Hybridization and Breeding Success of Collared and Pied Flycatchers on the Island of Gotland

Hybridization and Breeding Success of Collared and Pied Flycatchers on the Island of Gotland

HYBRIDIZATION AND BREEDING SUCCESS OF COLLARED AND PIED FLYCATCHERS ON THE ISLAND OF GOTLAND RAUNO V. ALATALO, LARS GUSTAFSSON,AND ARNE LUNDBERG Departmentof Zoology,Uppsala University, Box 561, S-75122 Uppsala,Sweden ABSTRACT.--Twoclosely related Old World flycatchers,the Pied Flycatcher(Ficedula hy- poleuca)and the Collared Flycatcher(F. albicollis),are allopatricon most of the European mainland but have overlappingranges in centraland easternEurope and on the island of Gotland in the Baltic. On Gotland, the Collared Flycatcheris approximately 10 times as abundantas the Pied Flycatcher.The two specieshybridize (4% of all matings)at frequencies lessthan those predicted for random mating (13%). Mixed pairs produceas many offspring as pure Pied Flycatcherpairs and more offspring than Collared Flycatchers.The competence at courtshipand/or viability of hybrid offspring, however, is probably reduced, because fewer hybrids breed than would be expectedfrom the proportion of hybrid fledglings. SPECIES are defined as intrafertile but non- onthe Baltic islands of Gotland and Oland (Fig. interhybridizing populations,and interspecif- 1). Sympatric populations hybridize to some ic hybrids are rarely encounteredin most sam- extent (L6hrl 1955, Alerstam et al. 1978). ples of animals under natural conditions. Collared and Pied flycatchersare closely re- Among birds, interspecifichybrids occur at a lated. Both are highly sex-dimorphicin plum- frequencyof one in about 50,000 (Mayr 1970). age. Malesof the two speciesare easilydistin- We assume that numerous isolating mecha- guishedfrom one another, the male Collared nisms owe their existenceto selectionagainst Flycatcherdiffering from the Pied Flycatcherin interspecific hybridization, which wastes having a white collar and rump. On the other genes.Yet, somebird speciespairs remain dis- hand, femalesare almost indistinguishablein tinct over much of their commonrange despite the field by plumage characters.The songsand hybridization in certain regions, perhaps es- alarm calls, however, are highly species-spe- pecially those regions in which habitats have cific. recentlybecome greatly modified and/orwhere On the island of Gotland, where this study sympatry is recent (e.g. West 1962, Short 1969, was performed, the total populationsof Col- Mayr 1970, Gill 1980). Similarly, largely allo- lared and Pied flycatchershave recently been patric speciesmay hybridize in narrow zones estimated at 4,000 and 500 pairs, respectively of overlap, in which case it is sometimes a (Gustafssonand H6gstr6m in press).The near- matter of taste whether the two are best re- est CollaredFlycatcher population is found ap- garded as speciesor subspecies.While situa- proximately 600 km southeast of Gotland, tions of this kind are commonly described in while the Pied Flycatcherbreeds abundantly the literature, quantitative information about on the mainland of northern, central, and east- the rate of hybridization and the reproductive ern Europe (see Fig. 1). Collared Flycatchers output of mixed, as opposed to pure, pairs is thus predominate in this peripheral, isolated almost always wanting. Here we report such area within the range of the more northerly details for a case of two European allospecies Pied Flycatcher. interbreedingin a zone of sympatry. In the present paper, we attempt to deter- The speciesinvolved are the Pied Flycatcher mine whether or not (1) birds mate assorta- (Ficedulahypoleuca) and the Collared Flycatch- er (F. albicollis).These Old World flycatchers tively and (2) breeding successof interspecific occuras allospecies(semispecies), with ranges pairs is reduced,as would be expectedfor true overlappingin centraland easternEurope and species. 285 The Auk 99: 285-291.April 1982 286 ALATALO,GUSTAFSSON, AND LUNDnEa6 [Auk, Vol. 99 • Pied flycatcher :.•!• Collaredflycatcher 0 500 1000 k m Fig. 1. Breedingranges of Collaredand Pied Flycatchers(based on Alerstam et al. 1978, but modified from Creutz 1970 and Glowacinski 1974). The island of Gotland is encircled. STUDY AREA AND METHODS Male hybrids can easily be identified from their vague whitish collars. In the hand, hybrid females Collared and Pied flycatchersare especially suit- can be distinguishedby the amount of white on their able for study becausethey use tree holes as nest sites. With nest boxes, one is able to control the neck feathers(see Svensson 1975). Collared Flycatch- er males were classifiedas old or yearlings on the whole population,because the birds seldomnest in basis of the color of all primary coverts (Svensson natural cavities when nest boxes are present. We 1975), while Pied Flycatchermales were aged from erecteda total of 435 nest boxesin two major forest the innermost primary covert (Alatalo et al. MS). habitats in southern Gotland (57ø10'N, 18ø20'E). In deciduouswoodlands the nest box density was 6/ha and in coniferous forest, 1.5 nest boxes/ha. The de- RESULTS ciduous woodlands are dominated by oak (Quercus robur)and ash (Fraxinusexcelsior), with a denselower Hybridizationfrequency.