Sex-role reversal in willow tit nest defence

Summary The roles of sexes in the nest defence behavior of monogamous willow tit (Parus montanus) were studied near Oulu, northern Finland, in 1988–1990. The relative roles of the sexes changed during the breeding cycle: females defended their nests more vigorously before hatching and males defended more after hatching. This sexual asymmetry was studied by means of the cost/benefit model of optimal parental investment (PI). Because of the monogamous breeding system, sexual differences in future benefits were not a likely explanation for the asymmetry. This was also supported by preliminary results of DNA-fingerprinting analyses. Behavioral observations — sex-role reversal, high correlation between the mates, and equal variations in male and female behavior — indicated the same. In the beginning of the breeding cycle, higher female defence was related to renesting potential. The extra costs of renesting are considerably higher for females than for males, and therefore, females appeared to benefit more than males in keeping the first clutch alive while renesting was still possible. Such a female strategy was possible in this northern population, since the time for renesting was rather short and never lasted beyond the hatching of the first nests. After this point, the male can only invest in the present clutch. By the end of the breeding cycle, increased defence of the nest by males may be a consequence of males being larger and/or in better condition than females. Therefore, in the latter half of the breeding cycle, the sexual differences in nest defence did not indicate any sexual difference in PI. The asynchronous arrival of the parents at the nest also affected individual responses, indicating that defending the nest is a dynamic process.     

Extra-pair paternity, offspring mortality and offspring sex ratio in the socially monogamous coal tit (Parus ater)

Females of many socially monogamous bird species commonly engage in extra-pair copulations. Assuming that extra-pair males are more attractive than the females’ social partners and that attractiveness has a heritable component, sex allocation theory predicts facultative overproduction of sons among extra-pair offspring (EPO) as sons benefit more than daughters from inheriting their father’s attractiveness traits. Here, we present a large-scale, three-year study on sex ratio variation in a passerine bird, the coal tit (Parus ater). Molecular sexing in combination with paternity analysis revealed no evidence for a male-bias in EPO sex ratios compared to their within-pair maternal half-siblings. Our main conclusion, therefore, is that facultative sex allocation to EPO is absent in the coal tit, in accordance with findings in several other species. Either there is no net selection for a deviation from random sex ratio variation (e.g. because extra-pair mating may serve goals different from striving for ‘attractiveness genes’) or evolutionary constraints preclude the evolution of precise maternal sex ratio adjustment. It is interesting to note that, however, we found broods without EPO as well as broods without mortality to be relatively female-biased compared to broods with EPO and mortality, respectively. We were unable to identify any environmental or parental variable to co-vary with brood sex ratios. There was no significant repeatability of sex ratios in consecutive broods of individual females that would hint at some idiosyncratic maternal sex ratio adjustment. Further research is needed to resolve the biological significance of the correlation between brood sex ratios and extra-pair paternity and mortality incidence, respectively.     

Rank-dependent hoarding effort in willow tits (<Emphasis Type="Italic">Parus montanus</Emphasis>): a test of theoretical predictions

Many birds and mammals store energy as hoarded food supplies. A supply of stored food is beneficial during periods when food is scarce, but building up and managing such a supply also entails costs. The optimal number of caches will be reached when the net benefit is at its maximum. If dominants can steal more stored food from subordinates than the other way around, the optimum will differ between these categories. A previous theoretical model of hoarding in groups with dominant and subordinate members produced three testable predictions: (1) hoarders should store more food as anticipated future conditions get worse; (2) subordinate flock members should store more food than dominants; and (3) dominants should increase hoarding relatively more than subordinates as conditions get worse. Here we present a field experiment on willow tits (Parus montanus) designed to test these predictions. We found support for all three. Hoarding increased as conditions got worse, subordinates stored at a higher rate than dominants, and dominants increased their hoarding effort relatively more than subordinates as conditions worsened. These results support the incorporation of information on dominance and food availability into models predicting food storage behaviour.Communciated by J. Dickinson     

Temporal increase in nest defence intensity of the willow tit (Parus montanus): parental investment or methodological artifact?

