Niche separation and Hybridization -are nestling hybrid flycatchers provided with a broader diet?

Transcription

1 Niche separation and Hybridization -are nestling hybrid flycatchers provided with a broader diet? Nilla Fogelberg Degree project in biology, 2006 Examensarbete i biologi 20p, 2006 Biology Education Centre and Department of Animal Ecology, Uppsala University Supervisor: Anna Qvarnström

2 Table of contents Abstract..3 Introduction...4 Objectives...5 Material and Methods 6 Study system...6 Methods...7 Data analysis...8 Results 9 Discussion.14 Conclusion..16 Acknowledgements...17 References

3 Abstract The principle of competitive exclusion states that two species that co-occur have to develop separate niches, considering all of their resources. If they compete over the exact same niche one of the species will eventually outcompete the other in that area. Hybridization between co-occurring species often leads to offspring that are infertile or have reduced fertility. This is costly for the individuals that interbreed. One factor that could possibly reduce this cost is if separated niches between two species are beneficial for the hybrid offspring. I have studied this factor at an early life history stage. The Collared flycatcher (Ficedula albicollis) and the Pied flycatcher (F. hypoleuca) are closely related species that occasionally hybridize in their areas of sympatry. To test the prediction that the hybrid nestlings can somehow benefit from having two niches available, I studied if the two species differ in their diet and if the hybrid nestlings are provided with a broader diet than nestlings in a pure collared or pied pair. I did find the pattern expected but for other reasons than I had expected. Hybrid nestlings are provided with a more diverse diet, but there is no difference in the diet between the pied flycatcher and the collared flycatcher. Instead, individual parents in mixed pairs each provide a more diverse diet. A possible explanation to this is that it is a result of the environment where mixed pairs breed. The broader diet had no effect on the growth of the nestlings and can therefore not be regarded as a factor that lowers the cost of hybridization. I also studied the feeding frequency of males in different pair types and found that males in mixed pairs have a slightly higher feeding frequency than males in pure pairs. Direct benefits of hybridization associated with hybrid offspring receiving broader diets might be a more likely scenario in hybrid zones where the two species differ more in their dietary niches. 3

4 Introduction When two species that are morphologically and ecologically similar co-occur, the interactions between them are likely to influence their ecology and evolution. Their similar ecological requirements may result in competition over resources. Furthermore, if they use similar cues for mate recognition hybridization may also occur. This makes the dynamics of the interactions between the species even more complex (Saetre et al. 1999a). There are many theories concerning interspecific competition. A central theory is the principle of competitive exclusion (Gause 1934). It states that two species that are using the same resources cannot coexist in a stable way. Eventually the stronger species will outcompete the weaker one, which will go extinct in that area. In order to coexist, the species need to establish separate niches, such as slight differences in diet and foraging patterns. Animals divide environmental resources between themselves in three basic ways: temporally, spatially and trophically. Species differ in times of activity, the places they exploit, or in their diet. This way competition is reduced and the coexistence of a variety of species is allowed (Pianka 1973). Since closely related species often are ecologically similar and have overlapping requirements, it has been suggested that interspecific competition may be particularly likely to influence population dynamics and competitive exclusion should occur more frequently between congeneric species than between more distantly related species (Saetre et al. 1999b). Hybridization is widespread among birds and the consequences of it differ. Hybrids are usually at a disadvantage when compared to the parental species. They may be completely sterile or be partially sterile and have smaller broods. Sometimes they appear to survive and breed as successfully as members of the parental species. The genetic incompatibilities between hybridizing species sometimes appear in the first generation of offspring, and sometimes with the production of recombinants in the second generation (Grant 1992). There are many examples of both plants and animals in which a few members of a population hybridize with individiduals of a closely related species and gene flow occurs through back crossing to the parental species. Occasionally new species are formed by hybridization, but more often the genetic and ecological characteristics of the interbreeding species are changing to varying extent. Some of these examples occur naturally while others occur as a consequence of human alteration of the environment that brings closely related species back into contact after a long period in separation (Grant 1996). Even though numerous studies of both animals and plants have shown that hybrids may be of equal fitness or even of higher fitness than their parental species in certain habitats (Pierotti & Annett 1993), most hybridization events lead to a fitness cost. It should be strongly selected against when hybrid offspring have reduced fitness (Veen et al. 2001). Through selection against unfit hybrids many mechanisms have developed to prevent interbreedings and to strenghten reproductive isolation between species. Hybridization in birds is the consequence of an active choice of a mating partner. The sex that invests more in offspring production, usually the female, is the more discriminating one. Mate choice can be based on various factors. It may be an inherited preference for certain physical traits, such as color or size. If choice can be based on traits that serve as indicators of parental quality females will gain adaptive benefits. In birds this may be quality of the territory or quality of food provided by the male (Pierotti & Annett 1993). 4

