Do female pied flycatchers seek extrapair copulations with ... - UiO

Behavioral Ecology Vol. 12 No. 4: 412–418

Do female pied flycatchers seek extrapair copulations with familiar males? A test of the incomplete knowledge hypothesis Tore Slagsvold,a Arild Johnsen,b Helene M. Lampe,a and Jan T. Lifjeldb Department of Biology, University of Oslo, P.O. Box 1050 Blindern, N-0316 Oslo, Norway, and bZoological Museum, University of Oslo, Sars gate 1, N-0562 Oslo, Norway

a

In birds with biparental care, great variation exists in the frequency of extrapair paternity. Several hypotheses have been proposed to account for this variation. We tested the incomplete knowledge hypothesis, which states that females are constrained in their knowledge of male quality and that this influences their willingness to engage in extrapair copulations (EPC). By selective removal and release of female pied flycatchers Ficedula hypoleuca, we created a situation where females finally settled with a social mate close to the site where a former social mate was breeding. According to the incomplete knowledge hypothesis, this would lower the threshold for females to seek extrapair copulations in cases where their former social mate was of higher quality than the one finally chosen. The hypothesis was not supported because manipulation of female settlement did not increase frequency of extrapair paternity, not even in cases where the female nested close to the previous mate and the current mate apparently was of lower quality because he was younger and more dull colored. However, we found that when extrapair paternity did occur, the cuckolder tended to be a familiar male (i.e., the female’s initial social mate). Key words: cuckoldry, extrapair paternity, Ficedula hypoleuca, pied flycatcher, polygamy. [Behav Ecol 12:412–418 (2001)]

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n birds with biparental care, great variation exists in the frequency of extrapair paternity (EPP) between species, between different populations of a species, and within populations (e.g., Møller and Briskie, 1995). This variation is poorly understood, and several hypotheses have been proposed (Birkhead and Møller, 1992; Gowaty, 1996; Petrie and Kempenaers, 1998). We tested the incomplete knowledge hypothesis (Slagsvold and Lifjeld, 1997), which states that females may be constrained during the period of mating, nest building, and egg laying so that they have limited opportunity to assess the quality of the various males present in the breeding area. It is assumed that females control engagement in extrapair copulations (EPC) and that they are only willing to perform EPCs when it is likely that the extrapair male is of higher genetic quality than their own social mate. Some observations are consistent with the hypothesis; in particular, the frequency of EPP often is higher at high than at low breeding densities (Westneat and Sherman, 1997). However, this is only circumstantial evidence and experimental tests are needed. Here we report a study of occurrence of EPP in the pied flycatcher Ficedula hypoleuca. The species is suitable for a test of the incomplete knowledge hypothesis because females may be constrained in their knowledge of male quality. This is because the species is migratory, nonsocial outside the breeding season, site fidelity is low, and females arrive late in the breeding season and only a short time before egg laying (Lundberg and Alatalo, 1992). Females spend from a few hours to a few days prospecting for a breeding site and a mate, visiting only a limited number of males (Dale and Slagsvold, 1996; Dale et al., 1992; Hovi and Ra¨tti, 1994). The frequency of EPP is low (Bru¨n et al., 1996; Ellegren et al., 1995; Lifjeld et al., 1991; Ra¨tti et al., 1995, 2001; but see also Gelter and Tegelstro¨m,

Address correspondence to T. Slagsvold. E-mail: tore.slagsvold@bio. uio.no. Received 14 April 2000; revised 18 September 2000; accepted 27 September 2000. 2001 International Society for Behavioral Ecology