Summary Nest defence responses of experimentally naive and revisited willow tits (Parus montanus) towards a predator model were studied near Oulu, northern Finland, in 1988–1989. Intensity was measured with three variables in 228 trials at 90 nests. Two hypotheses explaining the temporal changes in the nest defence behavior were examined. In naive birds the nest defence intensity was positively correlated with the brood age, supporting the parental investment hypothesis. The number of previous trials did not cause additional variation in nest defence behavior in comparison between naive and revisited birds. Similarly, experiences in a previous breeding attempt did not affect the nest defence behavior during the course of the subsequent brood. Thus, the temporal increase in multiply visited nests was not due to the birds becoming familiar with the nest threat, i.e., no support was found for the so-called positive reinforcement hypothesis. We think that methodological problems in avian nest defence studies can hide the adaptive significance of the behavior, not explain it. Offprint requests to: S. Rytkönen     

Changes in singing behavior of male black-capped chickadees (Parus atricapillus) following mate removal

We removed the mates of ten male black-capped chickadees (Pares atricapillus) during the nest-building period to determine the effect of female presence on dawn singing. During the first dawn chorus following mate removal, males sang significantly longer, increased movement within their territory, and increased the percentage of their territory covered while singing. After the female was returned, these parameters returned to the pre-removal values. Males did not alter the frequency range or modal frequency of their songs when the mate was removed, nor did they change the degree of frequency shifting in the fee-bee song. We conclude that dawn singing in the black-capped chickadee acts, in part, as an intersexual signal, and that the behavior of frequency shifting in the song may be directed more toward rival males than females. Correspondence to: K. Otter     

Manipulation of nest-box density affects extra-pair paternity in a population of blue tits (<Emphasis Type="Italic">Parus caeruleus</Emphasis>)

We tested the effect of manipulation of breeding density on the occurrence of extra-pair paternity in a blue tit (Parus caeruleus) population during two consecutive years. In a homogeneous oak forest, nest-box manipulation provided a high density plot (plot A, 1.10 and 1.32 pairs/ha) and a low density plot (plot B, 0.43 and 0.46 pairs/ha). Microsatellite analysis on 91 broods revealed a higher proportion of extra-pair paternity in broods in plot A (mean of 17.2%) than in plot B (mean of 11.4%). A correlative approach showed that the proportion of extra-pair young in broods was affected by the number of breeding neighbours within 100 m around the nest-box, by the distance to the nearest breeding neighbour and by an additional plot effect. However, the nearest neighbours accounted for only 39.3% of extra-pair paternities and distance to extra-pair fathers was significantly higher than the nearest neighbour distance in both plots. This implies that the effect of density on the occurrence of extra-pair paternities is associated with active female choice to enhance the brood fitness. Although there were more extra-pair young in broods when density was high, the number of extra-pair fathers did not increase and stayed close to one. We suggest that density increases the cost of mate guarding by males, thereby increasing the possibility for females to solicit extra-pair paternities to the cuckolding male they have chosen. Finally, we discuss why correlatives approaches do not always show evidence for an effect of breeding density on extra-pair paternity occurrence.Communicated by M. Soler     

Brood size manipulation affects frequency of second clutches in the blue tit

The reproductive trade-off hypothesis predicts that the investment made in current reproduction determines the breeders’ future fitness as a consequence of intra-or inter-generational reproductive costs. Long-lived species are expected to favour their own reproductive value at the expense of their offspring, hence incurring in inter-generational costs, whereas short-lived species are expected to invest in the current breeding attempt even at the expense of their own survival, thus incurring in intra-generational costs. We tested whether intensity of current reproductive effort has intra-or inter-generational costs in a short-lived bird, the blue tit Parus caeruleus, with a brood size manipulation experiment. We expected more intra-generational (parental reproduction and/or survival) than inter-generational (offspring quality and survival) reproductive costs. We found that parental effort, measured as the hourly rate of parental visits to nests, increased gradually with experimental manipulation. Brood size manipulation resulted in a gradual increase in the number of fledglings per nest from reduced to increased treatments. We found an effect of the manipulation on the probability of making a second clutch, with adults rearing enlarged broods being less likely to undertake such a second reproduction during the season compared to those rearing control or decreased broods. We found no evidence of other reproductive costs; neither as adult weight after manipulation, apparent parental local survival, apparent offspring local survival or local recruitment. Although the results seem to support the a priori expectations, alternative explanations are discussed.Communicated by M. Soler     

