Extra-pair Fertilizations Research Articles

Copulatory Behavior of Semi-Colonial Montagu's Harriers

The copulatory behavior of the monogamous Montagu's Harrier (Circus pygargus) was studied between 1992 and 1995 near Madrid, Spain. Copulations started around 20 days before laying, and typically stopped at the end of the laying period. Copulation frequency peaked just before the start of the laying period, averaging 2.2 copulations day -1 . Females copulated on average 39 times per brood. Copulation rates apparently were not related to food provisioning by males except in the early pre-laying period. At least 5 and up to 11 (4-8%) of the copulations observed were extra-pair copulations (EPCs), and all occurred within the fertile period. Montagu's Harriers increased their copulation rate in situations of increased EPC risk: solitary pairs copulated for a shorter period of time and at an overall lower frequency, whereas pairs nesting in clumps showed higher copulation rates with increasing number of neighboring pairs. This increase was most marked during the laving period, when there is a higher risk of EPCs producing extra-pair fertilizations.

Species Differences in Male Parental Care in Birds: A Reexamination of Correlates with Paternity

Avian species differ markedly in the extent to which males contribute to pre- hatching and posthatching parental care. In a recent comparative study, Moller and Birkhead (1993) concluded that diversity in male parental care was associated with differences among species in extrapair paternity. Specifically, their results showed a significant inverse rela- tionship between extrapair paternity and male contributions to feeding of nestlings. We used a revised and updated data set in an attempt to replicate their study. In contrast to their results, we found no evidence that the evolution of male posthatching care was strongly cor- related with paternity. Instead, our results showed that male participation in incubation tends to be negatively associated with extrapair fertilization rates, thereby providing ten- tative support for Ketterson and Nolan's (1994) hypothesis that this particular form of pa- rental care may be especially restrictive to male extrapair mating activities. Received 15 Sep-

Within-Clutch Patterns of Egg Viability and Paternity in the House Sparrow

One of the potential benefits of extra-pair copulation is an enhanced likelihood of successful fertilization if the pair male is unable to produce sufficient viable sperm to ensure the fertility of an entire clutch. Low male fertility can be a transient phenomenon associated with higher rates of sperm usage than production, leading to a progressive depletion of sperm reserves. Evidence of a correlation between hatching success and the occurrence of extra-pair fertilizations has been reported in House Sparrows. In this study we have used single-locus minisatellite and microsatellite profiling to investigate the distribution of extra-pair fertilizations within House Sparrow broods to determine whether there are changes in the level of hatching failure or extra-pair paternity during the course of producing a clutch. Embryonic material was collected at mid-incubation to minimize loss of data through pre-sampling mortality and to ensure that laying order was known. A significant clustering of both infertile eggs and extra-pair young was noted in the earliest eggs in a clutch. Possible causes of this phenomenon are discussed.

Breeding synchrony and EPF rates: the key to a can of worms?

Given that roughly 95% of bird species are classified as ‘monogamous’1, some might claim that the class Aves is short on mating system diversity. Whatever the group might lack in this respect, however, is offset by rather impressive species differences in the prevalence of covert matings by females. By a recent count2, extra-pair fertilization (EPF) rates range from 0 to 76% across avian species, with the mean for all species at about 13%.

Multi-locus DNA fingerprinting supports genetic monogamy in Florida scrub-jays

Extensive behavioural and pedigree data on a colour-marked population of Florida scrub-jays (Aphelocoma coerulescens) suggested that this cooperatively breeding species is monogamous, with extremely rare exceptions in which males have two mates. We used multi-locus DNA fingerprinting to test these observations by determining genetic parentage. Despite restricted dispersal and high relatedness between behavioural parents and non-breeding members of the group, DNA fingerprints provided sufficient variability to determine parentage unambiguously in almost all cases. We found no evidence of extra-pair fertilisation of females or egg dumping, and confirmed a suspected case of polygyny in which a mother and daughter laid and incubated in the same nest. Our results confirm that detailed behavioural data allow accurate assignment of genetic parentage in this species. In Florida scrub-jays, large territory size may limit opportunities for cuckoldry, and persistent intense competition for limited breeding space may lead to low variance in the quality of established male breeders. These factors would reduce both the opportunity for, and benefits of engaging in extra-pair fertilisations. Delayed dispersal and cooperative breeding in this species have not evolved as avenues for direct reproduction by unpaired individuals.