1992). We asked whether this is caused by constraints on female knowledge of male quality. We manipulated female settlement so that females finally settled with a social mate close to a site where their former social mate was breeding. The focal female had been mated to the latter male earlier within the same breeding season, but he became occupied by another female because of our temporary removal of the focal female. We expected our manipulation to lower the threshold for females to engage in EPCs because they now would have proper knowledge of the quality of at least one potential extrapair mate. We assumed that the focal female was able to find her former mate because female pied flycatchers are able to identify and locate a male from his song even after having been exposed to the song only for a short time (Lampe and Slagsvold, 1998). The following predictions were tested: (1) The level of EPP would be higher for experimental than for control females. (2) In cases of EPP, the cuckolder would primarily be the female’s initial mate. (3) The level of EPP would be negatively related to the distance between the nest of the previous and current mate. This is because of increased costs of visiting and being visited by males at longer distances as assumed by the hypothesis (Slagsvold and Lifjeld, 1997). (4) EPP would primarily occur when the inital mate was of higher genetic quality than the final mate. The male characteristics that females use to assess male genetic quality are unknown. Hence, we included several phenotypic traits which at least seem to be of importance in female choice of social mate (e.g., females prefer older males [Lifjeld and Slagsvold, 1988], males that arrive at breeding sites early [Alatalo et al., 1984; Dale and Slagsvold, 1990], males with bright plumage color [Ja¨rvi et al., 1987; Lifjeld and Slagsvold, 1988; Sætre et al., 1994], and males singing complex songs [Lampe and Sætre, 1995]). (5) Local recruitment of young to the breeding population may be higher from broods with EPP than from broods without EPP because in the former cases the females may have succeeded in improving the quality of at least some offspring.

Slagsvold et al. • EPP in pied flycatchers

METHODS Study area and study species The study was performed in a mixed deciduous–coniferous woodland area near Oslo, during spring in 1996–1998, under license from the Directorate for Nature Management in Norway. The study area covered about 55 ha and was provided with about 260 nest-boxes each year. The pied flycatcher is a small, hole-nesting, insectivorous passerine bird which usually is monogamous, but in which some 5–10% of the males may be polygynous. The decision of female settlement seems to be made by the females, not by the males (Dale and Slagsvold, 1994), and choice is based both on the quality of the nest site (Alatalo et al., 1986; Dale and Slagsvold, 1996; Slagsvold, 1986) and the quality of the male, including his display activity (Slagsvold and Viljugrein, 1999), song quality (Lampe and Sætre, 1995), plumage color ( Ja¨rvi et al., 1987; Lifjeld and Slagsvold, 1988; Sætre et al., 1994), and mating status (Dale and Slagsvold, 1996; Slagsvold and Dale, 1994). The species is single-brooded but may renest if the first nesting fails. Females were removed after mating and released soon after the initial mate had attracted a new female. This procedure was necessary because earlier experiments showed that females tend to return and mate with the same male if released within the same breeding season (Slagsvold and Dale, 1991). However, we have also shown that returning females have difficulty in displacing a new female if the latter has been present for a day or more (Dale and Slagsvold, 1995). In such cases, the returning female tends to settle with another male in the neighborhood (Slagsvold and Dale, 1991). Knowledge of this behavior formed the basis for the present experiment. Experimental females We removed and then released 48 females; 18 disappeared, 5 settled with the initial male, and 25 settled with a new male (final male) in the study area. Of the latter females, two were excluded from the analysis because the identity of the final social mate was uncertain, and one was excluded (but included as a control; see below) because the initial mate had disappeared when the female was released. We also included three females that switched mate probably because of our trapping and handling. These females were never removed, but they had the opportunity to obtain information on the quality of the initial male. Hence, we were left with 25 females for the study (referred to below as ‘‘experimental females’’; 3 in 1996, 9 in 1997, and 13 in 1998). In three cases, the initial mate was the same for two different focal females in the same year. All six females were included because they settled with final males that differed in quality and in distances to the nestbox of the initial male. One male was final male both in 1997 and 1998. Both cases were accepted because the male differed in age and experience between the two breeding seasons and because the initial males were different. Focal females were only included once, except one female that was manipulated both in 1997 and 1998. She was included because initial and final mates were different in the two years. Females were always removed before onset of egg laying, 0– 13 days after having started nest building with the initial male (mean ⫽ 4.3, SD ⫽ 3.1, n ⫽ 25). The nest was also removed. The females were kept in captivity for 0–21 days (mean ⫽ 7.0, SD ⫽ 6.3, n ⫽ 25) and provided with mealworms and water. They were released close to their initial nest-box and started nest building with the final male 0–3 days after release (mean ⫽ 1.4, SD ⫽ 1.0, n ⫽ 25). Egg laying started 4–9 days after onset of nest building (mean ⫽ 5.9, SD ⫽ 1.4, n ⫽ 25), which was from 6 days before to 14 days after the onset of egg laying