5 Direct benefits of mate choice are very important for sexual selection within a species, but their role in hybridization is ignored. Because closely related co-occuring species are likely to differ in their niche, hybrid offspring might receive an advantage by receiving a broader range of food types from parents of two species. Direct benefits such as these might be important in reducing the genetic costs of hybridization. The collared flycatcher Ficedula albicollis and the pied flycatcher F. hypoleuca are two closely related species of small migratory passerine birds. They co-occur on the Baltic islands of Öland and Gotland. These flycatchers are a good system to address this study to. The two species are very similar ecologically. They breed in nest boxes and have similar habitat preference. Since they breed in the same area at the same time of the year they are expected to utilize different diets if they can continue to coexist. In the zones of sympatry, hybridization between the two species occasionally occurs. On Öland and Gotland, the frequency of hybrids in the breeding population has been estimated to 1-4% (Gelter et al. 1992, Alerstam et al. 1978). Many studies show that hybrid males have slightly reduced fertility relative to males of the parental species, whereas the eggs of hybrid females fail to hatch (Veen et al. 2001). In spite of these known genetic costs of hybridization, direct benefits of pairing with a heterospecific male have never been investigated. Although choice of a heterospecific mate is typically regarded as an error or a failure of an isolating or specific mate recognition mechanism, some ecologists argue that in certain circumstances, there may be benefits to such choices (Pierotti & Annett 1993). Veen et al.(2001) suggested that hybridization may under some circumstances be the best option and represent adaptive mate choice. One of the factors that may substantially lower the cost of hybridization for the female is extra pair copulations. Females may mate with a conspecific male even though she has chosen a heterospecific male as a social mate. DNA tests demonstrate that nestlings in a single clutch often have multiple parentages (Gelter et al 1992). Objectives In this study I am examining if offspring in a mixed pair receive a more diverse diet than offspring in pure collared flycatcher and pied flycatcher pairs. The background to this theory is that according to the principle of competitive exclusion, the collared and pied flycatchers should have some differences in their use of resources. If they are separated in their diet then it is possible that the nestlings in a mixed pair will receive a more diverse diet than nestlings with both parents of the same species. To find out how differentiated the two species are I will first examine their overlap in diet. If their niches are separated it may have a result on the diet of nestlings in mixed pairs. If their diets are overlapping to a great extent this may mean that competitive exclusion of one species becomes more likely. If I find a difference in diet diversity between nestlings in pure and mixed pairs I will test if this has any effect on the growth of the nestlings. If niche differences constitute a direct benefit for hybrid chicks in flycatchers, I expect that (1) the diet in mixed nests should be more diverse, (2) high dietary diversity should increase the success of the chicks, and (3) the high dietary divesity should be caused by the two species bringing different food-types and not be a result of the environment where mixed pairs breed. The cost of hybridization for the female may also be reduced if heterospecific males provide the offspring with higher quality paternal care than the conspecific males. I will compare feeding frequency between the males of the two species. 5

