Seasonal decline in reproductive performance varies with colony size in the fairy martin, <Emphasis Type="Italic">Petrochelidon ariel</Emphasis>

Reproductive success within populations often varies with the timing of breeding, typically declining over the season. This variation is usually attributed to seasonal changes in resource availability and/or differences in the quality or experience of breeders. In colonial species, the timing of breeding may be of particular importance because the costs and benefits of colonial breeding are likely to vary over the season and also with colony size. In this study, we examine the relationship between timing of breeding and reproductive performance (clutch size and nest success) both within and between variable sized colonies (n = 18) of fairy martins, Petrochelidon ariel. In four of these colonies, we also experimentally delayed laying in selected nests to disentangle the effects of laying date and individual quality/experience on reproductive success. Within colonies, later laying birds produced smaller clutches, but only in larger colonies. The general seasonal decline in nest success was also more pronounced in larger colonies. Late laying birds were generally smaller than earlier laying birds, but morphological differences were also related to colony size, suggesting optimal colony size also varies with phenotype. Experimentally delayed clutches were larger than concurrently produced non-delayed clutches, but only in larger colonies. Similarly, delayed clutches were more likely to produce fledglings, particularly later in the season and in larger colonies. We suggest that the reduced performance of late breeding pairs in larger colonies resulted primarily from inexperienced/low quality birds preferring to settle in larger colonies, possibly exacerbated by an increase in the costs of coloniality (e.g., resource depletion and ectoparasite infestations) with date and colony size. These findings highlight the importance of phenotype-related differences in settlement decisions and reproductive performance to an improved understanding of colonial breeding and variation in colony size.     

Multiple mating opportunities boost protandry in a pied flycatcher population

Protandry, the earlier arrival of males than females to breeding areas, is widespread in birds, but its underlying mechanisms are far from well understood. The two, not mutually exclusive most highly supported hypotheses to explain avian protandry postulate that it has evolved from intrasexual male competition to acquire the best territories (“rank advantage” hypothesis) and/or to maximize the number of mates (“mate opportunity” hypothesis). We studied for two consecutive years the relative importance of both hypotheses in a population of pied flycatchers (Ficedula hypoleuca), a territorial songbird with a mixed mating strategy. We measured territory quality using a long-term dataset on nest occupation and breeding output, and we used molecular techniques to assess male fitness across the range of social and genetic mating options. Territory quality was unrelated to breeding date and had no influence on extra-pair paternity or social polygynous events. However, males breeding early increased their chances of becoming socially polygynous and/or of attaining extra-pair paternity and, as a consequence, increased their total reproductive success. These results support the “mate opportunity” hypothesis, suggesting that sexual selection is the main mechanism driving protandry in this population.     

Effects of sex-associated competitive asymmetries on foraging group structure and despotic distribution in Andean condors

Phenotype-limited interference models assume competitive asymmetries among conspecifics and unequal sharing of resources. Their main prediction is a correlation between dominance status and patch quality: dominant individuals should preferentially exploit better-quality habitats. We tested assumptions and predictions of the phenotype-limited interference model in Andean condors (Vultur gryphus), a New World vulture with strong sexual size dimorphism (males are 30–40% heavier than females). We recorded searching birds in habitats differing in quality: mountains and plains. We also observed scavenging behaviour at 20 sheep carcasses, and videotaped 5 of them. Intraspecific hierarchy at carcasses was based on size: males dominated females and, within each sex, older birds dominated younger ones. Adult males and juvenile females occupied extreme positions in the feeding hierarchy. Aggression was directed at those individuals belonging to lower hierarchical levels. In high-quality areas (mountains), more condors arrived at carcasses. Juvenile females were more often observed searching in low-quality areas (plains), far from breeding areas and main roost sites. GLM analyses of individual behaviour showed that the hierarchy did not influence time of arrival, but low-ranking individuals spent more time at carcasses, especially if the number of condors at arrival was high. Additionally, low-ranking condors spent less time feeding at carcasses when individuals of higher hierarchical levels were present. On the other hand, the number of condors present had a positive effect on feeding rates of dominant individuals, probably because of a reduction in individual vigilance. These results support most of the assumptions and predictions of the phenotype-limited distribution model, although a spatial truncated distribution between phenotypes was not observed. Asymmetric feeding pay-off, unequal parental roles and sexual selection constraints could favour sexual divergence in body size in Andean condors. Received: 6 April 1998 / Accepted after revision: 11 July 1998     