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by the replacement female of the initial male (mean ⫽ 2.0, SD ⫽ 3.8, n ⫽ 25). Control females We analyzed paternity for 82 control nests, consisting of 3 groups. The first group consisted of females that had been temporarily removed after intial mating in the study area but that renested with the same male (n ⫽ 5). In the second group, females that had started nest building 2–29 km away were kept in captivity and then released into the study area (n ⫽ 31). These birds nested with a new male and they had no initial mate present in the study area. We also included the case mentioned above where a female had her first and repeat nesting attempt in the study area but where the initial mate had disappeared. The third group consisted of females that were never removed (n ⫽ 46). We excluded three cases where there was a switch of social mate in the female’s fertile period, probably caused by predation of the first male from European sparrowhawk Accipiter nisus. Male characteristics and mating status Soon after arrival, the males were color-ringed, and plumage color, ranging from black and white (score 1) to brownish and femalelike (score 7; Drost, 1936) was recorded. We used halfscores in case of intermediate types, and mean values were used if there were repeat measurements during the same year. Body mass was measured to the nearest 0.1 g with a Pesola 50-g balance, and wing length was measured to the nearest 0.5 mm using a flattened and straightened wing. Males were aged as yearlings or older according to Karlsson et al. (1986). We also have data on the song of both mates for 18 of the experimental females. Song recordings were done during the unmated stage (29 males), after female removal (4 males), or in their secondary territory (3 males). Song repertoire size has been found to be similar for individual males singing to attract a female in their first and secondary territory (Lyngby, 1996). Songs were recorded using a Sony TC-D5 Pro(II) cassette recorder with a Telinga II Pro parabolic microphone and headphones. The recordings were analyzed using Canary 1.2.4 (Bioacoustic Research Program, Cornell Laboratory of Ornithology). We measured song repertoire size as the number of different song figures in 25 consecutive song strophes. Daily visits to the nest-boxes made it possible to observe arrival and mating date of the males and date of onset of egg laying. Arrival time of males varied significantly between years, and values were standardized by using the deviation (in days) from the annual mean for the initial and final males involved. We applied a similar procedure for date of first mating. We classified females as monogamously mated or mated as primary or secondary female to a polygynous male. All nestlings were banded, which enabled a study of recruitment to the breeding population. We also visited the study area daily in the breeding season of 1999 and 2000 to search for recruits. We included recruits found in the study area (n ⫽ 14) and in other nest-box plots (n ⫽ 7). Distances between nest-boxes were log-transformed in statistical tests. Statistical tests are twotailed. Parentage analyses A sample of 25–50 ␮l blood was taken by puncturing the brachial vein, suspended in 1 ml Queens lysis buffer (Seutin et al., 1991), and stored in a refrigerator for later analysis. Blood was collected from the 25 experimental females, from their 127 chicks, and from their initial and final social mates. We also collected blood from the 82 control families (426 chicks).