6 Materials and Methods Study system The flycatcher populations on Öland and Gotland have been studied extensively for over 20 years. Most data used in this study came from the flycatcher population on Öland (57 10 N, E), Sweden, where the proportions of the two species are more equal than on Gotland. Both the collared and the pied flycatchers are sexually dimorphic with black and white males and less conspicuous brown females. The males are easy to distinguish from one another. Collared males have a white collar and larger white forehead patch than pied males. Hybrid males are identified by their incomplete collar (figure 1). Females on the other hand are hard to distinguish in the field (Alatalo et al. 1982), but can be identified in the hand by white in the neck feathers of collared flycatchers. Figure 1. Male collared flycatcher (left), male pied flycatcher (centre) and male hybrid flycatcher (right) (Mullarney et al. 1999). Both species have female mate choice, whereas males of both species show no mate discrimination (Saetre et al. 1997). The first birds return from Africa, where both species overwinter, in the end of April. The males return shortly before the females, to aquire a territory. The collared male is socially dominant in competitive interaction over territories (Alatalo et al. 1994). Collared and pied flycatchers are allopatric on most of the European mainland but there are two separate areas where the two species live in sympatry: the islands of Gotland and Öland in the Baltic Sea, and an area across Central Europe (figure 2). In both areas the collared flycatcher is numerically dominant. On Gotland and in Central Europe they are very dominant, 95% and 85% respectively, whereas on Öland the collared flycatchers make up approximatly 60% of the flycatcher population (Veen et al. 2001). 6

7 Figure 2. Map showing the distrubution of the two species, Pied flycatcher (F. hypoleuca) and Collared flycatcher (F. albicollis). The overlapping areas are the dark areas on the map, Öland, Gotland and a few areas across Central Europe. Ö= Öland and G= Gotland (Alerstam et al. 1978). The hybrid males have reduced fertility, whereas the hybrid females seem to be completely sterile. The origin of this hybrid sterility is probably genetic and caused by incompatibility between genes from the two species (Gelter et al. 1992). The observed fertility pattern in hybrids between pied and collared flycatchers is in accordance with Haldane s rule (Haldane 1922). According to this rule, a higher degree of hybrid sterility is expected in the heterogametic sex than in the homogametic sex in matings between two genetically differentiated populations. In birds the female is the heterogametic sex with ZW chromosomes and the male the homogametic sex, with ZZ chromosomes (Gelter et al. 1992). Methods In the summers of 2004 and 2005 birds were video recorded using IR-light cameras (YOKO model YK-3045B, f = 3.6 mm broad lens). The video camera was placed inside the nest boxes and filmed the birds from above. It was left in the box for one day prior to recording to let the birds get used to it. Each recording were one hour long. Most nests were filmed twice, during different hours of the day. To analyze the videotapes a digital video cassette recorder (Panasonic, DVCPRO model AJ-D230) was used (figure 3). All birds were caught, identified, ringed and weighed using standard methods. The nestlings were 8-10 days old. 7

8 Figure 3a. Still from the video tapes. A collared flycatcher male feeding the nestlings. Figure 3b. Still from the videotapes. A female flycatcher feeding the nestlings. Observations were made from viewings of the video tapes. The feeding frequency of the male and female was noted, as well as which type of prey they brought to the nest. The types of prey were divided into four categories: (1) winged insects, (2) larvae, (3) spiders and ants and (4) unidentified prey. About 55 nests were filmed. For the feeding frequency analysis a larger data set was used adding data from nest watches where feeding rates, i.e. number of visits to the nest by males and females, were observed during one hour periods. The observer sat at least 15m from the nest box and observed with binoculars. Data analysis To test if nestlings of mixed pairs received a greater diversity of diet, Shannon s diversity index (Shannon 1948) was calculated for each nest. In this formula P i is the proportion of all feeds that involved food category i. The three categories of resources used were (1) winged insects, (2) larvae and (3) spiders and ants. The value of H goes from zero and up where zero means no diversity (i.e. only one food type given). Nests with less than 10 feeding events were excluded from the analyses, to prevent them from being assigned low diversity. 8