Duration of paternal care in pine engraver beetles: why do larger males care less?

Male pine engravers, Ips pini (Coleoptera: Scolytidae), assist their mates during reproduction by removing the debris that accumulates while females excavate oviposition tunnels in the phloem tissue of host tree bark. Although duration of paternal care and male reproductive success are positively correlated, large males leave their mates and brood sooner than small males. We address two hypotheses to explain the earlier departure of larger males from their breeding galleries: (1) females oviposit most rapidly when paired with large males, thereby reducing the length of time that paternal care increases male reproductive success, (2) larger males have better prospects for future reproduction, and thus leave their galleries in search of new breeding opportunities sooner than smaller males. Contrary to the first hypothesis, when females were paired either with large or small males, there was no effect of male size on their rate of oviposition. Consistent with the second hypothesis, males that initiated breeding galleries were larger than males from the general population. In addition, large males flew farther and faster on flight mills than small males, which may indicate that large males have an advantage in locating suitable breeding sites. Thus, we suggest that large male pine engravers leave their galleries earlier than small males because large individuals have better prospects for future reproduction. Received: 30 November 1997 / Accepted after revision: 23 May 1998     

A seventeen-year study of the rose star Crossaster papposus population in a coastal bay in southeast Alaska

A 17 yr study of a population of the rose star Crossaster papposus (Linnaeus, 1767) in Auke Bay, near Juneau, Alaska, used SCUBA to follow a total of 956 individually marked stars. Population density was relatively constant over the 17 yr of study, although the influx of smaller individuals varied by an order of magnitude. Growth in diameter of individual stars was inversely related to size, with the largest individuals showing negligible growth for periods of months or years. Annual survivorship was relatively high (from 76.3 to 90.4%), and some individuals persisted throughout the study, indicating longevities of at least 20 yr in this species. Observations of food habits indicated that the principal prey item is the reddish scallop Chlamys rubida. In summary, this rose star population was characterized by a relatively stable number of long-lived, slow-growing individuals. Received: 7 April 1998 / Accepted: 5 October 1998     

Brood sex ratio is dependent on female mating status in polygynous great reed warblers

Females capable of adjusting the sex ratio of their offspring should be more fit than females lacking such an ability. In polygynous birds where breeding success in males is more strongly influenced by body size and/or attractiveness than in females, females might produce more sons when predicting good conditions or when mating with attractive males. Polygynous great reed warbler, Acrocephalusarundinaceus, males direct most of their feeding effort to the primary (first-hatching) nest and in these nests increase their feeding effort in relation to the brood sex ratio (proportion of sons). Therefore, with the expectation of well-nourished sons, we would predict that females which start breeding first within harems might produce more sons than those which start breeding later, and in anticipation of sons with good genes, that females mated to polygynous males might produce more sons than females mated to monogamous males. I took blood samples from hatchlings and determined the sex using DNA markers. The sex ratio of primary (monogamous and polygynous primary) broods is more male-biased (mean 0.58 males, n = 50) than that of secondary (polygynous secondary and tertiary) broods (mean 0.46, n = 25). Moreover, in the secondary broods with the largest clutch (five eggs), in which offspring are most likely to suffer food shortage, the sex ratio was distinctively female biased (mean 0.33, n = 10). In the primary broods, sex ratio was correlated to harem size. The results suggest that great reed warbler females modify the brood sex ratio to produce both well-nourished sons and sons with good genes, but the former effect is probably stronger than the latter factor. Received: 11 March 1998 / Accepted after revision: 23 May 1998     

