Parental care of nestlings by male red-winged blackbirds

Summary Nestling feeding by males is less common among birds with polygynous mating systems than in monogamous species, because of the pronounced trade-off between parental behavior and the attraction of additional mates. In this study, however, we found that male red-winged blackbirds (Agelaius phoeniceus) commonly assisted females in feeding nestlings in several Ontario marshes. Male parental care was additional to that provided by females, and it significantly enhanced the fledging success of nests (Table 2). Male redwings did not help to feed all nests on their territories: primary and secondary nests were much more likely to receive male parental care than tertiary and later nests. Contrary to expectation, male parental care was not restricted to the nests of primary females: a greater proportion of secondary than primary nests were assisted (Tables 4a and b). The presence of new females settling on the territory at the same time that a resident female had nestlings, resulted in males deferring parental care until a later brood. This suggests that males trade off the recruitment of females against parental care to an existing brood. Although the number of nestlings fledging from a male's territory was strongly influenced by the number of females in the harem, males could additionally increase their reproductive success by feeding nestlings in one or more nests on their territories (Fig. 2). The reproductive success of females was significantly enhanced by male assistance in feeding nestlings (Table 3). However, those females not receiving male assistance on territories of feeding males did not suffer a significantly reduced reproductive success in comparison to females on territories of non-feeding males (Table 2). Males varied considerably in the quality of brood care given. We therefore suggest that the quality of male parental care may be a factor considered by females in choosing a breeding situation.     

Antipredation role of clumped nesting by marsh-nesting red-winged blackbirds

Summary Red-winged blackbirds, Agelaius phoeniceus, breed in marshes in high densities and their nests are frequently clumped. Because predation is consistently the most important cause of redwing nesting mortality, high densities of breeding individuals could be an anti-predation adaptation. In our study site predation by marsh wrens, Cistothorus palustris, was the main cause of redwing nesting losses. In situations when marsh wrens were near, predation rates on redwing nests decreased with increasing female density. Group life could reduce predation because of improved nest defense, selfish herd effects, or predator dilution effects. We differentiated between these possibilities by introducing experimental colonies consisting of 3, 6, and 9 artificial nests near and away from active redwing nests. The experimental colonies near active nests suffered less predation, but predation rates were not correlated with colony size or a nest's location within the colony. Therefore, the advantage of group life in this population is probably mutual nest protection.     

Function of male courtship vocalizations in red-winged blackbirds

Summary During courtship, male red-winged blackbirds (Agelaius phoeniceus) produce long sequences of simple, single-note precopulatory calls, punctuated with occasional songs. Male song has previously been shown to stimulate copulation solicitation in captive, estradiol-implanted females of this species (Searcy and Brenowitz 1988; Searcy, in press). Here I show that sequences of precopulatory calls also stimulate copulation solicitation in female redwings, but that they are not as stimulatory as song. In tests contrasting mixed bouts of song and precopulatory calls with bouts of song alone, the mixed bouts elicited significantly more female solicitation in one instance, more but not significantly more in a second, and an exactly equal amount as song alone in a third. Neither song nor precopulatory calls could be shown to affect proximity of females to the speaker.     

Owner and floater red-winged blackbirds: determinants of status

Summary We used sequential removal experiments to test whether the resource-holding potential (RHP) of territory owner red-winged blackbirds, Agelaius phoeniceus, was superior to that of their first replacements (shallow floaters) and subsequent replacements (deep floaters). Among the removals were secondyear males, which were morphologically inferior to adults and which also tended to be competitively inferior in aviary contests. The highest proportion of secondyear males occurred in the deep floater class. Thus, the RHP of some deep floaters was inferior to that of owners and shallow floaters. However, among adults, owners, shallow floaters, and deep floaters had equivalent morphological and competitive RHP. Furthermore, replacement males that had defended territories for many days were neither morphologically nor competitively superior to males that had defended territories for only a few days. Our results suggest that RHP distinguishes adults from second-year males, but does not separate owners from floaters. The only hypothesis that is potentially supported by our observations is that owners have a greater expected payoff from their territory than intruders, and in this way owners are able to maintain site dominance. The nature of that payoff remains to be determined. Offprint requests to: P.J. Weatherhead     