Response of Female Hooded Mergansers to Eggs of an Interspecific Brood Parasite

mous Tree Swallows. Animal Behaviour 45:213229. LIFJELD, J. T., AND R. J. ROBERTSON. 1992. Female control of extra-pair fertilizations in Tree Swallows. Behavioral Ecology and Sociobiology 31:89-96. MORTON, E. S., L. FORMAN, AND M. BRAUN. 1990. Extra-pair fertilizations and the evolution of colonial breeding in Purple Martins. Auk 107:275283. ROBERTSON, R. J., B. J. STUTCHBURY, AND R. R. CoHEN. 1992. Tree Swallow (Tachycineta bicolor). In The birds of North America, no. 11 (A. Poole, P. Stettenheim, and F. Gill, Eds.). Academy of Natural Sciences, Philadelphia, and American Ornithologists' Union, Washington, D.C. SHIN, H-S., T. A. BARGIELLO, B. T. CLARK, F. R. JACKSON, AND M. W YOUNG. 1985. An unusual coding sequence from a Drosophila clock gene is conserved in vertebrates. Nature 317:445-448. SHUTLER, D., AND P. J. WEATHERHEAD. 1991. Owner and Red-winged Blackbirds: Determinants of status. Behavioral Ecology and Sociobiology 28:235-241. SMITH, J. N. M., AND P. ARCESE. 1989. How fit are floaters? Consequences of alternate territorial behaviors in a non-migratory sparrow. American Naturalist 133:830-845. SMITH, S. M. 1978. The underworld in a territorial sparrow: Adaptive strategy for floaters. American Naturalist 112:571-582. STUTCHBURY, B. J. 1991. Floater behaviour and territory acquisition in male Purple Martins. Animal Behaviour 42:435-443. STUTCHBURY, B. J., AND R. J. ROBERTSON. 1985. Floating populations of female Tree Swallows. Auk 102:651-654. STUTCHBURY, B. J., AND R. J. ROBERTSON. 1987. Behavioral tactics of subadult female floaters in the Tree Swallow. Behavioral Ecology and Sociobiology 20:413-419. VENIER, L. A., P. O. DUNN, J. T. LIFJELD, AND R. J. ROBERTSON. 1993. Behavioural patterns of extra-pair copulation in Tree Swallows. Animal Behaviour 45:412-415. VENIER, L. A., AND R. J. ROBERTSON. 1991. Copulation behaviour of the Tree Swallow, Tachycineta bicolor: Paternity assurance in the presence of sperm competition. Animal Behaviour 42:939-948. WEATHERHEAD, P. J., AND P. T. BOAG. 1995. Pair and extra-pair mating success relative to male quality in Red-winged Blackbirds. Behavioral Ecology and Sociobiology 37:81-91. WETTON, J. H., R. E. CARTER, D. T. PARKIN, AND D. WALTERS. 1987. Demographic study of a wild House Sparrow population by DNA fingerprinting. Nature 327:147-149. ZACK, S., AND B. J. STUTCHBURY. 1992. Delayed breeding in avian social systems: The role of territory quality and floater tactics. Behaviour 123:194-219.

Adult Survival and Imperfect Assessment of Parentage: Effects on Male Parenting Decisions

Applications of molecular methods to assess parentage have revealed that the distribution of reproductive success among individuals often differs, sometimes dramatically, from expectation based on observation of behavioral association. Much theory exists on whether and when males should reduce parental care in response to level of paternity. Life‐history theory predicts that trade‐offs in reproductive effort should be influenced by adult survival. We used a dynamic programming approach to address how level of paternity, ability to assess paternity, and adult survival rate interact to affect male tolerance of reduced parentage in a given brood. Adult survival has the greatest influence on male decisions such that, for any given cost of reproduction and value of male care, tolerance of extrapair fertilizations (EPFs) decreases as adult survival increases. An unexpected result of these models is that an optimal response also depends on a male's ability to predict probability of parentage (i.e., uncertainty). These models better characterize the nature of paternity uncertainty and its effect on EPF tolerance than have previous models and add to our understanding of the complex relationship between uncertainty, mating strategies, and adult survival.