Reproduction by subordinates in cooperatively breeding Arabian babblers is uncommon but predictable

In a genetic analysis of the mating system of cooperatively breeding Arabian babblers (Timalidae: Turdoides squamiceps), we identified which individuals in the population are breeding, and how reproductive success was distributed among group members with respect to their dominance rank, for both males and females. The population was characterized by an asymmetrical distribution of reproductive success; behaviorally dominant males produced 176 of 186 (95%) of the offspring in 44 social groups analyzed, and alpha females produced 185 of 186 (99.5%). We evaluated models of reproductive skew by examining genetic and demographic correlates of reproduction by␣subordinates. Subordinate (beta) males that sired young were more likely to be recent dispersers from their natal groups or members of newly formed groups than betas that did not reproduce. Breeding beta males had spent smaller proportions of their lives with the current alpha male and female as alphas than had beta males that did not sire young. One consequence of the linkage of dispersal with breeding in newly formed, nonnatal groups is that beta males that sired young had significantly lower genetic similarity to the alpha males in their groups (based on band-sharing coefficients using multilocus minisatellite DNA fingerprinting) than those that did not sire young. This pattern may occur generally in species in which group membership accrues both through nondispersal of young (forming groups of relatives) as well as through dispersal involving coalitions that sometimes include nonrelatives. Received: 22 July 1997 / Accepted after revision: 5 February 1998     

Plasticity in size and age at maturity in a monogamous fish: effect of host coral size and frequency dependence

The number and maximum body size of the gobioid fish, Paragobiodon echinocephalus, increase with the size of its obligate host coral, Stylophora. Only the largest two individuals breed monogamously in each coral head, and the reproductive success of each spawning is positively correlated with body size. In this study, the plasticity in size and age at maturity in P. echinocephalus was examined. We analyzed life history data from gobies 15–20 mm TL (total length) at their initial marking. Gobies found in larger corals were of lower rank in size order and began to breed later at a larger size, usually upon moving to other corals. The size at maturity ranged widely from 17.2 to 36.0 mm TL. After maturation, growth rates decreased. Mortality, however, was not affected by the timing of maturation. The host coral size did not affect the growth and mortality of marked fish, but the mortality rate of juveniles prior to marking appeared to be higher in smaller corals. The estimated lifetime reproductive success did not differ between the gobies inhabiting corals of different size. Thus the plasticity in size and age at maturity in this species may be maintained by frequency-dependent selection in choosing a host coral size that affects an individual’s social status. Received: 5 April 1995/Accepted after revision: 18 February 1996     

Synchrony in egg-laying and reproductive success of neighboring common murres,Uria aalge

Common murres (Uria aalge) are highly colonial; pairs often breed at the highest possible densities, in bodily contact with neighbors. At Bluff, a colony in western Alaska, we tested for synchrony in egg laying at various spatial scales and found little evidence for higher synchrony, either within study plots of 15–195 pairs, or within subplots containing several pairs, than among plots in a 5-year study. Egg laying of neighbors generally was more synchronous than expected based on overall frequency distributions in laying dates, however. Breeding success was positively correlated with the number of breeding neighbors and the number of neighbors tending eggs or nestlings at the time of egg laying. Breeding success of pairs with neighbors was positively related to the breeding success of neighbors. Pairs that produced eggs synchronously with at least one neighboring pair had higher success than those that began breeding either before or after their neighbors. Most reproductive failures at Bluff are due to accidental egg loss and predation on eggs by common ravens, Corvus corax, soon after laying. By occupying space where a raven might otherwise land and defending their own eggs, active breeding neighbors locally reduce the probability of egg predation. Active breeding neighbors also are less likely to flush and accidentally dislodge nearby eggs when disturbed than are nonbreeders. Murres breeding synchronously with neighbors have the highest assurance of the presence of active breeding neighbors both at the time of egg laying and throughout their reproductive attempts. Groups of neighboring murres can be considered small “selfish herds,” demonstrating by-product mutualism through their continued presence and defense of their own eggs and nestlings. Despite the advantages of breeding synchronously with neighbors, early breeding may often be favored, however. Received: 22 January 1996/Accepted after revision: 16 June 1996     