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For most males this was done soon after their arrival, for females in the incubation period, and for chicks when about 6– 9 days old. DNA was extracted using the QIAamp blood kit (Qiagene). We analyzed parentage by microsatellite typing, using five pied flycatcher markers (FhU1–5; Ellegren, 1992; Primmer et al., 1996) that have been used successfully in previous studies of the species (e.g., Ellegren et al., 1995; Lifjeld et al., 1997). Refer to Ellegren et al. (1995) for details of the polymerase chain reaction protocol. Allele frequencies, calculated from the genotypes of all adults (separately for the 1996 sample and 1997 and 1998 combined), were used to estimate the exclusion power of the marker set ( Jamieson 1994). The combined exclusion probability was .998 and the probability of identity was 5 ⫻ 10⫺8, both for 1996 and for 1997 and 1998 combined. Excluding the FhU1 marker only reduced the exclusion probability by .05%. Thus, for the second half of the 1997 material and the complete 1998 material, we only used the remaining four markers. Probability of false inclusion was calculated according to Jeffreys et al. (1992). Of the 553 analyzed nestlings, 517 showed no mismatches with either parent and had a low mean probability of chance inclusion (.0020 ⫾ .0058 SD). One nestling showed one paternal mismatch but had a relatively low probability of false inclusion for the father after excluding the locus with the mismatch from the calculations (.016). All these 518 nestlings are considered legitimate. Thirty-one nestlings had 2 or more paternal mismatches, whereas they matched a female allele in all loci, thus being sired by extrapair males. Two nestlings had one paternal mismatch and a relatively high probability of chance inclusion, excluding the mismatched locus (.020 and.043, respectively). Furthermore, the mismatched alleles of these nestlings were the same size as the ones of brood mates showing mismatches on other loci as well. We therefore consider 33 nestlings to have resulted from EPCs. Two nestlings (both in 1998) showed mismatches with both parents at two or more loci, and thus probably resulting from egg dumping. These nestlings were excluded from all further analyses. For 30 of the 33 extrapair young (EPY), a male present in the study area matched the paternal genotype completely (mean probability of false inclusion, .0026 ⫾ .0041 SD). Separated for the 3 years 1996–1998, the proportion of EPY was 6% (n ⫽ 328), 6% (n ⫽ 107), and 7% (n ⫽ 116), and the proportion of nests with EPY was 10% (n ⫽ 63), 9% (n ⫽ 22), and 14% (n ⫽ 22), respectively. Because there was no significant annual variation, we combined the data in the analyses below. RESULTS Frequency of EPP in experimental and control broods EPP occurred in only 4 (16%) of the 25 broods where an experimental female finally mated with a male different from the one initially chosen and where the latter male was still present in the study area (1 in 1996, 1 in 1997, and 2 in 1998). The frequency of broods with EPP for the three control groups was 0% (n ⫽ 5) for females that returned to their initial mate after capture and release; 16% (n ⫽ 31) for females that had been kept in captivity but where the initial male was not present in the study area; and 4% (n ⫽ 46) for unmanipulated females. Hence, the frequency of broods with EPY was similar for experimental females (16%, n ⫽ 25) and for all control females combined (9%, n ⫽ 82; ␹2 ⫽ 0.49, df ⫽ 1, p ⫽ .48) and similar for experimental females and for control females kept in captivity (control groups 1 and 2 combined; 14%, n ⫽ 36; ␹2 ⫽ 0.02, df ⫽ 1, p ⫽ .89).

Figure 1 Characteristics of renesting female pied flycatchers. Time elapsing between onset of egg laying of focal female and onset of egg laying in the nest of her initial mate is plotted against the distance between the two nest sites. Filled circles, some chicks sired by initial male; filled squares, some chicks sired by an extrapair male but not the initial male.

Identity of cuckolders In two of the four experimental broods with EPP, four chicks were sired by the initial mate and one chick by the final mate. In the two remaining cases of EPP, the cuckolder was evidently not the initial mate. In the first case, one chick was sired by the final male and one chick by a male that had his own nest 360 m away. In the second case, two chicks were sired by the final male and three chicks by an unknown male. In the two cases of EPP where the initial male was the cuckolder, the distance between his final nest and the final nest of the focal female was 72 m and 203 m (Figure 1). Considering 203 m as a maximum distance for males to engage in EPC with the focal female, which may be a conservative value (see below), we found 10 and 13 males to defend nest-boxes within this range in the two cases of EPP (inital mates and unmated males included). In the two other cases of EPP, there were 10 and 12 males present within 203 m from the focal female’s nest. If females engaged in EPC with a single random male within this range (203 m) from their nest, we can calculate the probability that they would have copulated by chance with their initial mate in each of four cases (p ⫽ .00006), in three of the four cases (p ⫽ .00263) and in two of the four cases (p ⫽ .04019). Combining these values, the probability that the four females would have copulated with two or more of the initial mates by chance would be only p ⫽ .043. The analysis is sensitive to the number of male neighbors included and so to the distance used (203 m). However, as mentioned above, cuckolders may have their own nest as much as 360 m away, and so a value of 203 m may be conservative. EPP and distance to initial mate Initial mates remained in the same part of the study area when focal females were removed and finally nested on average only 32 m (SD ⫽ 33, range 0–102, n ⫽ 25) away; in 11 cases using the same nest-box. The distance between the site


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Table 1 Characteristics of initial and final mates of female pied flycatchers and of final mates that were cuckolded Initial males (n ⫽ 25)

Final males (n ⫽ 25)

Cuckolded males (n ⫽ 4) a

Variable

Mean

SD

Mean

SD

t

Mean

SD

tb

Plumage color (Drost score) Song repertoire sizec Wing length (mm) Body mass (g) Arrival dated First mating dated Premating period (days)