Costs of switching social groups for dominant and subordinate dark-eyed juncos (Junco hyemalis)

I quantified the costs of switching from a familiar to an unfamiliar flock for captive dark-eyed juncos (Junco h. hyemalis) by measuring several physiological and behavioral variables before and after flock switching. Birds that were initially dominant dropped in status in unfamiliar flocks, and experienced increased metabolic rates, while subordinate birds appeared to undergo less physiological change when switching flocks. This difference occurred despite a lack of any rank-related differences in the effects of joining a new flock on rates of aggression, weight change, access to food, or plasma corticosterone levels. These results suggest that for dominant, but not subordinate, individuals there is a measurable metabolic cost to joining a new social group, even in the absence of adverse factors such as food limitation. Dominant individuals may be less likely than subordinates to leave familiar flocks because of their higher metabolic costs when joining a new social group.     

Benefits and costs of earwig (Forficula auricularia) family life

The evolution of parental care and family group formation critically depends on offspring survival benefits and parental fecundity costs of care under given ecological conditions. Investigations of the functional significance of care in insect species that exhibit facultative parental care have been relatively rare but may be of particular interest for better understanding of benefit and cost schedules at an early evolutionary stage. In this study, aspects of benefits and costs of care were addressed in the sub-social European earwig (Forficula auricularia; Dermaptera: Forficulidae) by manipulating the presence of tending mothers and brood size in a fully crossed experimental design. Larvae growing in broods tended by their mother or of reduced size showed a higher survival probability than larvae growing in untended or large broods, as predicted if maternal care is beneficial and shaped by a trade-off between number and quality of offspring. Analysis of patterns of food consumption and developmental time further suggested that the benefit of maternal attendance is mediated by the maternal provisioning of food, while the quality–quantity trade-off seemed to be driven by sibling rivalry. Further, tending mothers delayed the production of a second clutch, indicating a potential cost of care in terms of lifetime fecundity. This study experimentally shows benefits and potential costs of maternal care and family group formation in the European earwig. More detailed behavioural experiments will be required to fully understand how behavioural interactions among family members mediate these reproductive outcomes.     

The scent of dominance: female field crickets use odour to predict the outcome of male competition

Pheromones may convey information about mate quality and social status. In the field cricket Gryllus integer, females mount the males for copulation, such that males cannot coerce females to mate. We examined whether virgin G. integer females preferred the scent of potentially dominant males to that of subordinate males. First, we collected pheromones by confining males on filter paper. Next, we offered filter paper from each of two size-matched males and control paper to females that had never been exposed to males, and measured the time spent by the female on each kind of paper. Finally, dominance status of the males in each size-matched pair was determined by pitting the two males against one another in agonistic contests. When offered filter paper from subsequently dominant versus subsequently subordinate males, females spent more time on the paper from the dominant male than the subordinate male, and much less time on control paper. Thus, pheromones may inform female G. integer about a male's potential to achieve dominant social status. Male pheromones were also associated with the female's tendency to mount a male. In contrast to cockroaches, where females prefer the scent of subordinate males (presumably to avoid risk of injury), female crickets prefer the scent of potentially dominant males and are more likely than males to wound their mating partners.     

