood parasitism by cowbirds (Molothrus spp.) in a population of Brown‐and‐yellow Marshbirds (Pseudoleistes virescens) in the Buenos Aires province, Argentina. We examined the possible effects of nest detectability, nest accessibility, and nest defense on rates of parasitism and nest predation. We also compared rates of parasitism and nest predation and nest survival time of marshbird nests during the egg stage (active nests) with those of the same nests artificially baited with passerine eggs after young fledged or nests failed (experimental nests). Most nests (45 of 48, or 94%) found during the building or laying stages were parasitized, and 79% suffered at least one egg‐predation event. Cowbirds were responsible for most egg predation, with 82 of 107 (77%) egg‐predation events corresponding to eggs punctured by cowbirds. Nests built in thistles had higher rates of parasitism and egg predation than nests in other plant, probably because cowbirds were most active in the area where thistles were almost the only available nesting substrate. Parasitism rates also tended to increase as the distance to conspecific nests increased, possibly due to cooperative mobbing and parental defense by marshbirds. The proportion of nests discovered by cowbirds was higher for active (95%) than for experimental (29%) nests, suggesting that cowbirds used host parental activity to locate nests. Despite active nest defense, parental activity did not affect either predation rates or nest‐survival time. Thus, although nest defense by Brown‐and‐yellow Marshbirds appears to be based on cooperative group defense, such behavior did not reduce the impact of brood parasites and predators.     

Brood parasitism,relatedness and sociality: a kinship role in female reproductive tactics

Conspecific brood parasitism (CBP) is a reproductive tactic in which parasitic females lay eggs in nests of other females of the same species that then raise the joint brood. Parasites benefit by increased reproduction, without costs of parental care for the parasitic eggs. CBP occurs in many egg‐laying animals, among birds most often in species with large clutches and self‐feeding young: two major factors facilitating successful parasitism. CBP is particularly common in waterfowl (Anatidae), a group with female‐biased natal philopatry and locally related females. Theory suggests that relatedness between host and parasite can lead to inclusive fitness benefits for both, but if host costs are high, parasites should instead target unrelated females. Pairwise relatedness (r) in host–parasite (h‐p) pairs of females has been estimated using molecular genetic methods in seven waterfowl (10 studies). In many h‐p pairs, the two females were unrelated (with low r, near the local population mean). However, close relatives (r = 0.5) were over‐represented in h‐p pairs, which in all 10 studies had higher mean relatedness than other females. In one species where this was studied, h‐p relatedness was higher than between nesting close neighbours, and hosts parasitized by non‐relatives aggressively rejected other females. In another species, birth nest‐mates (mother–daughters, sisters) associated in the breeding area as adults, and became h‐p pairs more often than expected by chance. These and other results point to recognition of birth nest‐mates and perhaps other close relatives. For small to medium host clutch sizes, addition of a few parasitic eggs need not reduce host offspring success. Estimates in two species suggest that hosts can then gain inclusive fitness if parasitized by relatives. Other evidence of female cooperation is incubation by old eider Somateria mollissima females of clutches laid by their relatives, and merging and joint care of broods of young. Merging females tended to be more closely related. Eiders associate with kin in many situations, and in some geese and swans, related females may associate over many years. Recent genetic evidence shows that also New World quails (Odontophoridae) have female‐biased natal philopatry, CBP and brood merging, inviting further study and comparison with waterfowl. Kin‐related parasitism also occurs in some insects, with revealing parallels and differences compared to birds. In hemipteran bugs, receiving extra eggs is beneficial for hosts by diluting offspring predation. In eggplant lace bugs Gargaphia solani, host and parasite are closely related, and kin selection favours egg donation to related females. Further studies of kinship in CBP, brood merging and other contexts can test if some of these species are socially more advanced than presently known.     