Breeding synchrony and extra-pair mating in red-winged blackbirds

Using data from a 6-year paternity study of red-winged blackbirds, I tested the hypotheses that increased nesting synchrony should either promote extra-pair mating by increasing the advantage of extra-pair mating to females, or decrease extra-pair mating by constraining males from seeking extra-pair copulations. Contrary to these hypotheses, the occurrence of extra-pair paternity did not vary with nesting synchrony over the breeding season, or vary with the number of synchronous nests within territories or within marshes, or with nesting order on territories. However, for nearly all nests with extra-pair young, there were fewer females synchronous with that nest on the cuckolder's territory than on the territory of the cuckolded male. This “advantage” of a synchrony difference was less pronounced for older males that cuckolded younger males, particularly when the two males were not neighbors. Collectively, these results suggest that breeding synchrony affects extra-pair mating by affecting mate guarding, but that breeding synchrony alone can not be used to predict which females are more likely to engage in extra-pair mating, nor with which extra-pair males they will mate. Understanding why extra-pair mating by older males is less affected by breeding synchrony may explain much about both the proximate and ultimate causes of extra-pair mating in red-winged blackbirds. Received: 7 June 1996 / Accepted after revision: 25 November 1996     

An experimental study of paternal behavior in red-winged blackbirds

Summary The effect of brood size and female nesting status on male parental behavior was investigated in red-winged blackbirds Agelaius phoeniceus using brood size manipulation experiments. Male redwings allocated parental effort on the basis of brood size and nestling age. Males began assisting females only at nests with at least three offspring older than three days. Female nesting status had no singificant influence on male parental care. When females were unable to meet a brood's demand for food, males assisted females with nestling feeding. Females did not reduce the amount of food delivered to nestlings when males assisted. The amount of food brought to nestlings by the male was additional to the amount of food provided by the female. Male assistance increased fledgling success. When female provisioning was sufficient to meet a brood's demand for food males did not assist. The value of male parental care varied inversely with the ability of the female to meet nestling food demands. The ability of unassisted females to provide sufficient food and to raise a brood of nestlings successfully appeared to be influenced by resource abundance.     

Competition for territories in red-winged blackbirds: Is resource-holding potential realized?

Summary In resource-defence mating systems we intutively expect that the most competitive males should win the best resources so that, in territorial species, male quality and territory quality will be highly correlated. In the polygyny threshold model (PTM) of mate choice this expectation has become an assumption. We performed a removal experiment using red-winged blackbirds to test the validity of this assumption. On the basis of two morphological and two behavioral indices of competitive ability we found only weak correlation between male competitive ability and territory quality. Factors potentially contributing to this result include habitat quality perception, site dominance, and site fidelity. However, both our experimental design and measurement of male competitive ability may have caused us to underestimate the real correlation between male quality and territory quality. Nevertheless, our results suggest that male quality is not perfectly correlated with territory quality as is assumed in the PTM. Therefore, male quality and territory quality should be treated independently when modelling mate choice in this and other territorial species.     

Bilateral symmetry and social dominance in captive male red-winged blackbirds

There has been much recent interest in subtle departures from perfect symmetry in bilaterally paired morphological characters, and the extent to which such departures reflect aspects of individual quality. We used data from aviary contests involving pairs of wild-caught male red-winged blackbirds (Agelaius phoeniceus) to test the hypothesis that comparatively symmetrical males are disproportionately successful in intra-sexual competition for food. Although paired contestants showed clear and consistent differences in competitive ability, there was no indication that symmetrical males were competitively superior. Winners and losers of aviary contests were indistinguishable based on asymmetry measures made on each of four bilateral characters (tarsus length, wing chord, and two epaulet dimensions), and for a fifth character (length of outer retrix), asymmetry differences, though significant, occurred in the direction opposite to that predicted. Furthermore, there was no detectable association between male competitive ability and a composite measure that combined asymmetry information across all five characters. Our results, in combination with those of several other recent avian studies, suggest that symmetry is generally a poor predictor of social dominance in birds. This finding is inconsistent with the proposal that symmetry provides a readily obtained, reliable measure of phenotypic quality. Received: 15 May 1997 / Accepted after revision: 26 September 1997     

Within-brood patterns of paternity and paternal behavior in red-winged blackbirds

We video-taped male and female red-winged blackbirds (Agelaius phoeniceus) feeding individual chicks in order to test the hypothesis that food might be differently allocated to within-pair offspring and extra-pair young. We found no evidence that paternity influenced the allocation of food by either males or females. Both males and females fed male offspring significantly more, but there was no tendency for paternity to be skewed by gender. Females fed older offspring significantly more, whereas males did not; extra-pair fertilizations, however, were not associated with lay or hatch order of the chicks. Given that males do not appear to discriminate within-pair from extra-pair offspring directly, these results are consistent with current theory on the effect of paternity on paternal behavior. We discuss briefly some of the possible reasons why discrimination might be lacking in red-winged blackbirds and in other species in which the possibility of discrimination of paternity and allocation of paternal behavior has been studied.     

Color bands function as secondary sexual traits in male red-winged blackbirds

Summary We present the first experiment to assess band color effects in a natural bird population. 38 territorial male red-winged blackbirds (Agelaius phoeniceus) were randomly assigned to 1 of 2 banding treatments. They received either all red bands (to match their epaulets) or all black bands (controls). Over half the red-banded males lost their territories while all black-banded males retained their territories. Red and black-banded males did not differ morphologically. However, among red-banded males, those that lost their territories had larger epaulets and were in poorer condition than those retaining their territories. Red-banded males suffered much higher intrusion rates, particularly by neighbors, than black-banded males. We propose that red color bands exaggerated the males' natural aggressive signal beyond the point where the signal was reliable. Our results suggest that signal reliability is maintained by regular testing, particularly of those males most likely to be signalling dishonestly. Offprint requests to: K.J. Metz     

Use of the song repertoire in intersexual and intrasexual contexts by male red-winged blackbirds

Summary Live and stuffed male and female red-winged blackbirds (Agelaius phoeniceus) were presented to territorial males in order to study the repertoire organization of males during intersexual and intrasexual encounters. Territorial male red-winged blackbirds switched song types more frequently and sang more song types in response to presentation of a female stimulus than during control periods, but decreased their switching frequency when a male stimulus was presented. Switching frequency in response to the female stimulus was three times that in response to the male stimulus. These results support the hypothesis that song repertoires of male red-winged blackbirds evolved primarily in response to intersexual rather than intrasexual selection. They also suggest that male red-winged blackbirds have been selected to de-emphasize their song repertoires during encounters with conspecific males as a result of some as yet unidentified cost. Offprint requests to: W.A. Searcy     

Patterns of courtship behavior and ejaculate characteristics in male red-winged blackbirds

Sperm competition in birds is likely to have important effects on the behavior and physiology of reproduction in both sexes. For males, such competition should select for large sperm reserves and behavioral adjustment of copulation when reserves are low. We investigated both these possibilities in free-living red-winged blackbirds (Agelaius phoeniceus), a highly polygynous species with apparently strong sperm competition. We found that the recent copulatory behavior of males did not affect the propensity to copulate with a model female. Ejaculates collected from individual males at 1-h intervals showed no evidence of sperm depletion, yet repeated ejaculates collected less than 10 min apart did. Male ejaculate size was significantly larger if it was the first one of the day (i.e., after an overnight rest). The average ejaculate size was 12.5 (±12.5 SD) million sperm. Males captured during the breeding season had an average of 111.7 (±52.8) million sperm stored in their seminal glomera. Because males average a peak copulation rate of six per female per day, in one day a male might utilize all the sperm in his seminal glomera if more than two females on his territory are fertilizable. We hypothesize that polygyny and sperm competition in this species have combined to select for rapid replenishment of the seminal glomera throughout the day, in contrast to other species that have been studied. Testis size and sperm reserves of male red-winged blackbirds are intermediate between monogamous species and species with intense sperm competition. Several possible explanations for this are discussed. Received: 21 October 1997 / Accepted after revision: 15 February 1998     

Parasite faunas,testosterone and secondary sexual traits in male red-winged blackbirds

Summary We examined associations among parasite infections, secondary sexual traits and testosterone in male red-winged blackbirds sampled at the start of the breeding season. Parasites quantified included ectoparasitic lice and mites and endoparasitic blood protozoans, nematodes, trematodes and cestodes. Secondary sexual traits that we quantified included body size, epaulet size and color, song repertoire size and song switching rate, and behavioral responses to male and female models. Overall we found few significant associations between parasites and secondary sexual traits, between secondary sexual traits and testosterone, or between parasites and testosterone. In addition, most parasite taxa appeared to infect birds independently, although the low prevalence (<50%) of many of the parasites meant that our sample sizes were too small to detect weak associations. Our most promising results were obtained for ectoparasitic mites, which tended to occur on birds uninfected with other parasites, on birds with longer epaulets, and on birds with higher levels of testosterone. Epaulet length and testosterone are both probable correlates of dominance in this species. Further research will be required to determine whether there is a causal link between the immunosuppressive effects of testosterone and the mite infections, and between testosterone, epaulet length and male mating success. Correspondence to: P. Weatherhead     

Ideal dominance distributions: a test using red-winged blackbirds (Agelaius phoeniceus)

Summary The ideal dominance distribution model predicts that competition between individuals of a species for territories will result in socially dominant individuals acquiring territories in higher quality habitat than their subordinates. Although the dispersion and relative reproductive success of male red-winged blackbirds (Agelaius phoeniceus) across habitats in eastern Ontario appears to conform to the ideal dominance distribution model, data from a study of three captive groups comprised of males from both high (marsh) and low (upland field) quality habitats failed to support the prediction that males from marsh habitat are dominant to those from upland habitat. Contrary to the prediction males from uplands were generally dominant to males from marshes. We found a significant positive correlation between dominance and both increased epaulet size and increased body size. Controlling for these positive effects, upland males remained generally dominant to marsh males. Measurements of independent samples of males from both habitats indicated that the overall distribution of males does not conform to ideal dominance. We suggest that the strong between-year territory fidelity shown by male red-winged blackbirds and chance events when they initially acquire territories may contribute to this lack of conformity.     

Pair and extra-pair mating success relative to male quality in red-winged blackbirds

We tested whether the reproductive success of male red-winged blackbirds (Agelaius phoeniceus) varied with male secondary sexual traits or with haematozoa prevalence, and whether these patterns were consistent with females preferring genetically superior males. We also determined whether the traits that correlated with male success on their own territories were also correlated with male success at siring young on other males territories. Our analysis included data from a 6-year study involving 617 nestlings for which paternity was determined by DNA profiling. Larger males sired more young on their territories, principally because they obtained larger harems. The success of larger males at acquiring more mates did not appear to be a consequence of larger males holding larger or better-quality territories. Older and longer-lived males sired more young by extra-pair fertilizations. Larger males sired the most offspring overall (on territory + off territory). Variation in epaulet size and color, responses to male and female models, nest defence and parasitism was not correlated with male success either on or off their territories or overall. Male success in a given year was significantly correlated with success the previous year, as expected if females were selecting genetically superior males. The male that was by far the most successful individual in this study was highly consistent from year to year. Because male body size is positively correlated with survival in this population (although not within the sample of males included in this study), female preference for larger males may have reflected a general preference for males with superior Survival ability. We propose that the direct advantage realized by older males in extra-pair matings might indicate that experience is important, such that experienced males are better at creating or exploiting the opportunities for extra-pair mating. This hypothesis is consistent with a pairwise analysis of cuckoldry that showed that cuckolders were most often older than the males they cuckolded.     

The secondary adjustment of clutch size in red-winged blackbirds (Agelaius phoeniceus)

More than a half century ago, the British ornithologist David Lack suggested that parent birds may use brood reduction to track uncertain food, a process facilitated by the asynchronous hatching of their young. Lack sketched the logic of asymmetric sibling rivalry: the phenotypic handicap imposed upon last-hatched marginal offspring renders their growth and survival conditional upon uncertain ecological conditions while buffering first-hatched core offspring from the inimical effects of overcrowding during periods of stringency. Though subjected to numerous indirect tests in short-term studies, the central prediction of Lack's hypothesis - that parents use marginal offspring to track unpredictable brood-rearing conditions and thus achieve a secondary adjustment of clutch size - has never been tested directly. Here we present the results of a 7-year study of marsh-nesting red-winged blackbirds (Agelaius phoeniceus) showing that (1) brood size tracks interannual variability in growth and survival of nestlings, (2) the growth and mortality of marginal but not core offspring is contingent upon stochastic environmental conditions (mean air temperature) during brood rearing, (3) the mortality of marginal but not core offspring is strongly affected by developmental uncertainty in the form of both experimental and natural alterations of brood size, (4) the phenotypic handicap of hatching asynchrony buffers core offspring from poor growth conditions, but (5) its effects upon marginal nestlings are reversible when growth conditions are favourable and especially when brood size is reduced either experimentally or via hatching failure. The presence of marginal offspring ensures that blackbird parents are not left with a too small brood when brood-rearing conditions are favourable. Parents create two castes of progeny: marginal offspring that are strongly affected by both ecological and developmental stochasticity, and core offspring that are not.     

Seasonal variation in ejaculate traits of male red-winged blackbirds (Agelaius phoeniceus)

Many reproductive traits, including ejaculate characteristics, usually show remarkable seasonal variation, but the potential for such dynamics in sperm morphology has been overlooked. Several studies have revealed high within-male repeatability in sperm morphology, but samples have typically been collected within a few hours or days, and the consistency of sperm morphology over longer periods remains unexplored. Here, we tested whether ejaculate traits, including sperm morphology, sperm number, and sperm velocity, exhibit seasonal phenotypic plasticity in a long-lived seasonal breeder, the red-winged blackbird (Agelaius phoeniceus). We found absolute and/or relative flagellum length and sperm velocity to increase across the season, whereas sperm numbers within ejaculates declined. Sperm morphological traits were further positively associated with harem size or the total number of offspring that fledged in each male’s territory, suggesting that sperm morphology is likely to be linked to male reproductive quality. The underlying mechanisms of these patterns of seasonal variation remain unresolved, but we discuss our results in the context of dynamics of reproductive hormones, testicular structures and function, and reproductive behavior.     

Asymmetric sibling rivalry and nestling growth in red-winged blackbirds (Agelaius phoeniceus)

Many birds hatch their offspring asynchronously, and the adaptive significance of this trait, if any, is controversial. David Lack suggested long ago that by facilitating brood reduction when resources are scarce, hatching asynchrony provides relief from the effects of overcrowding. Some field workers interpret this to mean that the growth and survival of survivors should rise following partial brood loss. Here we show in a 6-year study of red-winged blackbirds (Agelaius phoeniceus) that the presence or absence of marginal offspring in experimentally manipulated broods had virtually no effect upon the growth of core offspring, whereas alterations of the size of core brood had strong and significant effects. Nestling growth was, not surprisingly, slower in broods with partial brood loss. Intriguingly, marginal offspring showed significantly greater variation in mass. Core offspring are less sensitive to, but not exempt from, the inimical effects of resource shortfall than are marginal offspring. The phenotypic handicap appears to marginal offspring a caste of high-variance progeny whose fitness prospects rest upon levels of parental input (stochastic resources) and the size of the core brood (stochastic development). Received: 21 June 1999 / Revised: 5 June 2000 / Accepted: 25 June 2000