Prior encounters with the opposite sex affect male and female mating behavior in a wolf spider (Araneae,Lycosidae)

Mate availability can vary widely in nature depending upon population density and sex ratio and can affect the ability of individuals to be selective in mate choice. We tested the effects of prior encounters with the opposite sex (i.e., exposure to the opposite sex either with or without mating) on subsequent mating behavior in two experiments that manipulated mate availability for both males and females in the wolf spider, Hogna helluo. The probability of mating in the experimental trial depended upon whether the prior encounter involved mating or not, and males and females responded in opposite directions. Exposure without mating resulted in a higher subsequent frequency of mating for females and a lower subsequent frequency of mating for males, while prior mating experience resulted in a lower frequency of female remating and a higher frequency of male remating. Prior exposure without mating did not affect female aggression. However, mated females engaged in precopulatory cannibalism more frequently than virgins. Mated males escaped postcopulatory cannibalism more frequently than virgins. Our results show that males respond to exposure without mating in the expected manner. However, prior mating (1 week earlier) had unexpected effects on males, which may be due to mated males being of higher quality. There were little or no effects of the size of the prior exposure individual or mate on subsequent mating behaviors. Further research is needed to determine why different species use different degrees of prior information in mate choice.     

Shifting foraging strategies in colonies of the social wasp Polybia occidentalis (Hymenoptera, Vespidae)

Social insect colonies can be expected to forage at rates that maximize colony fitness. Foraging at higher rates would increase the rate of worker production, but decrease adult survival. This trade-off has particular significance during the founding stage, when adults lost are not replaced. Prior work has shown that independent-founding wasps rear the first workers rapidly by foraging at high rates. Foraging rates decrease after those individuals pupate, presumably reducing the risk of foundress death. In the swarm-founding wasps, colony-founding units have many workers, making colony death by forager attrition less likely. Do swarm-founding wasps show similar shifts in foraging rates during the founding stage? We measured foraging rates of the swarm-founding wasp, Polybia occidentalis at four stages of colony development. At each stage, foraging rates correlated with the number of larvae present, which, in the founding stages, correlated with the number of cells in the new nest. Thus, foraging rates appear to be demand-driven, with the level of demand in the founding stage set by the size of nest that is constructed. During the founding stage, foraging rates per larva were high initially, suggesting that colonies minimize the development times of larvae early in the founding stage. Later in the stage, foraging rates decreased, which would reduce worker mortality until new workers eclose. This pattern is similar to that shown for independent-founding wasps and likely results from conflicting pressures to maximize colony growth and minimize the risk of colony death by forager attrition.     

Sexual selection in the northern pintail (Anas acuta): the importance of female choice versus male-male competition in the evolution of sexually-selected traits

We experimentally studied the relative importance of plumage, dominance status, and courtship behavior in determining male pairing success in the northern pintail Anas acuta and assessed whether these traits function in female choice, male-male competition or both. In an experiment (experiment IA) that eliminated the confounding effects of male-male competition and social courtship, females chose males with pure white breasts and colorful scapular feathers. When the same group of birds were free to interact (experiment 1B), male behavior was more important: females chose males that courted them intensely and were attentive to them, although preferred males again had whiter breasts and more colorful scapulars. In a second experiment (experiment 2), testing the effect of age on pairing success, females showed a significant preference for 2-year-old males over yearlings: 2-year-old males courted more and were more attentive to the female than yearlings; they were also more colorful than yearlings in a number of plumage measurements. Although males (in both experiments 1B and 2) were aggressive to one another while courting the female and dominant males were sometimes able to exclude subordinates from social courtship, contrary to expectation, we found no relationship between initial dominance rank and pairing success or dominance rank and age. In addition, dominance was not correlated with any of the morphological traits measured. Once chosen, however, subordinate males typically initiated fights with the higher-ranked male(s) and quickly achieved dominance. These results suggest that (1) females choose males based on a suite of morphological and behavioral characteristics, (2) male dominance relationships do not constrain active female choice, (3) a male's position in a dominance hierarchy is largely a result rather than a cause of female choice, and (4) female choice plays a more significant role than male-male competition in the evolution of several secondary sexual traits in male northern pintails.     

