Territoriality among male red-winged blackbirds

Summary We examined male site fidelity and territorial movements in a population of individuallyidentifiable red-winged blackbirds (Agelaius phoeniceus) over an 8-year period. Based on general theory and knowledge of the ecology of this population, we predicted that males would be site conservative, and that they would voluntarily change territories only when they could expect to improve significantly their reproductive success. Because males are absent from their territories for only short periods, and probably have accurate comparative information only about nearby territories on which to base their decisions, we predicted that most moves would be over short distances. About 70% of males that bred for more than 1 year retained their original territories between breeding years. Most of the 30% of males that changed territories moved less than 200 m, often to adjacent territories. As predicted, males moving less than 200 m tended to have better reproductive success after moving than before, whereas long distance movers did not improve their success after moving. Territorial male redwings appear to monitor breeding activity on nearby territories and move when significant potential benefits are perceived and opportunities exist. Long-distance moves may be involuntary ones. The amount of information possessed about present and alternative sites, and the time and opportunity required to collect the information, are probably major constraints influencing site conservatism in this and other territorial species.     

Alternative mating tactics in dimorphic males of the harvestman <Emphasis Type="Italic">Longiperna concolor</Emphasis> (Arachnida: Opiliones)

Intense male–male competition for females may drive the evolution of male morphological dimorphism, which is frequently associated with alternative mating tactics. Using modern techniques for the detection of discontinuous allometries, we describe male dimorphism in the Neotropical harvestman Longiperna concolor, the males of which use their elongated, sexually dimorphic legs IV in fights for the possession of territories where females lay eggs. We also tested three predictions related to the existence of alternative mating tactics: (1) if individuals with relatively longer legs IV (majors) are more likely to monopolize access to reproductive resources, they are expected to remain close to stable groups of females more than individuals with relatively shorter legs IV (minors) do; (2) if minors achieve fertilization by moving between territories, they are expected to be less faithful to specific sites; and (3) majors should be observed in aggressive interactions more often. We individually marked all the individuals from a population of Longiperna during the reproductive season and recorded the location of each sighting for males and females as well as the identity of males involved in fights. Majors were more likely to have harems, and large majors were even more likely to do so. Majors were more philopatric and all males involved in fights belonged to this morph. These results strongly suggest that the mating tactic of the majors is based on resource defense whereas that of the minors probably relies on sneaking into the territories of the majors and furtively copulating with females.     

Site fidelity and patterns of short- and long-term movement in the brilliant-thighed poison frog <Emphasis Type="Italic">Allobates femoralis</Emphasis> (Aromobatidae)

We studied movement and site fidelity of males and females of the territorial frog Allobates femoralis (Aromobatidae) in a population in the Nature Reserve “Les Nouragues” in French Guiana, South America. Observations during 3 months in 2006 ascertained intra-seasonal site fidelity for males and females. Males actively defend large multi-purpose territories whereas females retreat to small resting sites from where they commute to neighbouring males for courtship and mating. Female short-term movement corroborates the previous assumption of a polygynous or promiscuous resource-defence mating system. Year-to-year recaptures from 2005 until 2008 revealed distinct patterns of inter-annual movement for males and regional site fidelity for females. Males abandon their territories and have to re-negotiate them when reproduction starts again at the end of the dry season. Females are not subject to intra- or inter-sexual territorial competition and as a result move significantly less between reproductive seasons than males. Male long-term movement reflects spatial structure and prevailing social interactions and is a reliable indicator for tadpole deposition sites. The combined effects of intra- and inter-seasonal movement promote the diversity of mates for both sexes.     

Site fidelity and territorial movements of males in a rapidly declining population of yellow-headed blackbirds

We tested several hypotheses to explain low between-year territory fidelity in a breeding population of yellow-headed blackbirds (Xanthocephalus xanthocephalus). During a 5-year study the population of territorial males declined by two-thirds and some of the marshes that supported territories significantly deteriorated. Individual males held territories and bred for an average of 1.9 years. Of males that bred for at least 2 years, 30% skipped owning a territory in the study area during at least 1 year of their breeding lifetimes. Our information suggests that they may have bred outside of the area in those years. Of males with territories in two or more breeding seasons, 60% changed breeding marshes at least once. Males changed territories during 42.9% of between-year opportunities to do so. We found no support for the hypotheses that male yellow-headed blackbirds: (1) are more likely to move when territory density is low; (2) are likely to abandon territories that are deteriorating; or (3) change territories to improve their reproductive success. We suggest three non-mutually exclusive explanations for the yellow-headed blackbird's weak site fidelity: (1) it is a response to habitat deterioration and to other factors that may be causing the population's decline; (2) the males, being migratory, make fresh settlement decisions each year after they arrive on the breeding grounds in the general vicinity of their previous year's breeding; (3) yellow-headed blackbirds may have evolved in, and be adapted to, highly unstable habitats, moving frequently in response to changes in local breeding site conditions. Correspondence to: L.D. Beletsky     