Sexual selection in the zebra finch Taeniopygia guttata : condition, sex traits and immune capacity

The aim of this study was to test two hypotheses: (1) that condition-dependent secondary sexual traits reflect an individual's immune capacity and (2) that immune capacity and secondary sexual traits covary with primary sex traits, specifically ejaculate quality. We used the zebra finch Taeniopygia guttata as a study species, since the traits that females find attractive in males of this species, song rate and beak colour, are well established. A paired experimental design comprising 31 pairs of brothers was used; for each pair, one male was assigned to a control group provided with ad libitum food and no additional exercise, and the other male was assigned to an experimental group which experienced additional exercise and a reduced rate of food intake. After 11 weeks, the experimental group differed significantly from the control group in a range of variables, including body mass, haematocrit, granulocyte:lymphocyte (G:L) ratio and several primary sex traits, indicating that condition in this group was reduced. Birds in the experimental group showed a differential response to the treatment. We used the rank order in which birds could be captured by an experimenter as an index of condition. Birds easily caught were assumed to be in poorer condition than those which were more difficult to capture. Rank capture order was repeatable and was significantly correlated with the G:L ratio in the experimental group, but not in the control group. In the experimental group, rank capture order was correlated significantly with both secondary sex traits: birds in better condition had redder beaks and a higher song rate. However, beak colour and song rate did not covary significantly, suggesting that these two traits provide different types of information. Secondary sex traits did not covary with primary sex traits or any sperm features. Thus, there was no evidence for Trivers' sexual-competence hypothesis or the phenotype-linked fertility hypothesis. We used four assays of immune capacity, two general (G:L ratio and spleen mass) and two specific [antibody titres to sheep red blood cells (SRBCs) and Brucella abortus (BA)]. The G:L ratio was significantly higher in the experimental group, spleen mass (absolute and relative) did not differ between the groups, anti-SRBC antibody titres were significantly higher in the control group (contrary to expectation), and anti-BA antibody titres were close to being significantly lower in the experimental group. Within the experimental group, there was no evidence that antibody titres covaried with secondary sex traits. Although we demonstrated that beak colour and song rate were condition dependent, our experiment provided no evidence that either of these traits covaried with immune capacity or sperm features. Received: 9 February 1998 / Accepted after revision: 5 September 1998     

Divorce and extrapair mating in female black-capped chickadees (Parus atricapillus): separate strategies with a common target

Patterns of divorce and extrapair mating can provide insights into the targets of female choice in free-living birds. In resident, site-faithful species with continuous partnerships, the better options and the incompatibility hypotheses provide the most likely explanations for divorce. Extrapair mating can be explained by a number of hypotheses often making similar predictions. For example, the good genes and future partnerships hypo- theses predict similar patterns if males with good genes also make the best future partners. By considering both divorce and extrapair mating, it may be possible to distinguish between these comparable hypotheses. We examined natural patterns of divorce and extrapair mating in a long-term study of black-capped chickadees (Parus atricapillus). Out of 144 partnerships over 8 years, we observed 11 divorces and 38 faithful pairs between seasons. Females usually divorced between their first and second breeding seasons for males of higher social rank than their previous partners, had similar reproductive success prior to divorce as females who retained their previous partners, and did not divorce on the basis of previous reproductive success. These results confirm earlier experimental evidence that females divorce for better options. Females who divorced were significantly more likely to have had mixed-paternity broods prior to divorce than females who stayed with their previous partners. There was no evidence that females divorced in favour of previous extrapair partners. These results support the good genes hypothesis for extrapair mating, suggesting that female chickadees use divorce and multiple mating as separate strategies sharing a common target. Received: 4 February 2000 / Revised: 20 July 2000 / Accepted: 4 September 2000     