9 H = 1 p i ln p i Pianka s index (Pianka 1973) was used to calculate niche overlap between the pied and the collared flycatcher. P i is the proportion of a certain resource used by each species; j and k are the two competing species. The same categories of resources were used here as above. P jk Pij 2 ij P ik = 2 ½ ( P P ik ) JMP 5.1 was used for the statstical analyses. Significance was accepted as p=0.05. The following analyses were performed: 1. Shannon diversity index (H) was calculated for three categories of pairs; pure collared pairs, mixed pairs and pure pied pairs. The difference in mean H between the three groups was analyzed using ANOVA. 2. I tested if greater dietary diversity in mixed pairs was due to differences between the species in their diets. The difference in diet between collared males, collared females, pied males and pied females were analyzed, using principal components analysis and MANOVA. The niche overlap in diet was calculated using Pianka s index. 3. I tested the alternate hypothesis that high diet diversity in mixed pairs was due to each individual in mixed pairs having broad diets. Nested ANOVA was performed to analyze the difference in diet diversity (H) between the three groups: pure collared pairs, mixed pairs and pure pied pairs, testing the individual birds in each pair. The effect of lay date and age category was also controlled for. 4. Measures of the nestlings mean weight on day 12 were used to analyze the impact of diet diversity on the fitness of the nestlings. Hatch date and category of pair were also included in the model as covariates. 5. Generalized linear model was used to test if males of the two species differed in the proportion of the feeding workload they carried out, and whether males in mixed pairs carried out more work than those of the same species in a pure pair. Results I compared the mean H for the three categories and found that in support of our hypothesis, the mixed pairs did bring a higher diversity of prey items to the nest than any of the pure pairs (ANOVA, F=4.31, DF=2, p=0.0185) (figure 4). 9

10 ,00 8 0,90 H 0,80 0, ,60 0,50 CF MIXED PF Figure 4. The difference in mean H (Shannon diversity index) between pairs of the three categories; pure collared pairs, mixed pairs and pure pied pairs. The bars show the standard errors. The underlying reason for expecting a higher diversity of diet in the mixed pairs would be because the two species should bring different prey items. I therefore tested if there was any difference in what the males and the females of the two species brought to the nest. However, a MANOVA indicates that there was no significant difference in the diet of either the species, or in the diet of the sexes or in the interaction between species and sex (see table 1). Table 1. The difference in diet between the species, the sexes and the interaction species and sex. Variable Exact F Num. DF Den. DF p Species Sex Species*Sex In fact, Pianka s index indicated that the diets of collared and pied flycatchers overlapped by 99.9%. Figure 5 illustrates this high overlap. Table 2 shows the eigenvectors (principal components) used in the analysis as well as the cumulative percent, which shows the percent of the difference they explain. 10

11 6 PC CF CM PF PM -4 PC 1 Figure 5. The overlap of diet in the four categories. CF =Collared Female, CM =Collared Male, PF= Pied Female, PM = Pied Male. Table 2. The eigenvectors and cumulative percent used in the principal components analysis. Eigenvectors 1 2 Winged insects Larvae and Caterpillars Spiders and ants Cumulative Percent Since there was no differences in the diet of the two species the next test was to see if there would still be a higher diversity in the nests of the mixed pairs if I looked at the birds individually. If this was significant, that would mean that it is not the male and the female that cause the high diversity by bringing different items, but that the individual birds change their diet into a more diverse one once they are in a mixed pair. I tested this the same way as with the pairs (but this time it was a nested ANOVA, to control for the fact that birds from each pair came from the same box and are not independent). There was a significantly higher diversity in the diet of individuals that were in mixed pairs (ANOVA, F=4.716, DF=2, 11