Host-parasite kinship in a female-philopatric bird population: evidence from relatedness trend analysis

Conspecific brood parasitism (CBP), an alternative reproductive tactic where some females lay eggs in the nests of other females of the same species, occurs in many animals with egg care. It is particularly common in waterfowl, for reasons that are debated. Many waterfowl females nest near their birthplace, making it likely that some local females are relatives. We analyse brood parasitism in a Hudson Bay population of common eiders, testing predictions from two alternative hypotheses on the role of relatedness in CBP. Some models predict host-parasite relatedness, others predict that parasites avoid close relatives as hosts. To distinguish between the alternatives, we use a novel approach, where the relatedness of host-parasite pairs is tested against the spatial population trend in pairwise relatedness. We estimate parasitism, nest take-over and relatedness with protein fingerprinting and bandsharing analysis of egg albumen, nondestructively sampled from each new egg in the nest throughout the laying period. The results refute the hypothesis that parasites avoid laying eggs in the nests of related hosts, and corroborate the alternative of host-parasite relatedness. With an estimated r of 0.12-0.14, females laying eggs in the same nest are on average closer kin than nesting neighbour females. Absence of a population trend in female pairwise relatedness vs. distance implies that host-parasite relatedness is not only an effect of strong natal philopatry: some additional form of kin bias is also involved.     

Relatedness and the evolution of conspecific brood parasitism

In conspecific brood parasitism (CBP), a parasitic female takes advantage of the parental care performed by a host female by laying eggs in the nest of the host. The host female raises the offspring of the parasitic female as well as her own. In species where local females are related, direct costs for the host might be more than compensated for by gains in inclusive fitness through increased reproduction of a related parasite, but the role of relatedness in CBP is debated. This inclusive-fitness model of parasitism, structured as a game between host and parasite, suggests that both females can gain inclusive fitness and that host-parasite relatedness can therefore facilitate the evolution of CBP. Crucial assumptions are that there is kin discrimination and a potential for host resistance to parasitism by unrelated females but close relatives are accepted. The cost of parasitism in terms of reduced clutch size or offspring survival for the host must not be large; otherwise, parasitism will reduce her inclusive fitness. Therefore, if these costs are high, it does not benefit a host to accept a parasite, even if the parasite is closely related. The secondary female may still have higher fitness from parasitism, but if the costs are high, she should parasitize an unrelated host, not a relative. This requires that the reduction in parasite success that a host can cause by resistance is not too large; otherwise, it will be better for the secondary female to parasitize an accepting related host or to nest solitarily. For these reasons, host-parasite relatedness is most likely to occur in animals where costs of being parasitized are low and host resistance can markedly reduce the success of an unrelated parasite. When costs are higher, parasitism of unrelated hosts may be better, and if host resistance strongly reduces parasite success, solitary breeding is preferable. In some cases, CBP is directly advantageous for the host, and it may sometimes evolve in close connection with cooperative breeding, which is also considered in the model. Some but not all empirical results support these ideas, and more detailed studies of behavior, relatedness, and reproduction of host and parasite are needed for critical tests.     

Alien eggs in duck nests: brood parasitism or a help from Grandma?

Intraspecific brood parasitism (IBP) is a remarkable phenomenon by which parasitic females can increase their reproductive output by laying eggs in conspecific females' nests in addition to incubating eggs in their own nest. Kin selection could explain the tolerance, or even the selective advantage, of IBP, but different models of IBP based on game theory yield contradicting predictions. Our analyses of seven polymorphic autosomal microsatellites in two eider duck colonies indicate that relatedness between host and parasitizing females is significantly higher than the background relatedness within the colony. This result is unlikely to be a by-product of relatives nesting in close vicinity, as nest distance and genetic identity are not correlated. For eider females that had been ring-marked during the decades prior to our study, our analyses indicate that (i) the average age of parasitized females is higher than the age of nonparasitized females, (ii) the percentage of nests with alien eggs increases with the age of nesting females, (iii) the level of IBP increases with the host females' age, and (iv) the number of own eggs in the nest of parasitized females significantly decreases with age. IBP may allow those older females unable to produce as many eggs as they can incubate to gain indirect fitness without impairing their direct fitness: genetically related females specialize in their energy allocation, with young females producing more eggs than they can incubate and entrusting these to their older relatives. Intraspecific brood parasitism in ducks may constitute cooperation among generations of closely related females.     