Structure and resilience of the social network in an insect colony as a function of colony size

Social interactions are critical to the organization of worker activities in insect colonies and their consequent ecological success. The structure of this interaction network is therefore crucial to our understanding of colony organization and functioning. In this paper, I study the properties of the interaction network in the colonies of the social wasp Ropalidia marginata. I find that the network is characterized by a uniform connectivity among individuals with increasing heterogeneity as colonies become larger. Important network parameters are found to be correlated with colony size and I investigate how this is reflected in the organization of work in colonies of different sizes. Finally, I test the resilience of these interaction networks by experimental removal of individuals from the colony and discuss the structural properties of the network that are related to resilience in a social network. This contribution is part of the special issue “Social Networks: new perspectives” (Guest Editors: J. Krause, D. Lusseau, and R. James).     

Distance to neighbours influences the trade-off between hiding after disturbance and defending food patches in convict cichlids (<Emphasis Type="Italic">Archocentrus nigrofasciatus</Emphasis>)

Territories are often aggregated. Because of this, distance to neighbours should influence how territory-holders balance safety from predators with the use and defence of resources. I examined the influence of distance to a neighbour on refuge use by pairs of convict cichlids (Archocentrus nigrofasciatus) faced with a conflict between hiding and defending food patches. Neighbours could reduce the rate of intrusions by strangers as a by-product of their own resource defence. This should allow fish with near neighbours to spend more time in the refuge. Neighbours could also steal from patches that are left undefended. This should lead to a reduction in use of the refuge. When one fish was confined to its refuge (so that its patch was undefended), theft by the other increased as inter-patch distance decreased. Distance between patches did not influence the rate of intrusion by non-territorial fish. When both fish defended patches, body mass influenced the effect of inter-patch distance on refuge use. Large fish rarely used the refuge, but small territory-holders spent more time in the refuge when patches were close together, as predicted. However, when one fish was dominant at both patches, distance between patches did not influence refuge use. These results suggest that, despite increased opportunity for theft, there is no realised foraging or defensive benefit to settling near neighbours that are of similar competitive ability.Communicated by J. Krause     

Benthic macrophyte communities in the Ghar El Melh lagoon (North Tunisia)

This work deals with the biodiversity and distribution of benthic macrophytes in the Ghar El Melh lagoon, a Mediterranean coastal lagoon located in the North of Tunisia. An inventory was made of the benthic flora and submerged macrophyte communities were mapped during two successive campaigns (the summer of 1999 and the winter of 2000). The following 24 macrophyte species were identified: seven red algae, two brown algae, 11 green algae, and four marine angiosperms. The results were compared with available data from the literature. Ruppia cirrhosa is the most dominant species. It is found in all lagoon parts, except in the west sector.Ruppia beds are usually associated withCladophora forming heterogeneous communities. During summerRuppia cirrhosa shows a large distribution, covering an area of ca. 21.4 km², with dense, extensive beds covering 80–100%. In winter, severalCladophora species have a very large distribution as well, covering nearly an area of 28.5 km² with an average cover of 46%. The green algaeCaulerpa prolifera is confined to the eastern part of the lagoon which is mainly affected by seawater. In comparison with previous situations, many transformations were observed in biodiversity and spatial distribution of the dominant communities. Thus,Cymodocea nodosa andZostera beds, which dominated in the 1970s, were replaced byZostera andCaulerpa prolifera in the 1980s and are currently succeeded byRuppia cirrhosa andCladophora. Restoration of the Ghar El Melh lagoon will enable an increase in the exchange with the open sea and the circulation of water, in particular in the confined zones. This should considerably improve the water quality and would positively influence the phytobenthic communities.     