Brood parasitism in the sand martin,Riparia riparia: evidence for two parasitic strategies in a colonial passerine

Sand martin parentage was studied at a large breeding colony in central Scotland. Multilocus DNA fingerprinting was used during the 3-year study to exclude some social parents as true parents, and thereby determine the frequency of monogamy, quasiparasitism (QP), intraspecific brood parasitism (ISBP) and extrapair fertilization (EPF) amongst 45 broods and 167 nestlings. Monogamous parentage characterized the majority of broods (60%), so most chicks were the offspring of their social parents (81%). QP (involving a male’s extrapair mate laying in his nest) was found in 9% of broods and 2.4% of chicks and ISBP (or ‘egg dumping’) in 4% of broods and 1.8% of chicks. A substantial proportion of offspring arose from EPFs, affecting 36% of broods and 14% of chicks. On present evidence, the relatively high frequency of QP found in sand martins is unusual. We propose that its observed frequency is unlikely to be due to chance events and may represent a female-driven strategy.

Experimental mate replacement does not increase extra–pair paternity in tree swallows

Before the onset of female fertility, we removed 12 early-settling male tree swallows (Tachycineta bicolor) from their nest-boxes and mates, and allowed replacement males which had been floaters to settle with the original female residents. We predicted that females which had their original mate choice altered (experimentals) would be more likely to obtain extra-pair fertilizations than females which remained paired with their original, early-settling mates (controls). The proportion of females obtaining extra-pair fertilizations, however, did not differ between controls and experimentals, indicating that mating tactics of female tree swallows were unaffected by mate replacement. However, differences between early-settling and replacement males did exist. Replacement males had shorter wing chords than early-settling males, suggesting that they were younger. Moreover, a significantly greater proportion of replacement males than early-settling males were unringed and thus new to the study site. Our results suggest that if females are choosing males for good genes, then early-settling males are not superior in genetic quality to the smaller replacement males which had originally been floaters.

Social monogamy and extra-pair fertilization in an Australian lizard, Tiliqua rugosa

This study investigates social monogamy in the Australian sleepy lizard, Tiliqua rugosa. At a 70-ha site near Mount Mary, South Australia, we radio tracked 55 adult female and 39 adult male lizards during their spring activity periods. Each lizard was observed in 1–5 years. Females were observed with a single male partner on an average of 10.8 days per year, although in 17.3% of cases, females were observed on 2 or fewer days with a male. The most intense pairing period each year was 15 September–15 November when females were with male partners on an average of 36% of observation days. Partnerships lasted an average of 43.3 days each year. After mating in early November, the pairs separated. Observations of females pairing with other males were rare. Most males (82%) were also consistently monogamous, although 7 were observed pairing with 2 females within one season. To investigate paternity, we allowed 21 gravid females to give birth to 42 offspring in the laboratory. We determined genotypes at five polymorphic microsatellite DNA loci for the females, their male partners and their offspring. Four litters (19%) and 6 of the offspring from those litters (14.3%) showed evidence of extra-pair fertilization (EPF). Although the sample sizes are small, females of polygynous males were more likely to experience EPF.

Sexual conflict over fertilizations: female bluethroats escape male paternity guards

Extra-pair copulations create a potential for sexual conflict in pair-bonding birds. Here we report an experimental study of the bluethroat, Luscinia s. svecica, in which the throat ornament of males was blackened with Nyanzol D in order to reduce their sexual attractiveness and thus increase the sexual conflict over fertilizations. In an earlier study, we showed that males blackened before pairing had a lower success in attracting social mates than controls, whereas males blackened after pairing guarded their mates more intensely and sang less than controls. Here we add behavioural data from one more year on males blackened after pairing and corroborate our previous finding that the manipulation caused males to guard their mates more intensely and advertise less for additional mates. Blackened males did not suffer more intrusions from neighbouring males than did controls. Paternity analyses of the combined data set, using multilocus DNA fingerprinting and microsatellite typing, revealed that blackened males lost significantly more paternity than controls. There was also a tendency for blackened males to show a lower success in achieving extra-pair fertilizations. These results indicate that females have the upper hand in the sexual conflict over fertilizations, as females paired with unattractive males can achieve more extra-pair paternity despite the greater constraint posed by the intensified mate guarding. Still, within the blackened group, there were some indications that males guarding more intensely and singing less had higher paternity than males guarding less and singing more, suggesting a marginal positive effect of guarding for unattractive males. Male mate guarding must nevertheless be considered a best-of-a-bad-job strategy in this species.

Non-aggressive mate guarding by the blue-footed booby: a balance of female and male control

Thirteen pairs of blue-footed boobies (Sula nebouxii) were observed on their colony. Pairs courted frequently and, on average, copulated 24 times during the 30 days before laying, with 38% of those copulations occurring in the last 5 days (presumed fertile period). Males and females increased attendance at the nest site as laying approached. Seven females performed an average seven extra-pair copulations, with 1–2 paired male neighbors, but these were less concentrated in the presumed fertile period than within-pair copulations, and the last two copulations of all 7 females were with their social mates. Rates of female extra-pair copulations were six times lower when their social mate was present, and during the presumed fertile period, no female performed an extra-pair copulation in the presence of her mate. Males did not respond to infidelity of social mates with aggression, prompt copulation, retaliatory copulation, or increase in copulation. Seven of 13 males performed an average of five extra-pair copulations, with 1–3 paired female neighbors, before their own mates began egg-laying. The males' extra-pair copulations represented only 4% of their total copulations during their own mates' presumed fertile periods. Females, the larger sex, apparently control sexual access and copulate with extra males to achieve extra-pair fertilization. Males pursue a mixed strategy: they copulate with extra females, mostly outside their own mate's presumed fertile period, and they copulate increasingly with their social mate as laying approaches, probably assuring some paternity by mate guarding, involving attendance and courtship. Behavior of males and females is also consistent with other hypotheses for extensive joint nest site attendance: pairbonding, copulation access, and territory acquisition.

Breeding synchrony best explains variation in extra-pair mating system among avian species

Stutchbury (1998) used new data and a comparative pairwise test to confirm that there is a strong correlation between breeding synchrony and extra-pair fertilization (EPF) frequency in passerines, as first reported by Stutchbury and Morton (1995). Although a pairwise test is a legitimate and powerful comparative method, Weatherhead and Yezerinac (1998) call for "better analyses to assess the correlation between synchrony and EPFs." There is a strong correlation between breeding synchrony and EPF frequency regardless of the comparative method used (Stutchbury and Morton 1995; Stutchbury 1998). Here I present a linear contrasts analysis on the 34 species given in Stutchbury (1998) using the phylogeny shown in Westneat and Sherman (1997) with other species added using the phylogeny in Sibley and Ahlquist (1990). I used version 2.0 of the Comparative Analysis by Independent Contrasts (CAIC) program (Purvis and Rambaut 1995), and assumed equal branch lengths. As expected, there was a strong positive correlation between contrasts in arcsinetransformed EPF frequency (proportion of broods with EPFs) and the synchrony index (r = 0.52, n = 33 contrasts, P < 0.002). The results for breeding density were similar to Westneat and Sherman (1997), with no correlation between contrasts of arcsine-transformed EPF frequency and nearest-neighbor distance (r -0.10, n 28 contrasts, P = 0.60). The relationship between breeding synchrony and EPFs across species demands explanation; either synchrony causally affects the evolution of extra-pair mating systems as argued by Stutchbury and Morton (1995), or both synchrony and EPF frequency are correlated with some unidentified causal factor. Weatherhead and Yezerinac (1998) favor the explanation that "both breeding synchrony and extra-pair mating [are] corre-

Female mate choice of extra-pair males: breeding synchrony is important

Recently, Stutchbury and Morton (1995) proposed that breeding synchrony among females favors the evolution of extra-pair mating strategies in birds, and reported that extra-pair fertilization (EPF) frequency is positively correlated with breeding synchrony in 21 genera of passerines. They then showed that tropical passerines, which typically have low breeding synchrony, have small relative testis size compared with temperate-zone species suggesting a low level of sperm competition and EPFs in tropical species. Weatherhead (1997) stated that there is reason to be skeptical of the breeding synchrony hypothesis because "the logic underlying the hypothesis may be faulty." Here, I respond to his assertion that the breeding synchrony hypothesis can be dismissed because (1) the correlation between species is spurious and (2) the hypothesis is based on faulty logic. First, does breeding synchrony really affect EPF frequency in a comparison among passerines? The empirical evidence indicates the answer is yes (Fig. 1). Data on synchrony are shown for 13 newly studied species in addition to those available to Stutchbury and Morton (1995). Among 34 species from 30 genera and 12 families, the correlation remains highly significant (r0.51, P = 0.002). A pairwise test is a simple way to control for phylogenetic effects, by comparing closely related species that differ in breeding synchrony. For nine pairs of related species, in eachl case EPF frequency was higher for the species with the higher breeding synchrony (Table 1; sign test P = 0.004). Weatherhead's study is among several that have found no correlation between synchrony and EPFs within a species. Based on this, he suggests that the correlation between species may be spurious, resulting from an additional variable, like breeding density. But in a comparative study using the linear contrasts method,

Breeding synchrony and extra-pair mating in birds

Recognition that extra-pair fertilizations (EPFs) are commonplace in birds demands hypotheses that can explain the variation in EPFs within and among species. Stutchbury and Morton (1995) offered such a hypothesis by proposing that synchronous breeding promotes extra-pair mating because it allows females to assess male quality more accurately and enhances opportunities for extra-pair mating for males. In response to criticism of this hypothesis, Stutchbury (1998) expressed concern that the hypothesis was being arbitrarily "brushed aside", even though. rejection of the synchrony hypothesis was based on detailed empirical research (Weatherhead 1.997; Yezerinac and Weatherhead 1997). Stutchbury (1.998) has offered new arguments and evidence in support of the synchrony hypothesis. Here we point out why neither the arguments nor the evidence are compelling, and we propose new proximate causes of EPFs that might account for correlations between synchrony and EPFs between species, without invoking the mechanism proposed by Stutchbury (1998). Weatherhead's (1997) suggestion that the logic of the original synchrony hypothesis might be faulty prompted Stutchbury (1998) to clarify the proposed mechanism by which breeding synchrony promotes extra-pair mating. Females are assumed to compare displaying males and to choose extra-pair mates whose display behavior signals greater relative quality. Because the intensity of male displays varies across the breeding cycle, Stutchbury argues that greater breeding synchrony allows females to assess and select extra-pair mates more reliably because synchrony tends to standardize the context of male displays.

Genetic Evidence for Low Frequency of Extra-Pair Fertilizations in Northern Goshawks

We collected blood for allozyme analysis from Northern Goshawks (Accipiter gentilis atricapillus) on the Kaibab Plateau in northern Arizona during 1991-1993 to quantify frequency of extra-pair fertilizations (EPFs). Northern Goshawks are socially monogamous, territorial, noncolonial, synchronous breeders, and their nests are relatively far apart, characteristics that suggest that EPFs should be low. We sampled 103 adults and 122 nestlings from 64 nests. Both putative parents were sampled at 39 nests, but we were able to capture and sample only one putative parent at 25 other nests (84%, female only; 16%, male only). Of the 39 complete families sampled, only 1 of 77 (1.3%) nestlings (or in 1 of 39 clutches, 2.6%) had a genotype not consistent with both parents; the male was excluded as the genetic father of this nestling. When this estimate was adjusted based upon the probability of excluding a putative father when he is not the actual father, the estimate of nestlings sired by EPFs was 9.4% in 1991 and zero in 1992 and 1993. There was no evidence of egg-dumping. Our data suggest that EPFs are infrequent in Northern Goshawks breeding in northern Arizona, a result consistent with characteristics of their mating system.

High Paternity without Paternity-Assurance Behavior in the Purple Sandpiper, a Species with High Paternal Investment

Among birds, males investing highly in paternal care should be under par- ticularly strong selection pressure to prevent extrapair paternity in their broods. Current literature implies that to obtain a high certainty of paternity, males should invest in pater- nity-assurance behaviors, and this makes them more likely to increase their investment in reproduction. The Purple Sandpiper (Calidris maritima) is a relatively long-lived arctic shor- ebird with a socially monogamous pair bond and strong mate and territory fidelity by both sexes. Paternal investment is high because males alone perform all of the brood care from hatching to fledging. We investigated the mating strategies of male and female Purple Sand- pipers through genetic analysis and examined the possibility that this species exhibits pa- ternity-assurance behaviors. We found no evidence that males exhibited behaviors such as frequent copulation or mate guarding during their mate's fertile period. Furthermore, high rates of copulation were not necessary for pairs to achieve complete fertility of the clutch. Multilocus DNA fingerprinting revealed little evidence for extrapair fertilizations: only 1 of 82 offspring was sired by an extrapair male. These results suggest that male and female Purple Sandpipers rarely seek extrapair copulations (EPCs). In contrast to several recent studies with similar findings, we believe that the apparent rarity of EPCs results because females have few genetic or other benefits to gain from seeking them. Thus, paternity-as- surance behaviors by male Purple Sandpipers were not necessary for achieving full paternity of the clutch, and such behaviors cannot be said to have led to the observed high level of paternal investment in reproduction. Uniparental brood care, however, is associated with a high certainty of paternity in this species. Received 3 June 1997, accepted 13 November 1997.

Do female black-capped chickadees prefer high-ranking males as extra-pair partners?

Previous studies have shown that some female black-capped chickadees (Poecile atricapillus) solicit copulations from males that rank higher in winter flocks than their social mates, and extra-pair paternity in nests occurs commonly enough to be considered a potential female mating tactic. This study uses blood samples collected in 1992–1995 from 58 families of black-capped chickadees to test whether females with extra-pair offspring have chosen extra-pair sires higher in social rank than their mates. Paternity was assessed with multilocus DNA fingerprinting in 1992–1994 nests and with microsatellite and single-locus minisatellite DNA typing in 1995 nests. Seventeen of 58 nests (29.3%) contained young genetically mismatched with their social father. In 11 of 15 cases where the identity of the extra-pair male was known, the extra-pair male was dominant to the social father. Using data from 29 nests located in 1994 and 1995 for which we had the most data on relative ranks of males, high-ranking males had greater realized reproductive success than low-ranking males as a result of extra-pair fertilizations. There was no significant difference between the number of nests containing extra-pair young of females mated to low-ranked versus high-ranked males. Two nests in 1995 contained young either genetically mismatched with both social parents (intraspecific brood parasitism) or, in one nest, genetically mismatched with the social mother but not the social father (quasi-parasitism). The implications of female strategies acquiring genetic benefits through extra-pair copulations are discussed.

Extra-Pair Fertilizations in the Sedge Warbler

Parentage of 201 young (from 44 broods) in a population of Sedge Warblers Acrocephalus schoenobaenus in South Central Sweden, 1990-1992, was determined by using multilocus DNA fingerprinting. The frequency of extra-pair young (EPY) was 7.5% and they occurred in 23% of the broods. For 11 out of 15 extra-pair young we could identify the true father; in all of the cases he was a close neighbour. Among ten broods with extra-pair young, nine contained only one extra-pair sire and the remaining brood two extra-pair sires. The frequency of EPF varied among years (1.8-11.8%). The seasonal timing of broods with and without extra-pair young did not differ, and the occurrence of extra-pair fertilizations (EPF) was not related to the length of the pair male's mate-guarding period. EPF were not related to breeding synchrony (estimated as the mean number of fertile females per day during a female's fertile period). Pairs whose nests contained extra-pair young had more territories within 100 m of their nest than pairs without extra-pair young. Females that engaged in EPF had fewer attractive (i.e. singing) males to choose among the day before and at the day they formed their pair bend. Moreover, in all but one case the extra-pair male was not singing (i.e. not available as pair mate) the day the EPF-female settled. Males that fertilized extra-pair young tended to arrive earlier and to have higher pairing success than both males that were cuckolded and other males. Hence, female Sedge Warblers engaged in extra-pair fertilizations with attractive male neighbours. These results are consistent with the hypotheses that females participating in EPF are seeking genetic benefits to their offspring, but we cannot exclude the alternative explanation that attractive males are more efficient in forcing females to accept EPF.

Does Breeding Density Covary with Extra-Pair Fertilizations in Hooded Warblers?

The extent of variation in the frequency of extra-pair matings among and within avian species could result from differences in breeding density, where breeding density is predicted to be positively correlated with extra-pair paternity. We combined paternity data and behavioural observations to evaluate this prediction using correlational data in relation to observed densities of Hooded Warblers Wilsonia citrina by comparing populations differing in (1) nearest neighbour distance which varied by more than an order of magnitude (range 50-600 m) and (2) the number of adjacent neighbours within 300 m (range 0-8). Spearman rank correlations demonstrated no relationship between extra-pair paternity and either inter-territory distance or the number of adjacent neighbours within 300 m. The rate of extra-territorial forays, female chip rates, the number of chipping bouts, time spent chipping, male song rates, the number of song bouts and time spent singing were similar among pairs breeding at different densities. Our results suggest that a simple linear relationship between density and extra-pair paternity may not exist in some avian species, possibly because the costs of inter-territory travel may be much lower than the benefits gained through extra-pair fertilizations, and also because of the conflicting mating tactics used by males and females.