Satellites and sneakers: opportunistic male breeding tactics in pupfish (Cyprinodon pecosensis)

Summary Manipulations of population density, availability of oviposition substrate, and size of breeding habitat affected the proportion of pupfish (Cyprinodon pecosensis) males adopting territorial and satellite breeding tactics. Satellites occupied stations above breeding substrates defended by territorial males. Both territorial and satellite males developed a bright blue breeding coloration. A third breeding tactic, sneak-spawning, was occasionally observed. Sneak-spawning males retained the cryptic female coloration and occasionally spawned on territories. The lower reproductive success of satellites and sneak-spawning males suggested that both are conditional breeding tactics adopted by competitively inferior males. Satellites were present in all treatments that favored a territorial breeding system, but were absent when the breeding system was a dominance hierarchy. Satellites were associated with territorial males that had higher reproductive success and larger territories than males without satellites. Satellites functioned as parasites rather than as mutualists, since they disrupted spawnings and stole copulations from territorial males. The occurrence and frequency of the conditional breeding behaviours in pupfish represent facultative responses of males to changes in the intensity of competition for breeding sites and females.     

Effectiveness of territorial polygyny and alternative mating strategies in northern fur seals, <Emphasis Type="Italic">Callorhinus ursinus</Emphasis>

We conducted a 6-year longitudinal behavioral and genetic investigation of a highly polygynous pinniped, the northern fur seal (Callorhinus ursinus), to determine the contribution of terrestrial polygyny to male fertilization success and to assess the occurrence of alternative mating strategies. Genetic samples from 37 adult males, 50 adult females, and 85 pups were collected and genotyped using five polymorphic microsatellite loci. Pup paternity was assigned using Cervus 2.0 at 99% confidence level. Paternity of 83 pups (98%) was assigned to terrestrial males who held territories or stayed temporarily in the study area during the breeding season when fertilization occurred. For 56 pups of which attendance records of their mothers were available, paternity of 45 pups (80%) was assigned to the associate males in whose territory their mothers stayed during the perioestrus period. In addition to defending breeding territories, territorial males have often been observed attempting to forcibly abduct adult females from adjacent territories (female stealing): We observed a total of 95 such cases, in which the stealers had significantly fewer females than the territorial males from whose territories they stole females. Our results indicate that terrestrial resource-defense polygyny is the major mating system in this species and that nonassociated paternity occurs mostly as a result of alternative mating strategies of less successful males. Male northern fur seals thus appear to adopt conditional alternative strategies that depend on their current social status to maximize their life-time reproductive success.     

Habituation in the green frog, Rana clamitans

Habituation and its role in the dear enemy effect was investigated in a population of green frogs, Rana clamitans. Green frogs have a prolonged breeding season, and males defend territories centered around suitable oviposition sites. We tested the prediction that male green frogs will habituate to broadcasts of synthetic conspecific stimuli. Our results indicate that male green frogs can discriminate familiar from unfamiliar stimuli. We suggest that habituation helps to mediate the territorial interactions between male green frogs. Strangers present a greater threat than familiar neighbors. By habituating to the advertisement vocalizations of their near neighbors, males avoid costly interactions with individuals that are not a major threat to their territories. Received: 20 July 1998 / Accepted after revision: 6 September 1998     

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.     

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     

Sexual selection on mature male parr of masu salmon (Oncorhynchus masou): does sneaking behavior favor small body size and less-developed sexual characters?

In many salmonid species, males exhibit morphological dimorphism associated with alternative mating behaviors. ”Precocious males” have a small body size with little or no development of sexual characters and adopt sneaking to gain access to females, while ”migratory males” of large body size and well-developed secondary sexual characters fight. We quantified selection on precocious male parr of masu salmon (Oncorhynchus masou) under simulated natural conditions to examine the contribution of morphology to sneaking success. In contrast to the prediction that sneaking behavior favors small body size, we detected selection favoring relatively large body size for sneaking success. This selection pressure was caused by the dominance hierarchy within parr and may have been facilitated by indifference of dominant migratory males to parr. Unlike the secondary sexual characters exhibited by migratory male salmon, such as the hooked snout and humped back, no morphological characters other than body size contributed to the reproductive success of masu salmon parr. This non-contribution may have been responsible for the lack of development of sexual characters in precocious males. Received: 15 November 1999 / Accepted: 20 May 2000     

Forced dispersal of juvenile guanacos (<Emphasis Type="Italic">Lama guanicoe</Emphasis>): causes,variation, and fates of individuals dispersing at different times

We examined adult-juvenile conflict in the guanaco (Lama guanicoe). During spring, territorial males become increasingly aggressive toward all juveniles born the previous year and begin expelling them from family groups. In an apparent effort to reduce aggression, juveniles display submissive crouches when being observed, approached, or attacked by the territorial male. Therefore, we assessed the influence of juvenile submissive behavior on the timing of dispersal and also examined if dispersal time was related to survival and reproductive performance as adults. We also evaluated hypotheses regarding the evolution of juvenile mammalian dispersal in the context of if and how each may favor the forced dispersal of juvenile guanacos by territorial males. Juveniles generally dispersed in late spring and early summer, and a nearly equal proportion of females (n=46; 48%) and males (n=49; 52%) dispersed. More-submissive animals generally dispersed later than less-submissive animals. Juvenile sex and dispersal time were not related to survival. In contrast, juvenile sex and dispersal time were related to reproductive performance. The probability of reproducing was highest when juveniles dispersed early and decreased with increasing time in family groups prior to dispersal. The largest proportion of juveniles was forced to disperse during a 2-week interval following the peak of the breeding season. Competition for food resources is likely very intense at this juncture and territorial males may force older juveniles to disperse in order to divert food resources to younger neonates. Additionally, juveniles may be forced to disperse after territorial males mate their mothers to prevent lost mating opportunities, because females leave territories when their offspring disperse and possibly prior to mating with males. We conclude that the forced dispersal of juvenile guanacos by territorial males is ultimately driven by competition for food resources on territories. The timing of dispersal, however, may be tempered by the chronology of matings between territorial males and particular adult females, and/or genetic relatedness between territorial males and juveniles.     

Interspecific aggression in Leucorrhinia dragonflies: a frequency-dependent discrimination threshold hypothesis

Summary Male Leucorrhinia dragonflies defend territories from conspecific and heterospecific intruders. Defense against heterospecifics is surprising, as mating and oviposition are the only activities that occur on the territories, and heterospecific males are not expected to pose a reproductive threat. L. frigida and L. intacta males respond aggressively with equal frequency and intensity against intrusions by conspecifics and heterospecifics. In contrast, L. proxima males respond more aggressively against conspecifics. The apparent lack of species discrimination shown by L. frigida and L. intacta males may result because territorial males that assess intruders (as do L. proxima) suffer a tactical disadvantage from hesitating when an intruder flies in. This assessment process may lead to reduced fighting success by the territorial male. Thus there is a tradeoff between assessment cost incurred when the intruder is a conspecific and benefit gained from avoiding conflict when the intruder is a heterospecific. Given this assessment cost, males of species that only rarely encounter conspecific intruders are more likely to evolve species discrimination than males of species that commonly encounter conspecific intruders.     

Sex-specific differences in site fidelity and the cost of dispersal in yellow-headed blackbirds

Male migratory birds tend to be more faithful than females to previous breeding sites, suggesting sex differences in costs or benefits of dispersal. In Illinois, greater site fidelity by male yellow-headed blackbirds (Xanthocephalus xanthocephalus) was associated with reduced reproductive success the following year for dispersers relative to non-dispersers. Dispersing females suffered no reduction in reproductive success the following year. Males that attracted few social mates, and thus had low reproductive success, were more likely to disperse, whereas females dispersed in response to low-patch reproductive success, regardless of their individual performance. Males that dispersed appeared to be successful acquiring territories because none was observed as a floater. The rate of dispersal by males in this low-density population was greater than in more dense populations where dispersing males may be less successful at acquiring territories. Despite success at obtaining territories, males that dispersed acquired territories on the periphery of wetlands where fewer females nested, resulting in lower reproductive success. In the second year after dispersing, however, males moved onto more central territories where they acquired larger harems. Thus, dispersal by males may be a long-term strategy requiring at least 2 years for benefits to be realized. Long-term success was enhanced because dispersing males moved to wetlands on which reproductive success was higher than on the wetlands they left. In addition to demonstrating that both individual and patch reproductive success affect dispersal decisions, these data indicate that when evaluating costs and benefits of dispersal, researchers should use a time frame beyond 1 year.     

An experimental study of song type function in the bobolink (Dolichonyx oryzivorus)

Summary We tested the hypothesis that the alpha and beta songs of male bobolinks (Dolichonyx oryzivorus) have separate intrasexual and intersexual functions by presenting caged males and caged females on the territories of male bobolinks. Although all males in our study population share both alpha and beta songs, we found no evidence for a specialized intersexual function of alpha song or a specialized intrasexual function of beta song. Territorial males increased their singing rate of both alpha and beta songs during both the male presentations and the female presentations. The proportions of the two song types did not change between control and experimental periods for either male or female presentations. We found no evidence in support of specialized intrasexual and intersexual functions of note types. Only 10% of the vocalizations initiated by territorial males were complete alpha or beta songs, while 87% were song fragments, and the remaining 3 % were compound songs. We did find that territorial males responded to simulated male intruders by increasing the proportion of fragmented songs and decreasing the proportion of compound songs. Moreover, males included fewer notes per song initiated and sang a higher proportion of songs that included only introductory notes during male presentations. We conclude that the vocal response of territorial male bobolinks to conspecific male intruders is to sing short, simple sequences of either song type. We found no consistent changes in vocal behavior of territorial males in response to female presentations other than increases in song rates.     

Mating system shift in a pronghorn population

Summary At the National Bison Range (western Montana, USA), pronghorn (Antilocapra american) maintained a territorial mating system from as early as 1965 through 1978. Preliminary observations in 1981 suggested that the mating system had changed. Data from the ruts, 1982–1984 revealed a progressive decay in territoriality. In this paper, data from the territorial years 1969–1978 are contrasted with data from the decay years 1982–1984, with results as follow: 1. In 1982–84, fewer territories were defended than in 1969–78. This was attributable to a smaller proportion of males defending territories in 1982–84, not to a smaller number of males. 2. In 1982–84, most territory owners either abandoned their territories early in ruts, or lost control of females on them following frequent, persistent intrusions by non-territorial males. Abandonment and loss of control did not occur in 1969–78. 3. In 1982–84, territorial males that maintained control of females on their territories did so by shrinking their zones of defense to small areas around female groups. 4. In 1982–84, following the disruption or severe disturbance of all territories, many females left territories, and mated elsewhere, with non-territorial males. In 1969–78, most females remained on territories throughout rut, and mated with territory owners. —The mating system change followed catastrophic winter mortality, 1978–79, that removed 75% of the males, including all males older than 5 years, and all male fawns, from the population. In 1982–84, the number of males present was not different from the number of males in 1969–1978, but the frequency distribution of male ages was strongly shifted toward younger ages. The small number of older males, 1982–84, likely resulted in smaller proportions of males initially defending territories, and in less effective territory defense. When females then clustered on the few territories where defense was at first successful, they attracted large numbers of non-territorial males. The resulting high rates of raids on these territories, coupled with reduced defense radii by territorial males, allowed females only 12% reclining time (summer percentage was 39%). This increased energy cost, plus an apparently greater risk of injury on weakly defended territories, appeared to prompt many females to seek calmer matings elsewhere. Also, if female pronghorn practiced mate selection based upon horn or body size, they may have reduced their efforts to remain on territories in 1982–84. Males from 1969–78 were larger than males from 1982–84, and showed greater variance in horn size. At least two conditions appear to influence the tendency of males to be territorial. First, males must be at least three years old before they attempt to defend a territory. Second, the declining proportion of males defending territories, 1982–84, that coincided with an increasing number of males three years and older, suggests that males also decide whether or not to attempt territory defense based upon the frequency of territorial defense in the population.     

Neighbours, strangers and male-male aggression as a determinant of lek size

Interactions between males on leks may play an influential role in lek formation and the regulation of lek size. In this paper I present the results of a playback experiment that simulated de novo settlement at sites adjacent to currently existing display territories of the ochre-bellied flycatcher, Mionectes oleagineus. In the study population, males displayed both solitarily and at small leks. A large proportion of males held no display territory at all. A stranger's song was played to both solitary and lekking males from 10 m outside their territorial boundaries. In separate playbacks, lekking males were also played neighbour's song. Both lekking and solitary territorial males reacted to the playback by decreasing their song rate, approaching the playback speaker and, on occasion, attacking the model. Solitarily displaying males responded more aggressively to playback of stranger's song than did lek males. Lek males were able to distinguish between their neighbour's and a stranger's song and did so irrespective of whether it was played from the neighbour's territory or from outside the lek. In addition to distinguishing between neighbours and strangers, lek males modified their responses to these different playbacks depending on where the playback originated. These results suggest that male-male interactions can be influential in structuring leks. In M. oleagineus, interactions between males are aggressive and act to limit rather than augment lek size. Received: 6 March 1996 / Accepted after revision: 9 December 1996     

Influence of territoriality on adult density in two rockfishes of the genus Sebastes