Characterization of the benthic vegetation in the Farwà Lagoon (Libya)

This study forms part of the Action Plan for the Conservation of Marine Vegetation in the Mediterranean SEa (United Nation Environmental Program). It was carried out in June 2000 in the Farwà Lagoon, Libya. The mapping of the main benthic vegetation was achieved by compiling the field observations (transect method), and remote sensing of SPOT satellite images. The phytobenthos in the Farwà lagoon covers an area of 1820 ha (65%). Three benthic macrophyte species dominate, namely the marine phanerogamsCymodocea nodosa andPosidonia oceanica, and the algaCaulerpa prolifera. DeadPosidonia oceanica leaves (litter) form veritable mounds in the vicinity of the openings leading to open sea. These leaves, which come from the coastal sea, are brought into the lagoon by currents and tides; their decomposition will lead to high oxygen consumption and the release of hydrogen sulphide. The phenological data ofPosidonia oceanica shoots sampled in the lagoon are similar to those from other stations in the Mediterranean. Conversely, the lepidochronological parameters of shoots sampled in the central part of the lagoon exhibit values that are substantially higher than those generally recorded in the Mediterranean. The mean number of leaves produced annually is 9.9 (mean value for the Mediterranean: 7.5) and the rhizome growth rate is of 35.7 mm.yr⁻¹ (mean value for the Mediterranean: 7.5 mm.yr⁻¹). This hypersaline environment would seem to provide optimum growth conditions for the speciesPosidonia oceanica.     

Song rate as a signal for nest site quality in blackcaps (Sylvia atricapilla)

In this study we examine male song output as a measure of nest site quality in blackcaps (Sylvia atricapilla). Song rate, breeding success, predation on nests and reaction to playbacks were investigated in individual males. Habitat features determining nest site and song post quality in terms of vegetation cover were compared between successful nests and nests that had suffered predation. We then related song rate of unmated males to habitat factors in territories and nesting sites in order to examine a possible predictor function of blackcap song for habitat quality. Several habitat features are responsible for variation in nesting success. These features also correlate with song rate of unmated males. The study indicates a potential role of song rate in the advertisement of territory quality. Furthermore, the data suggest that females use song rates rather than territory quality in mating decisions. The information females may gain about male quality in relation to territory quality are also discussed.     

Escape take-off strategies in birds: the significance of protective cover

Avian escape strategy is highly dependent on the ability to fly, and the success rate of raptor attacks is reduced if the prey gets fully airborne. Therefore, when escaping from predator attacks, the initial take-off is crucial and a rapid take-off, high velocity, and high angle of ascent maximize the chance of survival. However, due to the laws of gravity, birds face a trade-off between maximizing its linear acceleration and maximizing its rate of climb when taking off. The optimal policy between velocity and angle of ascent when a bird escapes from an attacking predator might depend on the detailed nature of the predator’s attack, the proximity to cover, and the presence of conspecifics. Many small birds rely on a quick dash to protective cover. In this study, we examine how the availability of protective cover affects take-off strategy in birds. Male great tits (Parus major) were subjected to a surprise attack by a model predator either in the presence or absence of protective cover. When attacked by the predator, great tits took off and perched in the cover when it was available. Birds subjected to the predator attack in the absence of cover took off in 10° higher angle of ascent and flew faster in the start of the take-off, compared to birds that took off in the presence of cover. Thus, this study for the first time shows that a bird do trade off speed with angle of ascent in an adaptive way, depending on the presence of protective cover.     

Less is more: social learning of predator recognition requires a low demonstrator to observer ratio in Arctic charr (Salvelinus alpinus)

Reintroduction programs for conservation purposes suffer from low post-release survival of captive-bred animals, especially so with fish reintroductions. High mortality is most often due to weakened antipredator skills, which could be enhanced through direct experience with predators and social learning. As yet, the relative efficiency of these learning methods has not been tested, and the significance of the factors determining how efficiently antipredator responses are transmitted socially from experienced (demonstrator) to predator naive (observer) individuals is largely unknown. Using hatchery-bred stock of the endangered Saimaa Arctic charr (Salvelinus alpinus), we show for the first time that: (1) contrary to theoretical predictions, acquired predator recognition was only socially transmitted from predator-experienced to predator-naive fish in groups with a smaller proportion of experienced than naive individuals, and that (2) socially acquired avoidance of the predator odors was as strong as in those fish conditioned with a combination of odors and predator exposure. We conclude that using social transmission procedures in training fish for reintroduction programs may provide considerable ecological, economical and ethical advantages.Communicated by J. Krause