2.9 32.7 79.6 12.2 ⫺3.0 ⫺4.7 3.8

0.9 12.9 1.3 0.5 3.5 4.5 3.6

3.3 33.7 79.7 12.3 2.2 2.9 6.2

1.3 14.1 1.6 0.6 9.6 8.9 5.9

1.43 1.06 0.22 0.61 2.97** 4.02*** 1.63

2.7 25.5 79.5 12.0 7.5 5.1 3.0

0.8 0.7 1.1 0.8 8.4 9.1 2.6

1.06 0.87 0.33 1.23 1.22 0.60 1.18

Between initial and final males (df ⫽ 24). Between final males that were cuckolded or not (df ⫽ 23). c n ⫽ 18 for initial and final males, and n ⫽ 2 for cuckolded males. d Relative to the annual mean. ** p ⬍ .01; ***p ⬍ .001. a

b

of the final nest of the focal female and the final nest of her initial mate was, on average, 250 m (SD ⫽ 224, range 32–754, n ⫽ 25; Figure 1). The distance was slightly shorter in case of EPP than in case of no EPP (Figure 1; mean values of 139 and 272 m), but this was not statistically significant (t ⫽ 0.79, df ⫽ 23, p ⫽ .44). EPP and male characteristics Final mates arrived and became mated about a week later than initial mates (Table 1). The plumage color of final mates tended to be slightly more brownish than that of initial mates but not significantly so (Table 1). Mean song repertoire size was similar for the two groups of males, as was mean wing

Figure 2 Plumage color of initial and final male chosen by female pied flycatchers (n ⫽ 25). Points above the y ⫽ x line indicate that the final male was browner than the intial male. Open circles, finale male sired all chicks; filled circles, some chicks sired by initial male; filled squares, some chicks sired by an extrapair male but not the initial male.

length and body mass (Table 1). In 16 cases, both males belonged to the same age class (14 older, 2 yearlings); in 7 cases the final male was yearling and the initial male older; and in 2 cases the final male was older and the initial male was yearling. Our manipulation did not necessarily result in great average differences in characteristics between intial and final males (Table 1); however, it resulted in great changes of mate characteristics for individual females between first and repeat nesting attempts. For instance, in one case the plumage color score was 2.5 and 5.5 for the initial and final male, whereas in another case scores were 5.5 and 3.0, respectively (Figure 2). The female that chose the initial male with the largest song repertoire of all the males analyzed (65 song figures) ended up with a final mate with one of the smallest song repertoires (17 song figures; Figure 3). In total, in 15 of 25 (60%) final

Figure 3 Song repertoire size of initial and final male chosen by female pied flycatchers (n ⫽ 18). Points above the y ⫽ x line indicate that the final male had a larger song repertoire than the intial male. Open circles, finale male sired all chicks; filled circles, some chicks sired by initial male.