--The occupancy of layer of hazel (Corylusaveliana) and hawthorn(Cra- nest boxes and the breeding densities of dif- taegusspp.). The coniferousforest is dominatedby ferent pair combinations in the two habitats pine (Pinussylvestris), but birch(Betula pubescens) is are given in Table 1. The breeding density of also common. flycatcherswas 20 times higher in deciduous The arrival of the flycatcher males and their oc- than in coniferousforest, and many more nest cupationof territorieswere monitoredby censuses boxes remained unoccupiedin the latter hab- everysecond day. For each nest we recordedthe on- itat. The Collared Flycatcher was almost 30 set of laying, clutch size, hatching success,and times as abundant as the Pied Flycatcher in fledglingnumbers. We capturedand bandedall fe- deciduous forest, while in the coniferous area males and almost all males during the nestling pe- riod. In some caseswe did not succeedin capturing we found only three times as many Collared the male,because polygamous males seldom feed the Flycatchers.The proportionof mixed pairs was youngof their secondnests. The speciesidentity of much higher in coniferous forest than in de- these males was clear, however, from our census rec- ciduousforest (17% and 2.5%, respectively). ords. In addition, we observed four nests in which April1982] Hybridizationin Flycatchers 287 TABLE71. Breeding densities of Pied (PF)and Collared(CF) flycatchersand of mixed pairs in differenthabitats on the island of Gotland. Percentage Size of occupancyby Density of breeding study Number Density of pairs/ha areas of nest Fly- All flycatcher Habitat (ha) boxes catchers species pairs/ha CF x CF CF x PF PF x PF Deciduous woodland 37 243 48.6 75.7 3.19 3.00 0.08 0.11 Coniferous woodland 116 192 9.9 24.0 0.16 0.10 0.03 0.03 one parentbird apparentlywas a hybrid; these habitat, so comparisonsmust take the time fac- pairs are not included in Table 1. tor into account. Our data are extensive for the If the individuals of the two speciesmated Collared Flycatcher, and we therefore use randomly, one would expect 13% of the pairs regressionequations for clutch size and fledg- to be mixed (Table2). The observedproportion ling numbers versus the laying date of this of mixed pairs was 4.4%, however, which de- species as a reference line. The regression viates significantly(X 2 = 7.66, P < 0.01) from equation for clutch size is: y = -0.078x + 6.77 random. Thus, there is an overrepresentation (SD = 0.66, n = 123), where x is laying date of intraspecificmatings, interbreedingoccur- (day 1 = 20 May). Pied Flycatchershad, on av- ring only in 36% of the number of casesex- erage,0.71 (SD = 0.84, n = 7) more eggsthan pectedif randommating is assumed.We may simultaneously laying Collared Flycatchers expect underrepresentationof mixed pairs, (Fig. 2). This differenceis significant(t = 2.70, simply becausethe frequenciesof occurrence P < 0.01). The clutch size of mixed pairs was of the two speciesare not identicalin the two intermediatebetween that of pure pairs of the habitats. In each habitat, however, the ob- two species(0.46 higher than for the Collared served number of mixed pairs was 29% (de- Flycatcher, SD = 0.65, n = 7, t = 1.81, ciduous: X2 = 5.11, P < 0.05) and 44% (conif- P < 0.10). erous:X 2 = 2.51,P > 0.10), lower than expected Hatching successof the flycatcherswas high, if random mating is assumed. In 5 of the 7 and no difference emerged among the pair mixed pairs (including 1 pair outside the main combinations(Table 3). The regressionline for study areas),the male was a CollaredFlycatch- fledgling numbers of the Collared Flycatcher er and the female a Pied Flycatcher.Moreover, decreasedmore rapidly with time than the line of all Pied Flycatchers,30% were involved in for clutch size (Fig. 3; regressionequation for interspecificmatings, whereas the correspond- number of fledglings: y = -0.128x + 6.43, ing figure for the CollaredFlycatcher was only SD = 1.71, n = 123). Early breeders presum- 2%. Of malesinvolved in interspecificmating, ably gained from an outbreakof caterpillars, 6 out of 7 (86%) were yearlings. For all intra- which levelled off in the second half of June. specificpairs this proportionwas lower (55%), but the differenceis not significant(Fisher ex- act, P = 0.12). Breedingsuccess.--Average clutch size and TABLE2. Observed and expected mating combina- tions of flycatchers. fledgling numbers of intra- and interspecific pairs are given in Table 3. We have combined Pure Pure the data from the two habitats, because we Collared Pied found no differencesin breeding successbe- Fly- Fly- tween habitats. Lundberg et al. (1981) found catcher catcher Mixed higher breedingsuccess for the Pied Flycatcher pairs pairs pairs in deciduous than in coniferous habitat, but Number found 123 7 6 the differencewas small. Laying date affects Percentagefound 90.4 5.1 4.4 breeding successmuch more profoundly than Percentageexpected 86.3 1.0 12.7 288 AL^•r^LO,GUS•r^FSSON, ^tad LUNDBERO [Auk, Vol. 99 •S4 i *%,0>% ß M,xed pa,r o Pied flycatcher ß Mrxed parr LAYING DATE 24 29 3 8 13 18 MAY JUNE LAYING DATE Fig. 3.

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