12 p=0.0125). See figure 6. The sample sizes are not double the sample sizes in figure 4, sinze I excluded all individuals that brought less than 10 items to the nest. 0,8 14 0,7 H 0, ,5 CF MIXED PF Figure 6. The difference in mean H (Shannon diversity index) between individuals of the three categories; in pure collared pairs, in mixed pairs and in pure pied pairs. The bars show the standard errors. It has been suggested that mixed pairs breed later in the season than the pure pairs. The females may have a problem finding a mate and therefore choose a heterospecific male, which may lead to a later start of breeding. One reason for the higher diversity of the mixed pairs could be that later in the season certain prey types are not as abundant as early in the season. I controlled for this but found that there was no correlation between diet diversity and lay date (ANOVA, F=1.325, DF=1, p=0.2551). Another possibility is that young birds are more likely to end up in mixed pairs and that their diet may be more diverse because of less experience in foraging. To check for this possibility I tested for the effect of age category on H. The age categories I used were (1) younger than one and (2) older than one. There was no difference in the diet diversity of the two age groups (ANOVA, F=0.144, DF=1, p=0.705). My original hypothesis was that the hybrid offspring may benefit from a higher diversity of diet. To test if this was true I analysed if higher diet diversity had any positive effects on the nestlings mean weight on day 12. I also tested the effect of hatch date and category of pair (pure collared, mixed or pure pied pairs) on the mean weight on day 12 (table 3). There was a significant effect of category of pair on the mean weight as expected. Pied flycatchers usually weigh more than collared. It was surprising that hatch date had no significant effect on mean weight since chicks born early in the season usually weigh more. The analysis showed that once category of pair was controlled for, higher diet diversity did not influence the weight of the chicks in a negative or a positive way. See table 3 for statistical values and figure 7 for the effect of diet diversity on mean weight. 12

13 Table 3. The effect of mean weight on diet diversity (H), hatch date and category of pair (pure collared, mixed and pure pied). Variable DF F p H Hatch date Category of pair Mean weight (g) ,2 0,4 0,6 0,8 1 1,2 H Figure 7. The effect of diet diversity (H) on the mean weight of the nestlings on day 12. In addition to high diet diversity, I also wanted to test an additional factor that could possibly reduce the high cost of hybridization for the female; if the male of one of the species provided the nestlings with better parental care or if the males did a higher proportion of the feedings when they were in a mixed pair. To do this analysis I used a larger dataset that, in addition to the observations from the video tapes, also consisted of observations of male and female feeding frequency from nest watches. Neither the species of the male nor whether they were in a mixed pair or not influenced the proportion of the workload they carried out (figure 8). However, there was a trend that males in mixed pairs had a higher feeding frequency than males in pure pairs. See table 4. 13

14 Table 4. Generalized linear model showing the difference in male feeding frequency of males in mixed pairs compared to males in pure pairs, and males in pure collared pairs compared to males in pure pied pairs. Variable t p Mixed vs Pure pairs Collared vs Pied males % male help CF Pure CF Mixed PF Pure PF Mixed Figure 8. The mean proportion of male feedings in the four pair categories; Pure collared pairs, Mixed pairs with a collared male, Pure pied pairs and Mixed pairs with a pied male. The bars show the standard errors. Discussion The aim of this study was to examine factors that could reduce the cost of hybridization. I tested if hybrid nestlings were provided with a broader diet than nestlings of pure collared or pied pairs. If this was the case, did it lead to any benefits for the hybrid nestlings? The results show that there is a difference in diet diversity between mixed and pure pairs. This higher diversity seems to have no effect on the growth of the nestlings. I had expected this difference in diversity to be because of different niches between the two parental species, but the diet analysis between them showed total overlap. My results showed that it is the individuals in mixed pair that are the reason for the higher diversity. Laydate and age of birds did not influence this result. 14