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mates were duller in color than the respective initial mates (Figure 2), and 7 of 18 (39%) had a smaller song repertoire (Figure 3). However, neither male color nor song repertoire size seemed to influence the rate of EPP because cuckolded final males were not more brownish, nor did they have smaller repertoires than the respective initial males (Figure 2 and 3). Moreover, the cuckolded final males were not more brownish, nor did they have smaller song repertoires than noncuckolded final males (Table 1). It should be noted, however, that the power of the latter tests were low due to few cases of EPP. Only four females settled with final mates that were both duller and had less complex song than the first mate; one of these females engaged in EPC with her initial mate. There was no significant correlation between color score of first and final mates (r ⫽ .12, n ⫽ 25, p ⫽ .56), nor of song repertoire size of first and final mates (r ⫽ ⫺.15, n ⫽ 18, p ⫽ .55), and hence no evidence for consistent female preferences with respect to male plumage color or song repertoires. For the experimental group, there was no significant difference between cuckolded and noncuckolded males in wing length, body mass, arrival date, mating date, and interval between arrival and mating (Table 1). In the two cases of EPP where the initial male was the cuckolder, the final male was a yearling in one case and an older male in the other case. The same was true for the two cases of EPP where identity of the cuckolder was unknown. In all four cases of EPP the intial male was older. Of the noncuckolded final males, 7 were yearlings and 14 were older. Shortest distance recorded between final nest of experimental females and nest of initial mate was only 32 m (Figure 1). In that case, the final male arrived late (10 June), was a yearling, and was brown (score 6). The initial male arrived much earlier (25 May), nested in the study area in a previous year, and was much darker in color (score 3.2). The focal female had spent 6 days with the initial male before our removal. Nevertheless, the final social mate sired all her four chicks. The initial mate sired all five chicks in his own nest, where egg laying had started 6 days earlier than in the nest of the focal female. Other patterns of EPP Occurrence of EPP was unrelated to the egg-laying interval between the two females (Figure 1; t ⫽ 0.02, df ⫽ 23, p ⫽ .98). In the two cases where the initial mate was the cuckolder, the time elapsing from removal of the focal female from the initial mate to her onset of egg laying was 7 and 14 days, respectively. There was a tendency that fewer experimental females (24%, n ⫽ 25) were mated with a polygynous male than controls (48%, n ⫽ 82; ␹2 ⫽ 3.45, df ⫽ 1, p ⫽ .063). However, the frequency of EPP was almost the same for females mated with monogamous (10%, n ⫽ 62) and polygynous (11%, n ⫽ 45) males (␹2 ⫽ 0.01, df ⫽ 1, p ⫽ .94). EPP and recruitment Of the nestlings reared in 1996–1998 later found as yearlings or older, 8 were offspring of experimental females (6.7% of young fledged, n ⫽ 119) and 13 were offspring of controls (3.5% of young fledged, n ⫽ 371); 1 was from a brood with EPP (2.0% of young fledged, n ⫽ 51), and 20 were from broods without EPP (4.6% of young fledged, n ⫽ 439). If we only include broods that produced at least one chick fledged, mean number of recruits per brood was insignificantly higher for experimental females (0.33, SD ⫽ 0.76, n ⫽ 24) than for controls (0.18, SD ⫽ 0.42, n ⫽ 72; t ⫽ 1.23, df ⫽ 94, p ⫽ .22) and insignificantly lower for broods with EPP (0.09, SD ⫽

0.30, n ⫽ 11) than for broods without EPP (0.24, SD ⫽ 0.55, n ⫽ 84; t ⫽ 0.85, df ⫽ 94, p ⫽ .40). The recruit from the brood with EPP was sired by an EPC. DISCUSSION We assumed that female pied flycatchers have restricted knowledge of male quality. This assumption was based on the fact that females are migratory and spend only a few hours or days prospecting for a breeding site and a mate, visiting only a few males (Dale and Slagsvold, 1996; Dale et al., 1992; Hovi and Ra¨tti, 1994). In an earlier experiment with the same population, we found that 28% of the females visited only a single male before settling; mean number of males visited was 3.1 (Dale and Slagsvold, 1996). This rapid settlement is caused by late arrival to the breeding ground and a short breeding season and also by strong competition for nest sites. Females restrict the extent of mate search if they encounter many prospecting females (Dale et al., 1992). Males also compete strongly for access to suitable nest sites, and this competition is primarily related to time of occupancy and less to male characteristics (Alatalo et al., 1986; Slagsvold, 1986). However, males with more complex songs tend to occupy more popular territories, and they also tend to be mated earlier than males with less complex songs (Lampe and Espmark, in press). In general, early-arriving males tend to be of higher quality (i.e., they are older, in better condition, have brighter plumage color, and have more complex songs; Lampe and Espmark, 1994). They potentially have the chance to occupy the best territories. However, a male occupying a good nest site may not always be of preferred genetic quality, and thus females may be expected to adjust initial mate choice by engaging in EPC (Birkhead and Møoller, 1992). We found that the frequency of broods with EPP was not significantly higher for experimental females (16%, n ⫽ 25) than for controls (10%, n ⫽ 82). The level of EPP was also similar to what we have found for the same population in two earlier breeding seasons: 15% (n ⫽ 27) in 1989 (Lifjeld et al., 1991) and 17% (n ⫽ 18) in 1992 (Ellegren et al., 1995). In the present study, we manipulated female settlement so that a familiar male was present in the neighborhood. However, females apparently did not take advantage of this by increasing the level of EPP, even when their final social mate appeared to be of much lower phenotypic quality than their initial mate. This lends little support to prediction 1 (see Introduction) from the incomplete knowledge hypothesis. The prediction is based on the assumption that the addressed aspects of male phenotype could be used by females to assess male genetic quality. This assumption was not tested, but circumstantial evidence comes from the fact that male plumage color is partly heritable in pied flycatchers (Lundberg and Alatalo, 1992; Slagsvold and Lifjeld, 1992). The frequency of EPP was so low that prediction 3 on a relationship with distances between initial and finale mate, and prediction 4 on a relationship with quality differences between the two mates, could not be critically tested. Support was only found for prediction 2 because, when EPP occurred, the cuckolder tended to be a familiar male. We conclude that the low frequency of EPP in this species does not seem to be explained by females having incomplete knowledge of male quality. It may be argued that the initial males were located too far away from focal females to be visited (e.g., because of risk of predation from European sparrowhawks; Slagsvold and Dale, 1996). This possibility can be rejected because distances between nest-boxes of inital and final mates were low in many cases (Figure 1). Moreover, initial mates remained in the same area when focal females were removed, so focal females presumably would have no difficulty