Nonterritorial Sebastes carnatus and S. chrysomelas existed, along with territorial individuals, at 3 tagging sites off southern California, USA, which were monitored for nearly 1 yr. To test the hypothesis that territoriality affected adult density in these species, territorial fish were removed and the subsequent utilization of vacated territories by other fish was monitored. Intrusion of other fish into vacated territories increased significantly in 90% of the removals. Other fish colonized both the feeding and sheltering parts of the vacated territories, indicating that the previous owners had successfully defended both parts of their territories. Many of the colonizers had already possessed territories; they expanded their territories or moved into presumably better havitat. Several previously-nonterritorial fish also moved into vacated areas, and at least some of them appeared to establish territories. These fish, then, had previously been capable of establishing territories, but were prevented from doing so by resident territory holders. Thus territoriality, rather than such other factors as predation or low recruitment, limited the number of territorial fish at each site. However, territorial fish did not inhibit the settlement of larval recruits, and the relative mortality rates of older territorial vs nonterritorial fish were not determined. Thus the question of whether territoriality was a major factor controlling total density remains unresolved.     

Territorial resource valuation deviates from habitat quality in an ecologically trapped,long-distance migratory bird

Preference measures are widely used in habitat selection studies to test an animal’s choice relative to particular habitat features, but most measures are subject to criticism as they fail to indicate the underlying behavioral motivation. Order of settlement on breeding sites has been proposed as an effective measure in migratory organisms, as it conceptually approaches a choice experiment. We tested the assumption that early red-backed shrikes (Lanius collurio) are more willing to defend their territorial resource than individuals arriving later. We earlier showed that shrikes arriving first settled in forest plantations that resulted in lower reproductive success compared to territories on farmland, suggesting an ecological trap. Therefore, individuals are expected to place higher value on the lower quality sites in forests. Within the context of resource valuation theory in animal contests, we used a simulated territorial intrusion experiment to measure territorial defense and to evaluate the perceived value of the territory during the settlement phase in both habitat types. Males arriving early were much more motivated to defend their territory than late birds. After correction for the disparity in the timing of arrival between habitat types, shrikes also more vigorously defended their territories in the forest habitat associated with the lowest reproductive returns. Although some resource valuation mechanisms remain unclear, our results show that early and late-arriving individuals strongly differ in behavioral motivation to hold their territorial resources. This study also demonstrates for the first time that organisms may exhibit a higher degree of territorial aggressiveness in a lower quality habitat.     

Alternative mating tactics in males of Polistes dominulus (Hymenoptera: Vespidae)

Summary In summer, males of Polistes dominulus form large aggregations at sunny landmarks. We identified two size-correlated behavioural categories: residents (R) and transient (T). R males, which constitute 20%–25% of the total population, are larger than T males, territorial, aggressive, and more site-faithful, while T males range more widely, are non-aggressive, and show little site tenacity. Field and laboratory data suggest that R males have an advantage in mating, particularly if they engage in frequent flights while on their territories. These alternative mating tactics within the same population are combined with behavioural flexibility in some individuals, which switch from one option to the other.     

Differential allocation of reproductive effort to territorial establishment and maintenance by male yellow warblers (Dendroica petechia)

Summary We tested the hypothesis that the basis of the variation in reproductive strategy among male yellow warblers (Dendroica petechia) is a tradeoff in the allocation of reproductive effort to intrasexual competition (territorial effort) and to parental care (parental effort). Since a negative correlation between level of parental effort and amount of brown streaking on the breast (plumage score) has already been demonstrated in this species, this study looked for evidence of a positive correlation of territorial effort with plumage score and a negative correlation with parental effort. Analysis of the spatial and temporal use of territories, prior to egg-laying, by males with different plumage scores supported the prediction that brighter (higher plumage score) males allocate more effort to territorial establishment and maintenance (Fig. 1). Male plumage score was positively and highly significantly correlated with six different measures of the relative amount of time spent, rate of energy expended, and risk of injury assumed by each male during territorial activities. The level of territorial effort was also found to be negatively correlated with the level of parental effort expended by the same males later in the season, confirming that there is a tradeoff in allocation of reproductive effort to these two major components. Further analysis revealed that territory quality was positively correlated with male plumage score (Figs. 3 and 4), while average nestling growth rate and other indices of reproductive success were not (Fig. 6). These results suggest that increased territorial effort by brighter males enables them to occupy higher quality territories where high levels of parental effort are not necessary to maintain high levels of reproductive success. Since reproductive success was not correlated with plumage score, this study further supports the differential allocation hypothesis that different males are using alternative strategies for the allocation of reproductive effort.