15 Another factor that could possibly reduce the cost of hybridization is if the heterospecific male spend more energy on paternal care than the conspecific. I tested this and found no significant difference, but a trend of higher male feeding frequency in mixed pairs compared to both types of pure pairs. The principle of competitive exclusion states that two related species cannot co-occur in the same area without any niche separation. In the two species of flycatchers in this study I found no separated niches in diet. According to Pianka s index, there is a 99.9% overlap in diet. Alerstam et al (1978) also used Pianka s index in a study of competition between collared and pied flycatchers. He tested for differences in foraging tactics and the result was a 91% overlap. The slight difference was that the pied flycatcher had a somewhat higher feeding rate on the ground whereas the collared flycatcher fed slightly more in the canopy. This study shows that not only do the two flycatcher species feed in the same way, but they target the same prey items. This suggests that there should be strong competition between the two species. In the zones of sympatry, the collared flycatcher is the numerically dominant species. The numbers of pied flycatchers in these zones seem to be decreasing. Studies have shown that the collared flycatcher is the more aggressive of the two and is dominant at acquiring a territory. The combination of earlier breeding and social dominance may seem enough for the collared flycatchers to outcompete the pied flycatchers in these areas. But other factors are also important. Saetre et al (1999b) studied the mixed population in the Czech Republic and found that there are differences in the main limiting factors for the two species. The pied flycatcher seemed to be limited mostly by interspecific competition as well as intrinsic nestling mortality. The collared flycatcher on the other hand was limited mostly by intraspecific competition, intrinsic nestling mortality and climatic variation. Interspecific competition did not influence the population dynamics of collared flycatchers significantly. Another study by Saetre et al (1999a) shows that the breeding density and distribution of the collared flycatcher correlated strongly with environmental conditions. It decreased from warm to cold habitats. The breeding densities of the pied flycatcher did not correlate with environmental conditions. The conclusion of this study was that during warm periods there is nothing in the way for the collared flycatchers to outcompete the pied flycatchers, but since the climate fluctuates the pied flycatcher can persist. Despite the fact that my results showed no differences in diet niche between the pied and the collared flycatcher, they did show a difference in diet diversity between hybrid nestlings and pure nestlings. Hybrid nestlings are provided with a broader diet. This was the pattern I had expected, but not for the reason I expected. The birds seem to change their diet when in a mixed pair. One possible explanation is that the habitats that mixed pairs inhabit differ from the habitats of pure pairs. The males in mixed pairs may be low status males that have been forced to inhabit a low quality territory. Another possible explanation is that mixed pairs only form in places where flycatchers are in very low densities and it is hard to find a mate. In these places, competition from other pairs is very low, and there is more food available to the mixed pair. My theory was that since two closely related species that co-occur must have different niches, their hybrid offspring may somehow benefit from this, at least at an early life history stage. My test of the effect of a broader diet on the nestling s growth showed no correlation. This was perhaps expected since the two parental species had very similar feeding modes. It is 15

16 possible that there were also other variables, such as the weather, which caused variation in chick mass that was not related to diet diversity. It is also possible that there is a difference in the diet niche between the two species, but that it would need a more detailed prey analysis to discover it. It would also be interesting to include the size of the prey since some birds may be specialized in taking only large prey. What about hybridization in populations where the parental species really have differentiated niches? Are there any other benefits associated with having parents specialized on two different niches? There are studies showing benefits at a later life history stage. A study by Grant (1994) showed that hybrids with intermediate characteristics were better adapted to their habitat than either parental species after environmental conditions suddenly changed. Normally people only think about niche differences being costly for hybrids because they are maladapted to either parental niche. Hybrids risk falling in between the niches, and being unable to use any niche optimally. However, more research is needed to find out if niche differences can be beneficial at an early life history stage. When a female chooses a heterospecific male it leads to a cost since the hybrid offspring have reduced fertility. But there is also a cost of the reproduction event. In pied flycatchers adult mortality peaks during the breeding season (Slagsvold & Dale 1996), suggesting that the cost of reproduction may be substantial in flycatchers. Since hybridization still occurs, the average fitness return from hybridization may outweigh the costs of reproduction so that mating with a heterospecific male is better than to not breed at all. I was interested in finding factors that could reduce the high cost of hybridizing. One such factor was if the male helped feeding the nestlings more in a mixed pair. My result was that there was no significant difference in male feeding frequency between collared and pied males, but the difference between male feeding frequency in mixed and pure pairs was close to significant. I was expecting the males of one of the species to put more effort in feeding the nestlings, whether in a mixed pair or not. One explanation for my results could be that the female in a mixed pair is not willing to invest as much energy in feeding the nestlings as she would in a pure pair. Perhaps the male compensates for this by feeding more. Males are less able to discriminate, and therefore may not be as knowledgeable of their pairing formation. Since this result was close to significant it would be interesting to include more data to see if the pattern is strong. A few other factors that can reduce the cost of hybridization for the female have been suggested. An important factor is the occurence of extra-pair copulations with conspecific males. This has been found in both pure and mixed pairs.whether females in mixed pairs pursue a more active strategy to increase the rate of extra-pair copulation is not known but would be interesting to study. The result is that females in mixed pairs produce a high proportion of non-hybrid offspring, meaning that the cost of heterospecific pairing is substantially reduced (Veen et al. 2001). Conclusion My conclusion of this study is that collared and pied flycatchers are extremely similar ecologically, with a total overlap in diet requirements. In spite of this, when they hybridize their offspring are provided with a broader diet than pure collared or pied offspring. The flycatchers did not seem to gain any direct benefits from mating with a heterospecific partner, but in species with smaller overlap the outcome may be different. The heterospecific males did not bring different food types; neither did they carry out a greater amount of the parental workload. Therefore, there is no evidence in flycatchers that 16