locating their former mates. On the other hand, the initial mate’s new female may not have tolerated intrusion by the experimental females, thus preventing these females in engaging in EPCs with their former mate. Female pied flycatchers have been shown to be very aggressive toward other females close to the nest site and also toward prospecting females in the mate’s second territory (Slagsvold and Sætre, 1991; Slagsvold et al., 1992; Ra¨tti, 1999). Hence, it is perhaps more likely that resident females choose among visiting extrapair males than that they leave their own territory to engage in EPCs, although in one study during the egg-laying period females spent, on average, about 35% of their time more than 100 m from the nest (Kilpimaa et al., 1995). Such visits away from the nest may mainly be motivated by foraging but may also provide opportunities for EPCs. We may ask if the quality differences between initial and final mates were large enough to be important. In general, final males arrived later than initial mates. However, in many cases the plumage color and/or song complexity of final mates was similar or better than that of initial mates. This may have been caused by better mating options available and/or from increased knowledge of the males in the area and improved ability to assess male quality. High quality of final males may explain why some females did not engage in EPCs. However, the incomplete knowledge hypothesis cannot explain why little EPP was found in cases where females ended up with low-quality males. The point should be further addressed when more is known about the possible benefits females gain from EPCs in this species. Because initial mates had their own social mate and nest, they may have been too busy to engage in EPCs. However, this explanation can be rejected because our manipulation resulted in many instances where females of initial mates had already passed the period when focal females were fertile (Figure 1). The period of peak female fertility seems to be short in pied flycatchers, only lasting from about 2 days before the first egg is laid until the day the first egg is laid (Birkhead et al., 1997; Lifjeld et al., 1997). It has been suggested that males may reduce their parental effort in response to cuckoldry (Møller, 1988; Westneat and Sherman, 1993). A previous study of the present population of pied flycatchers, in which mate switching was experimentally induced in the female’s fertile period by male removals, supported this idea (Lifjeld et al., 1998). However, such a risk would hardly prevent females from engaging in EPCs under natural conditions because intensity of male mate-guarding is low in the present population (Chek et al., 1996), making it difficult for males to know that their mate has copulated with another male. Finally, benefits supposed to be gained from EPC may be less important in pied flycatchers than in some other bird species. These benefits may include increased genetic quality of the offspring (e.g., Brown, 1997; Johnsen et al., 2000). We found that broods with EPY produced no more recruits to the subsequent breeding population than broods without EPY. A stronger test would be to compare recruitment of chicks from the same brood. There were too few broods available for such an analysis, but local recruitment was no higher for EPY than for their half-sibs in two species of tits (Parus major, P. caeruleus; Kempenaers et al., 1997; Krokene et al., 1998; Lubjuhn et al., 1999). In the collared flycatcher Ficedula albicollis, fledging condition was higher for EPY than for their half-sibs, but local recruitment was not studied (Sheldon et al., 1997). In two of the four experimental cases with EPP, the initial mate was identified to be the cuckolder. This was more than expected if the four females copulated with a randomly chosen male in the vicinity. We can probably reject the idea that these females had stored sperm from the mating period with

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the initial mate because of the long time interval from our removal of the focal female to her final egg laying (7 and 14 days; cf. Birkhead et al., 1997). We conclude that our manipulation of female settlement did not result in increased levels of EPP, but when EPP did occur the cuckolder tended to be a familiar male (the initial mate). We recommend that the latter finding be further tested in species where the frequency of EPP is higher than in pied flycatchers. We are indebted to the late H. Rinden for assistance in the field and also to W. Johansen, R. Fjeld Karlsen, and C. Listøen Lerberg.

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