Reproductive biology of the commercial sea cucumber Holothuria fuscogilva in the Solomon Islands

 Reproduction of Holothuria fuscogilva (Selenka, 1867) in the Solomon Islands was investigated over a 4 yr period (1994 to 1998) by macroscopic and microscopic examination of the gonad tubules, the gonad index (GI) method, histological examination of gametogenesis, and spawning-induction trials. The gonad consisted of numerous tubules that dominated the coelom of gravid specimens. New tubules appeared in March, and grew in size and extent of branching until they reached their maximum size and maturity in August. Spawning occurred from August to October, with the majority of gametes released during October, although it was only partial in many individuals. After spawning, the tubules appeared wrinkled and resorbed into the gonad basis. A five-stage gonad maturity scale based on the macroscopic appearance of the gonad tubules corresponded with discrete stages of gametogenesis identified by histology. Gametogenesis was initiated in mid-March, with oogenesis and spermatogenesis occurring in parallel, followed by the growing stage (May to July) which was marked by active gamete development. Successful induction of spawning during the breeding period corroborated the GI and histological data. The uniform growth of gonad tubules indicated that H. fuscogilva in the Solomon Islands does not conform to the progressive tubule recruitment model described for other holothurians. An important application of this study is that the appearance of gonad tubules, removed by biopsy, can be used to determine the gonad condition of wild adults or captive broodstock. Received: 28 June 1999 / Accepted: 8 March 2000     

Past reproductive success affects future habitat selection

Correlational studies have shown that an individual's past reproductive success often increases its breeding site fidelity (i.e., the tendency to return to a previously occupied location), suggesting that individuals use their reproductive experience to assess habitat quality. However, the causality of the relationship between reproductive success and site fidelity is still uncertain. In a field experiment, the effect of mating success on site fidelity was isolated from potential confounding variables in a territorial dragonfly, the eastern amberwing (Perithemis tenera). The experiment controlled for site quality, intrinsic characteristics of males, previous territorial experience at the site, arrival order, and territorial evictions. Males that were prevented from mating were much more likely to change sites the following day than control males that were allowed to mate. This result was not affected by age, the amount of time a male spent on the site, or mortality. These results imply that individuals use their own reproductive success to assess the quality of the habitat. The benefit to an individual of using its reproductive success to determine habitat quality is discussed relative to other sources of information. Received: 31 May 1996 / Accepted: 31 January 1997     

Sperm competition and sexual selection: a meta-analysis of paternity studies of birds

Sperm competition (the competition among the sperm of different males for fertilization of the eggs of a female) has been suggested to be an important component of sexual selection, but no general assessment has been made of this proposition. We used a meta-analytic approach to assess the extensive literature on paternity (the proportion of offspring in a focal nest sired by an attending male) in birds based on allozyme and molecular techniques. The relative variance in male mating success was on average increased by a factor of 4.6 over the variance in apparent male success. Males with more extravagant secondary sexual characters had higher paternity in their own nests than less adorned males. There was a weak effect of male age being positively associated with paternity in own nests. Male body size measured as the length of wing and tarsus was weakly positively associated with paternity in own nests. Male survival prospect was positively associated with paternity in own nests. Polygynous males generally had decreased paternity of their broods compared to monogamous males. Paternity of the resident male decreased with increasing population density and breeding asynchrony. The intensity of paternity guards such as within-pair copulation rate and mate guarding were not significantly related to extra-pair paternity. Sperm competition was thus an important component of sexual selection by increasing the variance in male mating success, and by being associated with the expression of secondary sexual characters, in particular in dense and asynchronously breeding populations of birds. Received: 12 February 1998 / Accepted after revision: 31 May 1998     

Larval paternity and male reproductive success of a broadcast-spawning gorgonian, Plexaura kuna

Male reproductive success of the broadcast-spawning gorgonian, Plexaura kuna Lasker, Kim and Coffroth, 1996, was measured in June 1994 and June and July 1995 at two sites in the San Blas Islands, Panamá in order to determine the environmental and biotic factors affecting individual reproductive success. Developing embryos were collected in the field during natural spawning events, and paternity determined using randomly amplified polymorphic DNA markers. Analyses of F₁ progeny from defined laboratory matings established that the markers were inherited in Mendelian fashion, and allowed the determination of the zygosities of the markers. P. kuna is clonal, but male reproductive success was not strictly proportional to clone size. Proximity to females appeared to have a greater effect on male reproductive success than clone size, and on both reefs the most successful male clone was the one closest to the spawning female clone. Current direction and transport of gametes by eddies explained variation in paternity assignments between nights. Clonal propagation allows clones to grow and spread toward each other, and may enhance male reproductive success. Received: 1 April 1997 / Accepted: 11 February 1998     

Frequency and timing of extrapair fertilisation in the polyandrous red phalarope (Phalaropus fulicarius)

In sequentially polyandrous bird species, where females mate with different males in succession during a single breeding season, sperm stored by females can occasionally lead to extrapair fertilisations (EPFs) in clutches cared for by the second and subsequent males. Thus, we predicted that in red phalaropes (Phalaropus fulicarius) – a sex-role-reversed, sequentially polyandrous, arctic breeding shorebird – EPFs would occur more frequently in clutches laid later in the breeding season. We used multilocus DNA profiling to examine the frequency and timing of EPFs in a population of red phalaropes breeding in the Canadian high arctic. Using a technique to determine parentage without maternal DNA, we inferred that 6 of 70 chicks in 18 broods resulted from EPFs – one extrapair chick in each of 6 broods. These results were supported by a further analysis using microsatellite DNA. As predicted, broods containing EPFs hatched from clutches laid significantly later in the season than did broods containing no EPFs. The difference in median hatch dates between broods with and without EPFs was 9.0 days, or 38% of the entire egg-laying period in that season. For the whole breeding season, we estimated that 6.5% of chicks were sired by extrapair males, which is similar to extrapair paternity estimates for other sex-role-reversed birds, but relatively low compared to the majority of socially monogamous species studied so far. Received: 24 August 1998 / Received in revised form: 18 January 1999 / Accepted: 24 January 1999     

Interannual, seasonal, local and body size variations in reproduction of the prawn Penaeus (Marsupenaeus) japonicus (Crustacea: Decapoda: Penaeidae) in the Ariake Sea and Tachibana Bay, Japan

 The reproductive biology of female Penaeus japonicus Bate was investigated in the Ariake Sea and Tachibana Bay (located outside the Ariake Sea), Japan from 1994 to 1996. Interannual, seasonal, individual female body size and spatial influences on the incidence of spawning were examined. The proportion of inseminated females, increased with increasing body size up to 170 mm body length (BL) and decreased thereafter. The minimum size at maturity was similar between years; 130 to 140 mm BL. The minimum size of individuals with developing ovaries or spermatophores differed from that of ripe females by 20 to 25 mm. Spawning occurred in the central part of the Ariake Sea and Tachibana Bay but rarely in the inner part. P. japonicus had a clear reproductive cycle. Spawning started earlier and ended later, occurring from mid-May to mid-October, in the eastern central part of and outside of the Ariake Sea compared with the western central part, where spawning occurred from mid-June to mid-September. Spawning dynamics differed across seasons, body sizes and areas but not across years. The seasonal peak in the proportion of ripe females varied with body size. In small individuals (130 to 169 mm BL), no peak was observed, whereas in large individuals (>170 mm BL) the proportion of ripe individuals peaked in June. The proportion of ripe individuals increased with increasing body size and was high outside the Ariake Sea. Factors causing the variation in spawning dynamics are discussed. Received: 30 November 1998 / Accepted: 12 October 1999     

Endocrine influences on parental care during a short breeding season: testosterone and male parental care in Lapland longspurs (Calcarius lapponicus)

In males of socially monogamous birds, plasma testosterone (T) typically declines to low levels during the parental phase. Studies on multiple-brooded species indicate that high T may be incompatible with high-quality paternal care. The length of the breeding season may affect the costs and benefits of high T and its effect on paternal care. We studied the effect of experimentally elevated T on paternal care in a single-brooded species with a short breeding season, the Lapland longspur (Calcarius lapponicus). We monitored T levels and parental behavior in 16 males with subcutaneous T implants, 14 males with empty implants, and 14 unimplanted males. We videotaped nests when nestlings were 2–3 days old and again at 4–5 days. T males with 2- to 3-day-old young visited nests and fed young less often than control males, and the mates of the T males compensated with elevated visits and feedings. However, when nestlings were 4–5 days old, T males visited their nests at normal rates – though feeding movements remained below normal – and T females visited and fed at normal rates. Nestling mass and nest success were similar in both groups. Overall, high T suppresses paternal care in Lapland longspur males. The partial improvement of paternal care when nestlings are older, despite high T, may be related to the short 6-week breeding season of this arctic species, and the consequently reduced benefits of sexual behavior late in the breeding season. Received: 2 February 1998 / Accepted after revision: 2 November 1998     

Supply and demand: causes and consequences of assortative mating in common terns <Emphasis Type="Italic">Sterna hirundo</Emphasis>

Although assortative mating is widespread among long-lived monogamous birds, the underlying mechanisms still remain unclear in many species. In this study, we analysed assortative mating by age and body mass in the common tern Sterna hirundo with special regard to mate choice decisions of newly paired birds. To assess whether assortative mating by age is an active decision involving either homotypic or directional preferences or rather a passive process due to restricted availability of potential mates, we analysed the influence of age-dependent arrival date and cohort size. Furthermore, we looked for direct benefits in terms of reproductive success. Common terns mated assortatively by age and arrival date but not by body mass at arrival. Assortative mating by age was age dependent and was detected mainly in younger birds, whereas birds older than 8 years rather re-paired with younger ones. The availability of same-aged mates was restricted by cohort size and arrival date. We found no general evidence for better reproductive performance of same-aged pairs. Instead, relative reproductive success was related to own age and the relative age of mate: It should be advantageous for any bird to acquire an old mate; hence, assortative mating by age seems to be beneficial only for old terns but not for young ones. Age-assortative mating in common terns occurred by both passive and active processes, which are not mutually exclusive. Our results do not indicate a homotypic but a directional preference and support the theory that high-quality (older/experienced) individuals tend to mate assortatively if same-aged mates are available, which leaves low-quality (young/inexperienced) individuals to mate among themselves.     

Is helping a beneficial learning experience for red-cockaded woodpecker (Picoides borealis) helpers?

Several hypotheses have been suggested to explain the evolution and maintenance of helping behavior in cooperatively breeding birds, one of which we investigated in the red-cockaded woodpecker (Picoides borealis). Helping may provide a learning experience that improves reproductive success once the helper becomes a breeder. We used data collected from a population of red-cockaded woodpeckers in the Sandhills of North Carolina to compare the reproductive success of 2-year-old breeders that were helpers at age 1 (helping experience) to those that were floaters or solitary males at age 1 (no helping experience). Reproductive success of the two groups was similar, indicating that helping provides no experience useful in reproduction. We reject the learning experience hypothesis as an explanation for the maintenance of helping behavior in red-cockaded woodpeckers. Received: 29 July 1996 / Accepted after revision: 27 April 1997