17 costs of hybridization are reduced by direct benefits. This study also suggests that in the future the two species might be unable to coexist without diverging in their dietary niches. Acknowledgements I would like to thank my supervisor Anna Qvarnström and Chris Wiley for ideas, advice and help with the analyses and for comments on the report. Thanks also to Joanna Sendecka for use of the stills from the video tapes. 17

18 References Alatalo, R.V. & Gustafsson, L. & Lundberg, A. (1982) Hybridization and breeding success of collared and pied flycatchers on the island of Gotland, The Auk 99: Alatalo, R.V. & Gustafsson, L. & Lundberg, A. (1994) Male coloration and species recognition in sympatric flycatchers, Proc. R. Soc. Lond. B256: Alerstam, T. & Ebenman, B. & Sylvén, M. & Tamm, S. & Ulfstrand, S. (1978) Hybridization as an agent of competition between two bird allospecies: Ficedula albicollis and F. hypoleuca on the island of Gotland in the Baltic, Oikos 31: Gause, G.F. (1934) The struggle for existence, New York: Hafner. Gelter, H.P. & Tegelström, H. & Gustafsson, L. (1992) Evidence from hatching success and DNA fingerprinting for the fertility of hybrid Pied Collared Flycatchers Ficedula hypoleuca albicollis, Ibis 134: Grant, P.R. & Grant, B.R. (1992) Hybridization of Bird Species, Science 256: Grant, P.R. & Grant, B.R. (1994) Phenotypic and genetic effects of hybridization in Darwin s Finches, Evolution 48: Grant, P.R. & Grant, B.R. (1996) High Survival of Darwin s finch hybrids: Effects of beak morphology and diets, Ecology 77: Haldane, J.B.S. (1922) Sex ratio and unisexual sterility in hybrid animals, J.Genet 12: Mullarney, K. & Svensson, L. & Zetterström, D. & Grant, P.J. (1999) Fågelguiden, Europas och Medelhavsområdets fåglar i fält, Albert Bonniers Förlag, Stockholm. Pianka, E.R. (1973). The structure of lizard communities, Annual Review of Ecology and Systematics 4, Pierotti, R. & Annett, C.A. (1993) Hybridization and male parental investment in birds, The Condor 95: Saetre, G-P. & Král, M. & Bures, S. (1997) Differential species recognition abilities of males and females in a flycatcher hybrid zone, J. Avian Biol 28: Saetre, G-P. & Král, M. & Bures, S. & Ims, R.A. (1999a) Dynamics of a clinal hybrid zone and a comparison with island hybrid zones of flycatchers (Ficedula hypoleuca and F. albicollis), J. Zool. 247: Saetre, G-P. & Post, E. & Král, M. (1999b) Can environmental fluctuation prevent competitive exclusion in sympatric flycatchers? Proc. R. Soc. Lond 266: Shannon, D.E. (1948) A mathematical theory of communication, Bell System Technological Journal 37: Slagsvold, T. & Dale, S. (1996) Disappearance of female pied flycatchers in relation to breeding stage and experimentally induced molt, Ecology 77: Veen, T. & Borge, T. & Griffith, S. C. & Saetre, G-P. & Bures, S. & Gustafsson, L. & Sheldon, B.C. (2001) Hybridization and adaptive mate choice in flycatchers, Nature 411: Wirtz, P. (1999) Mother species father species: unidirectional hybridization in animals with female choice, Animal behaviour 58: