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Male song sparrows sing repertoires of 4–13 distinct song types and have proved a valuable model for testing hypotheses concerning the function and evolution of song complexity. Captive female song sparrows solicit more copulations in response to playback of larger repertoires, yet it remains unclear whether male repertoire size influences female mate choice in natural situations. We used long-term data from free-living song sparrows inhabiting Mandarte Island, British Columbia, Canada, to investigate whether male song repertoire size predicted three components of reproductive performance during the first year: territory acquisition, mating success and laying date. Across males whose song was recorded, males with larger repertoires were not more likely to acquire a territory, to acquire a larger territory or to settle sooner. However, after we controlled for territory size and between-year variation in the population sex ratio, first-year males with larger repertoires were more likely to mate. This was because they were more likely to pair with newly settled females, not because they were more likely to acquire territories where older females were already resident. After we controlled for territory size and between-year variation in breeding date, newly settled females laid earlier when mated with males with larger repertoires. Together with the results of previous mate choice experiments, these patterns are consistent with the hypothesis that male song repertoire size is a sexually selected trait that influences female mate choice in song sparrows.

Scent marking is ubiquitous among the dwarf antelope and gazelles of Africa, but its function has been the subject of debate. This study examined preorbital gland scent marking in the oribi, Ourebia ourebi, a territorial African antelope. Several hypotheses for the function of scent marking by territorial antelope were tested with observational data. Of these, the hypotheses that scent marking is driven by intrasexual competition between neighbouring males, and that marks serve as an honest advertisement of a male's ability to defend his territory from rivals, were supported best. Thirty-three territorial male oribi on 23 territories marked most at borders shared with other territorial males, and territorial males marked more often at borders shared with multimale groups than at borders shared with a single male. This suggests that males perceived neighbouring male groups as a greater threat to territory ownership than neighbouring males that defended their territories without the aid of adult subordinates. Marking rate was unrelated to territory size or the number of females on adjacent territories, but males with many male neighbours marked at higher rates than those with fewer male neighbours. These results suggest that the presence of male neighbours has a greater effect on the scent marking behaviour of territorial antelope than has been considered previously. Copyright 1999 The Association for the Study of Animal Behaviour.

I observed free-ranging oribi, Ourebia ourebi, in Serengeti National Park, Tanzania, to determine whether group formation by males provides evidence for cooperative territory defence, a behaviour that is rare among male vertebrates. Socially dominant males that shared territories with subordinate auxiliary males were replaced by rivals less often than males that defended territories without auxiliary males. Auxiliary males marked territories with preorbital glands, dung and urine, and territories defended by male groups were marked more thoroughly than those defended by single males. Fifteen of 24 (62.5%) auxiliary males whose histories were known were born on territories defended by males that probably were their fathers. But 37.5% of auxiliary males probably were unrelated to dominant males, because male groups also formed when territory owners accepted adult male immigrants as subordinates, and when owners allowed young males to remain philopatric after evicting a male likely to have been the young male's father. All males in groups probably had some mating access to females, but dominant males may have minimized matings of auxiliary males by guarding fertile females. These results suggest that auxiliary male oribi aided dominant males in territory defence, and that dominants traded off the risks of losing matings to auxiliaries, or being overthrown by them, in exchange for a reduction in their chance of being evicted by rival neighbours or immigrants. Copyright 1999 The Association for the Study of Animal Behaviour.

Faeces may be ideal substances for scent marking because they have a minimal energetic cost to the signaller. However, marking with faeces is also constrained by the animal's ability to produce faeces. This study examined whether limits on the volume of faeces produced by oribi Ourebia ourebi, in Serengeti National Park, Tanzania, caused territorial males to regulate their output and prioritize the placement of faecal marks. Territorial males marked with faeces more often, and with a smaller volume per defecation, than did juvenile males and females. Territorial males also defecated only on established dung middens along borders shared with other territorial males or on top of a female's urine and faeces. In contrast, juvenile males and females defecated randomly with regard to their location in territories. Territorial males with larger harems marked with faeces at higher rates and less volume than males with few or no females. This difference suggests that when males overmark female excretions they reduce the amount of faeces available for marking other preferred sites, such as along territory borders shared with other males. Dominant males with adult subordinates marked with faeces less often, and with a greater volume per mark, than males that defended territories without the aid of subordinates. Dominant males also reduced the volume of marks less as the number of females on their territory increased than did males without subordinates. Territories occupied by more than one adult male also were marked with faeces at higher rates, and with marks of greater volume, than territories held by single males. These results suggest that the presence of subordinate males reduced the demand on dominant males to regulate the volume and placement of faecal marks. Overall, these results suggest that territorial male oribi regulate their faecal marking behaviour in response to a limited supply of faeces. Copyright 1999 The Association for the Study of Animal Behaviour.

Possession of a territory is essential to successful reproduction in male song sparrows, Melospiza melodia. However, the aggressive behaviour of territory owners frequently prevents some males from settling. Several factors that might affect the ability of territory owners to defend their territories against intrusion by non-territorial ‘floater’ males were investigated. Floaters lived secretively on the territories of owners, but were only occasionally detected by them. During consecutive breeding seasons, intrusions were most frequently observed on the territories of birds older or younger than 2 or 3 years. As males aged, intrusion pressure on their territories changed as predicted by patterns observed within seasons. Males experienced similar intrusion pressure in each year with respect to the means of their age classes. However, intrusion pressure on the same territory was not correlated across years, nor across different owners of a territory within years. Intrusion pressure was therefore an attribute of males rather than of territories. Males that experienced high intrusion pressure held territories for shorter periods, and they sometimes possessed handicaps that decreased their ability to evict floaters. Two-and 3-year-old males held their territories longest. Intrusion pressure was unrelated to floater density, territory size, to whether a female was present on the territory, or to the fertility of territorial females. This study shows that floaters intrude selectively against particular males, and that the ability to defend a territory against floaters first increases and then decreases with age in male song sparrows.

Male song sparrows,Melospiza melodia, contested for territories year-round in a non-migratory population. Over 5 years, most settlers were yearlings, but many were older floaters that had previously owned a territory. Most turnovers occurred when floaters evicted territory owners from all or part of their territories. This finding contrasts with the assumptions of most models of dispersal and habitat settlement, i.e. that owners are always dominant to floaters, and that floaters gain territories only after an owner's death. Territory owners often evicted neighbours in the breeding period, when they were also likely to gain access to additional females. Floaters usually gained territories just prior to breeding and as breeding began. These patterns are consistent with the idea that male territory defence primarily serves to provide access to females. Middle-aged males more often expanded their territories and regained them after being evicted than 1- or 4-year-old males. Birds that floated as yearlings were more likely to float after owning a territory as an adult than were birds that gained a territory before April of their first breeding season. Individual differences in competitive ability persist throughout life, but vary with age. Factors that affect patterns of aggressive versus passive territory acquisition between populations include: migratory tendency, habitat saturation, length of the breeding period, the potential to breed outside the natal population, the relative survival rates of territory owners versus floaters, and the mortality rate of territory owners in the absence of challenges by floaters. An understanding of the territorial system is crucial to predicting patterns of dispersal and habitat distribution.

This study focuses on the effect of intrasexual competition in each sex and its role in the occurrence of polygyny in a primarily monogamous song sparrow, Melospiza melodia, population. Polygyny resulted after males annexed neighbouring territories and after floating females settled on the territories of incubating females. Polygynous males raised more young than monogamous males. Females in polygynous groups often raised young without the aid of a male and consequently raised fewer young than monogamous or male-aided females. Two-year-old males were most likely to become polygynous, but the reverse was found for females, suggesting that age-related differences in competitive ability influenced the observed pattern of matings. Added supplemental food on 15 territories resulted in a reduction in foraging time, an increase in body mass and a greater portion of the time budget spent in surveillance for floaters by both males and females. Floaters settled on control territories more often than expected, even though reproductive success was four times higher on experimental territories, as expected if a territory owner's ability to engage in territory defence affects the likelihood of polygyny. Models that emphasize the role of female choice over that of female competition for resources that limit reproduction predict that females settle where they are most likely to maximize their reproductive success. Monogamy is probably the most frequent mating association in this population because males are usually unable to defend the territory of more than one female, and because territorial females are usually able to prevent settlement by floating females.

Song repertoire size, time to first territory acquisition, territory size, territory tenure and lifetime reproductive success were measured in males of known age in a wild, resident population of song sparrows, Melospiza melodia. One-year-old males had smaller repertoires than older males, but this apparent change in repertoire size was due to attrition of individuals with small repertoires rather than to an increase in the repertoire size of individuals with age. Males with large repertoires held territories longer and had greater annual and lifetime reproductive success than males with small repertoires. Half-hour repertoire size, which may represent an ecologically more relevant measure of repertoire size perceived by a transient listener, was also positively related to territory tenure and annual reproductive success. Thus listeners might be able to assess a singing male's competitive ability without sampling the entire repertoire.

This study examines relationships between natal dispersal and intrasexual competition, the mating system, inbreeding, inheritance and social status in five cohorts of song sparrows, Melospiza melodia, resident on Mandarte Island, B.C., Canada. The role of natal dispersal in population regulation is also considered. Several hypotheses of natal dispersal were tested with experimental and observational data. Dispersal distances were similar among the sexes, but distances depended strongly on point of origin on the island. Adding supplemental food on 15 territories dramatically reduced dispersal distance in each sex relative to controls, supporting the hypothesis that dispersal results from intrasexual competition for access to high-quality breeding territories. No evidence for genetic determination of dispersal distance, or for mechanisms that serve to prevent close inbreeding, was found. Social status was related to the time of settlement, but not to distance dispersed on Mandarte. Subordinate birds emigrated from Mandarte more often than dominants. Dispersal distance on Mandarte was unrelated to the probability of settling with a relative, but neither distance moved nor relatedness of the first mate were related to lifetime reproductive success. Disappearance of juveniles coincided with periods of territoriality in late summer through autumn and late winter through spring. The proportion of juveniles recruiting to the population declined as the number of territorial adults in the autumn increased. These results suggest a close link between intrasexual competition for breeding territories, natal dispersal and population regulation.