Colony kin structure and host‐parasite relatedness in the barnacle goose

Conspecific brood parasitism (CBP), females laying eggs in the nest of other ‘host’ females of the same species, is a common alternative reproductive tactic among birds. For hosts there are likely costs of incubating and rearing foreign offspring, but costs may be low in species with precocial chicks such as waterfowl, among which CBP is common. Waterfowl show strong female natal philopatry, and spatial relatedness among females may influence the evolution of CBP. Here we investigate fine‐scale kin structure in a Baltic colony of barnacle geese, Branta leucopsis, estimating female spatial relatedness using protein fingerprints of egg albumen, and testing the performance of this estimator in known mother‐daughter pairs. Relatedness was significantly higher between neighbour females (nesting ≤ 40 metres from each other) than between females nesting farther apart, but there was no further distance trend in relatedness. This pattern may be explained by earlier observations of females nesting close to their mother or brood sisters, even when far from the birth nest. Hosts and parasites were on average not more closely related than neighbour females. In 25 of 35 sampled parasitized nests, parasitic eggs were laid after the host female finished laying, too late to develop and hatch. Timely parasites, laying eggs in the host’s laying sequence, had similar relatedness to hosts as that between neighbours. Females laying late parasitic eggs tended to be less related to the host, but not significantly so. Our results suggest that CBP in barnacle geese might represent different tactical life‐history responses.     

Differential responses to related hosts by nesting and non-nesting parasites in a brood-parasitic duck

Host-parasite relatedness may facilitate the evolution of conspecific brood parasitism, but empirical support for this contention remains inconclusive. One reason for this disparity may relate to the diversity of parasitic tactics, a key distinguishing feature being whether the parasite has a nest of her own. Previous work suggests that parasites without nests of their own may be of inferior phenotypic quality, but because of difficulties in identifying these parasitic individuals, little is known about their host selection criteria. We used high-resolution molecular maternity tests to assign parasitic offspring to known parasites with and without their own nests in a population of Barrow's goldeneyes (Bucephala islandica). We determined whether parasite nesting status, host-parasite relatedness and distance between host and parasite nests affected the probability of parasitizing a host and the number of eggs laid per host. We also investigated whether nesting parasites, conventionally nesting females and non-nesting parasites differed regarding their age, structural size, body condition, nesting phenology or total brood size. The probability of engaging in parasitism increased with host-parasite relatedness and spatial proximity to host nests for nesting and non-nesting females alike. However, nesting parasites increased the number of eggs donated with relatedness to the host, while non-nesting parasites did not do so. Non-nesting parasites laid fewer eggs in total, but did not differ by any of the other quality measures from conventional nesters or nesting parasites. Our study provides the first demonstration that nesting and non-nesting parasites from the same population may use different host selection criteria.     

Decoding dumping ducks

Conspecific brood parasitism, where females of the same species lay eggs in each other's nests, is common in waterfowl, and is usually considered costly to host females, which are stuck looking after eggs and chicks that are not their own. However, since female waterfowl often exhibit an unusual propensity to nest near where they were born, there has been some uncertainty over whether, in ducks and geese, laying in nests of conspecifics really is parasitism. Do parasitic and host females tend to be related? And is parasitism actually a form of cooperation in disguise? In a population in Hudson Bay, Andersson & Waldeck (this issue) found that ‘parasitic’ eggs in nests of the common eider, Somateria mollissima sedentaria, are more closely related to host eggs than expected by chance. In fact, host and ‘donor’ eggs are more closely related than are females breeding at neighbouring nests. The Hudson Bay population of common eiders is unusual, because unlike in more benign climates, females do not tend to breed near their natal nest. Spatial proximity alone cannot account for the high relatedness between host eggs and ‘dumped’ or donor eggs. Instead, the high relatedness values are probably the result of active recognition, where females favour kin, either when dumping or accepting eggs. These new data, along with evidence indicating that the donor lays the first egg in the nest nearly half the time, suggest that what appears to be parasitism in common eiders may be a form of kin‐based cooperation.     

Evolution of social behaviour in the primitively eusocial wasp Ropalidia marginata: do we need to look beyond kin selection?

Ropalidia marginata is a primitively eusocial wasp widely distributed in peninsular India. Although solitary females found a small proportion of nests, the vast majority of new nests are founded by small groups of females. In such multiple foundress nests, a single dominant female functions as the queen and lays eggs, while the rest function as sterile workers and care for the queen''s brood. Previous attempts to understand the evolution of social behaviour and altruism in this species have employed inclusive fitness theory (kin selection) as a guiding framework. Although inclusive fitness theory is quite successful in explaining the high propensity of the wasps to found nests in groups, several features of their social organization suggest that forces other than kin selection may also have played a significant role in the evolution of this species. These features include lowering of genetic relatedness owing to polyandry and serial polygyny, nest foundation by unrelated individuals, acceptance of young non-nest-mates, a combination of well-developed nest-mate recognition and lack of intra-colony kin recognition, a combination of meek and docile queens and a decentralized self-organized work force, long reproductive queues with cryptic heir designates and conflict-free queen succession, all resulting in extreme intra-colony cooperation and inter-colony conflict.     

Relationship Between Nest Predation Suffered by Hosts and Brown-Headed Cowbird Parasitism: A Comparative Study

There are at least four main hypotheses that may explain how the evolution of host selection by avian brood parasites could be linked to nest predation among their potential hosts. First, selection may have favoured parasite phenotypes discriminating among hosts on the basis of expected nest failure. Second, parasitized nests may be more easily detected by predators and extra costs of parasitism may accelerate the evolution of host defences. Third, selection may have favoured predator phenotypes avoiding parasitized nests because parasitism enhances nest defence. Fourth, female brood parasites may directly or indirectly induce host nesting failures in order to enhance future laying opportunities. We collected data on brood parasitism and nest failure due to predation to test these hypotheses in a comparative approach using North American passerines and their brood parasite, the brown-headed cowbird Molothrus ater. Under the hypotheses 1 or 3 we predicted brood parasitism to be negatively associated with nest predation across species, whereas this relation is expected to be positive if hypotheses 2 or 4 are true. We demonstrate that independent of host suitability, nest location, habitat type, length of the nestling period, body mass and similarity among species due to common ancestry, species experiencing relatively high levels of nest predation suffered lower levels of cowbird parasitism. Our results suggest a previously ignored role for nest predation suffered by hosts on the dynamics of the coevolutionary relationships between hosts and avian brood parasites. Co-ordinating editor: Dr. F. Stuefer     

Small hosts infrequently disrupt laying by Brown-headed Cowbirds and Bronzed Cowbirds

ABSTRACT Brood parasites often must overcome host defenses that may include behaviors that serve other functions, such as deterrence of predators and nest attendance during laying and incubation. Host use by brood parasites may also be influenced by competitors in areas where more than one parasitic species occurs. We identified the degree to which behavior of potential hosts and potential competitors affected laying by Brown‐headed Cowbirds (Molothrus ater) and Bronzed Cowbirds (M. aeneus) at a site in south Texas where they co‐occur. We watched potential host nests during the presunrise period to record cowbird laying and document nest visitation, laying, cowbird‐host encounters, and nest attentiveness by hosts. Hosts were frequently at their nests when cowbirds laid eggs (83% of 121 watches among nests of five host species) and cowbirds regularly encountered hosts (43–74% and 40–77% of watches per species of host for Brown‐headed and Bronzed cowbirds, respectively). Host nest defense infrequently interfered with cowbird laying and cowbirds rarely interacted with one another during laying. Overall, 12% of the 42 cowbird laying attempts that elicited host nest defense failed, resulting in cowbird eggs either laid atop hosts as they sat in nests or laid outside the nest cup. We clearly documented that relatively small hosts can thwart parasitism by cowbirds. Thus, the potential for successful defense of nests should be considered when assessing the evolution of behaviors to deter the removal of host eggs by cowbirds and mechanisms leading to nest abandonment. Regarding the latter, the presence of a cowbird at a nest would be a poor indicator for parasitism as some laying attempts were thwarted and unparasitized broods were reared at those nests. Despite the potential for nest defense to affect host use by cowbirds, we did not detect an effect of nest defense. Because most host defense was ineffective, we examined hypotheses for the timing of cowbird laying and host nest attendance. Our analysis of time of day of laying by Brown‐headed Cowbirds at our site and data compiled from the literature suggests that laying time is best predicted by the time of civil twilight (first light) rather than sunrise.     

The role of kin recognition in the evolution of conspecific brood parasitism

Conspecific brood parasitism (CBP) is a common strategy in several species of birds. Currently, some studies suggest that relatedness between host and parasite enhances CBP, since indirect fitness benefits could select for acceptance of related eggs by hosts. Conversely, parasites should avoid laying eggs in nests of relatives if this is costly for the host. Based on the latter argument, kinship should not promote brood parasitism. A recent model clarified this relationship, and showed that kinship can promote brood parasitism, assuming kin recognition. However, in that model kin recognition was assumed perfect. Here we present a model that addresses the role of relatedness and kin selection in CBP, when kin recognition is not perfect and hosts do not always detect parasitism. We consider both the indirect fitness of the parasite and the possible responses of the host. Our results indicate that the existence and accuracy of a kin recognition system is crucial to the final outcome. When CBP represents a cost to the host, a parasitic female that has the choice should avoid parasitizing relatives, unless (1) the costs are not too high and (2) hosts can accurately enough recognize eggs laid by relatives, rejecting them less often than eggs laid by nonkin. But if ‘parasitism’ enhances the direct fitness of the host (which is possible in species with precocial young) parasites should choose relatives whenever possible, even if hosts do not recognize kin eggs. Copyright 2002 The Association for the Study of Animal Behaviour. Published by Elsevier Science Ltd. All rights reserved.     

Brood parasitism of Black‐capped Vireos: frontline and post‐laying behavioral responses and effects on productivity

The cost of brood parasitism favors the evolution of host behaviors that reduce the risk or expense of being parasitized. Endangered Black‐capped Vireos (Vireo atricapilla) have likely coexisted with brood‐parasitic Brown‐headed Cowbirds (Molothrus ater) for more than 10,000 yr, so it is likely that they have evolved anti‐parasitic behaviors. We monitored naturally parasitized and non‐parasitized vireo nests to evaluate factors that might explain parasitism risk and nest desertion behavior and also assessed whether behaviors that occurred after being parasitized improved reproductive output. Vireos reduced the risk of parasitism by initiating breeding early and nesting farther from open grasslands and edges of woody thickets. Post‐laying, nest desertion was common (70% of parasitized nests) and increased with both the presence of at least one cowbird egg in nests and clutch reduction by cowbirds. After accounting for these cues, desertion was also more likely at nests located closer to cowbird foraging habitat and below potential cowbird vantage points. Despite its regularity, desertion did not appear to provide reproductive benefits to vireos. Instead, accepting cowbird eggs was a more effective strategy because 42% of cowbird eggs did not hatch. Furthermore, cowbird eggs were somehow ejected from at least three vireo nests. Our results suggest that Black‐capped Vireos can behave in a variety of ways that reduce the impact of brood parasitism, with frontline behaviors appearing to provide the greatest benefit. Our results also suggest that habitat management should focus on providing Black‐capped Vireos with adequate breeding habitat that provides access to safe nesting sites, and with high‐quality wintering habitat that allows vireos to migrate and initiate nesting early.     

Reproductive interactions of the Shiny Cowbird Molothrus bonariensis and the Yellow-hooded Blackbird Agelaius icterocephalus in Trinidad

The reproductive interactions of the Shiny Cowbird Molothrus bonariensis , a brood parasite, and the Yellow-hooded Blackbird Agelaius icterocephalus , a host of the cowbird, were studied In Trinidad, West Indies. We gathered information on the breeding biology of the Shiny Cowbird and the Yellow-hooded Blackbird, the frequency of use of the host species, and the effects of brood parasitism on host breeding success. Yellow-hooded Blackbirds are polygynous for the most part; males build nests and attempt to attract females to lay in them by means of song and visual displays directed towards the nests. This behaviour probably makes it easy for cowbirds to locate breeding birds and their nests. Cowbird eggs were found in 153 of 377 (40–3 %) blackbird nests located before the nestling stage. Shiny Cowbird parasitism of the Yellow-hooded Blackbird had little negative impact on host reproductive success, whereas predation accounted for the majority of nest failures. Vigilant nest defense by male blackbirds combined with colonial breeding apparently also minimized the extent of host egg damage and removal by cowbirds, and the parasitized and unparasitized nests were equally successful at producing blackbirds. Cowbirds most frequently parasitized the first or only nesting attempts in blackbird territories, and first or only nests were also successful more frequently than subsequent nests.     

Why Wasp Foundresses Change Nests: Relatedness,Dominance, and Nest Quality

The costs and benefits of different social options are best understood when individuals can be followed as they make different choices, something that can be difficult in social insects. In this detailed study, we follow overwintered females of the social wasp Polistes carolina through different nesting strategies in a stratified habitat where nest site quality varies with proximity to a foraging area, and genetic relatedness among females is known. Females may initiate nests, join nests temporarily or permanently, or abandon nests. Females can become helpers or egglayers, effectively workers or queens. What they actually do can be predicted by a combination of ecological and relatedness factors. Advantages through increased lifetime success of individuals and nests drives foundresses of the social wasp Polistes from solitary to social nest founding. We studied reproductive options of spring foundresses of P. carolina by monitoring individually-marked wasps and assessing reproductive success of each foundress by using DNA microsatellites. We examined what behavioral decisions foundresses make after relaxing a strong ecological constraint, shortage of nesting sites. We also look at the reproductive consequences of different behaviors. As in other Polistes, the most successful strategy for a foundress was to initiate a nest as early as possible and then accept others as subordinates. A common feature for many P. carolina foundresses was, however, that they reassessed their reproductive options by actively monitoring other nests at the field site and sometimes moving permanently to new nests should that offer better (inclusive) fitness prospects compared to their original nests. A clear motivation for moving to new nests was high genetic relatedness; by the end of the foundress period all females were on nests with full sisters.     

Fitness consequences of ecological constraints and implications for the evolution of sociality in an incipiently social bee

Ecological constraints such as resource limitation, unfavourable weather conditions, and parasite pressure have long been considered some of the most important selective pressures for the evolution of sociality. In the present study, we assess the fitness consequences of these three ecological factors on reproductive success of solitary nests and social colonies in the socially polymorphic small carpenter bee, Ceratina australensis, based on 982 nests collected over four reproductive periods. Nest site limitation was predicted to decrease opportunities for independent nest initiation and increase the frequency of social nesting. Nest sites were not limiting in this species and the frequency of social nesting was consistent across the four brood‐rearing periods studied. Unfavourable weather was predicted to lower the frequency of female dispersal from their natal nests and to limit the brood‐rearing season; this would increase the frequency and fitness of social colonies. Daily temperature and precipitation accumulation varied between seasons but were not correlated with reproductive success in this bee. Increased parasite pressure is predicted to increase the frequency and fitness of social colonies because solitary bees must leave the nest unattended during foraging bouts and are less able to defend the nest against parasites. Severe parasitism by a chalcid wasp (Eurytoma sp.) resulted in low reproductive success and total nest failure in solitary nests. Social colonies had higher reproductive success and were never extirpated by parasites. The high frequency of solitary nests suggests that this is the optimal strategy. However, social colonies have a selective advantage over solitary nesting females during periods of extreme parasite pressure, and we suggest that social nesting represents a form of bet‐hedging against unpredictable fluctuations in parasite number. © 2011 The Linnean Society of London, Biological Journal of the Linnean Society, 2011, 103 , 57–67.     

Extrapair paternity, inclusive fitness, and within-group benefits of helping in western bluebirds

In central coastal California, USA, 3–16% of western bluebird ( Sialia mexicana ) pairs have adult male helpers at the nest. Demographic data on a colour-ringed population over a 13-year period indicate that helpers gain a small indirect fitness benefit through increases in the number of young fledged from nests of close kin. A small proportion of adult helpers (16%) that were able to breed and help simultaneously had higher annual inclusive fitness than males that only bred. These males comprised such a minor proportion of helpers that the mean fitness of helpers was still lower than the mean fitness of independent breeders. We used DNA fingerprinting to determine whether extrapair fertilizations alter within-group benefits enough to tip the balance in favour of helping behaviour. Overall, 19% of 207 offspring were sired by males other than their social father and extrapair fertilizations occurred in 45% of 51 nests. Intraspecific brood parasitism was rare so that mean mother-nestling relatedness approximated the expected value of 0.5. Extrapair paternity reduced putative father-offspring relatedness to 0.38. Mean helper-nestling relatedness was 0.41 for helpers assisting one or both parents and 0.28 for helpers aiding their brothers. Helpers rarely sired offspring in the nests at which they helped. Helping was not conditional on paternity and helpers were not significantly more closely related to offspring in their parents' nests than to offspring in their own nests. Although helpers may derive extracurricular benefits if helping increases their own or their father's opportunities for extrapair fertilizations, within-nest inclusive fitness benefits of helping do not compensate males for failing to breed. Breeding failure and constraints on breeding are the most likely explanations for why most helpers help.     

Host-parasite relatedness in wood ducks: patterns of kinship and parasite success

We investigated the role of kinship in intraspecific nest parasitismof wood ducks (Aix sponsa). Among waterfowl, female philopatrycreates the potential for female relatives to nest in proximity.Costs of intraspecific nest parasitism to host females may bereduced if parasites lay eggs with kin. However, previous observationsof marked wood ducks indicated that females avoided parasitizingclutch mates or the female that incubated them. To further examinethe role of kinship, we determined the genotypes of 27 host-parasitepairs at five microsatellite loci. Average relatedness betweenhosts and all females laying parasitic eggs was only 0.04 ±0.03. Parasites appeared to choose hosts randomly with respectto kinship from among females with nests in the neighborhoodand those within the entire study area. However, host relatednessto the parasite with the greatest number of young leaving thenest was 0.11 ± 0.03, which was greater than expectedif eggs were accepted randomly from neighboring females or fromfemales present on the entire study area (p = .03 and p = .02,respectively). These patterns may reflect parasitism of randomlyselected nests followed by differential acceptance by hosts,differential hatching success of related parasites (e.g., dueto greater laying synchrony), or a mixture of parasitic strategies,one with a focus on related hosts and the other on unrelatedhosts. Genetic data revealed that social relationships did notalways reflect true relatedness and that success of primaryparasites was associated with kinship to hosts.     

An experimental analysis of joint nesting in the salamander Hemidaetylium scutatum (Caudata: Plethodontidae): the effects of population density

Three nesting behaviour patterns are documented in the plethodontid salamander Hemidactylium scutatum. A female may lay eggs (1) in a solitary nest and brood them, (2) in a joint nest and brood them as well as eggs of other females, or (3) in a joint nest that is brooded by another female. The hypothesis that population density was positively associated with joint nesting was tested by following two populations for 5 years and by experimentally manipulating the population density of nesting females in artificial habitats for the latter 2 years. The proportion of joint nests did not vary with density, although joint nests tended to contain eggs of more females at the high population density. Joint nests were usually brooded by one female; thus, most females that laid eggs in joint nests did not brood them at high density. The reproductive success, as measured by survival of embryos, of solitary and joint nesters was equivalent. Joint nests were deserted less often, however, which decreased the probability of catastrophic mortality. The number of days of brooding was significantly positively correlated with loss of body mass of females, suggesting a cost to brooding behaviour. Joint nesting with solitary brooding is not explained by aggressive usurpation of nests or by brood parasitism.