Comparative reproductive success of Yellow-shafted, Red-shafted, and hybrid flickers across a hybrid zone

-Alternative hypotheses of hybrid zones make specific predictions about reproductive components of fitness in the hybrids. The dynamic-equilibrium and reinforcement hypotheses are premised on reduced hybrid fitness, which should be apparent as reduced clutch or brood size or as increased embryonic mortality. The hybrid-superiority and introgression hypotheses predict normal clutch and brood size and embryonic mortality. Reproductive success was measured at four study sites on a transect across the hybrid zone between the Yellow(Colaptes auratus auratus) and Red-shafted (C. a. cafer) subspecies of the Northern Flicker. Two additional clutch size samples representing pure Yellowand Redshafted flickers were obtained from museum egg collections. Mean clutch size did not differ significantly among the six samples. Factorial ANOVAs showed that early clutches and broods are larger than late clutches and broods, but no significant difference was detected between hybrid and parental study sites. Analyses of the effect of phenotype (yellow-shafted, red-shafted, hybrid) also suggest that neither clutch size nor brood size is affected, with the exception that hybrid males sired significantly smaller broods. Finally, there were no significant effects of type of cross (red-shafted male x hybrid female, etc.) on the ratio broodsize / clutch-size. The only evidence for reduced hybrid fitness was in the test where males with hybrid phenotypes appear to have sired small broods. This may indicate that abnormal behavior of hybrid males affects female fecundity, but it is also plausible that this marginally significant result is a type I statistical error. The overall lack of evidence for reduced hybrid fitness is inconsistent with either the dynamic-equilibrium or reinforcement models. Of the two remaining alternatives, the bounded hybrid-superiority model appears the more likely explanation of the Northern Flicker hybrid zone because earlier work (Moore and Buchanan 1985) showed that the hybrid zone is not becoming broader, as predicted by the introgression model. Received 20 February 1985, accepted 2 July 1985. HYBRID zones often occur between avian populations that have diverged to near the species level but retain contiguous distributions as a result of either secondary contact or parapatric divergence (Short 1969, Moore 1977, Rising 1983a). Typically, hybridization is rampant in the zone but the zone itself is very narrow and seems to act as a barrier separating markedly divergent plumage patterns or song types, or both. Furthermore, the biogeography of most avian hybrid zones suggests that they arose in antiquity, probably as a result of Pleistocene glaciation, and therefore seem to be evolutionarily stable configurations (Short 1970, Moore 1977; but see Barrowclough 1980). Three theories have been proposed to explain the existence of stable hybrid zones. Moore (1977) suggested that hybrid zones represent secondary contacts between taxa that had diverged in isolation but not to the extent that hybridization would disrupt distinctly coadapted gene complexes resulting in hybrid breakdown or hybrid unfitness. He further suggested that the secondary contact was established as distinct ecological communities expanded from refugia to form a "suture zone" in the sense of Remington (1968) and that the genes of the divergent characters are either adapted in their respective communities or closely linked to genes that are. The hybrid zone persists, then, because it occurs in an ecotone in which neither parental taxon is particularly well adapted. This was termed the hybrid-superiority model, although bounded hybrid superiority is more apt because it implies that hybrid superiority is restricted geographically. Barton (1979) and Barton and Hewitt (1981) 42 The Auk 103: 42-51. January 1986 This content downloaded from 157.55.39.28 on Thu, 15 Dec 2016 04:53:04 UTC All use subject to http://about.jstor.org/terms January 1986] Reproduction in Hybrid Flickers 43 developed an alternative hypothesis that explains stable hybrid zones by balancing opposing forces. The essential feature of the BartonHewitt dynamic-equilibrium model is hybrid unfitness, which prevents the hybrid zone from becoming broader through introgressive hybridization; less-fit hybrids form a narrower hybrid zone. Barton (1979) showed that hybrid unfitness itself can fix the width of the hybrid zone and further showed that variations in population density could prevent the hybrid zone from "flowing" geographically. The third alternative is that the hybrid zone appears stable only to a short-lived observer but actually is growing broader through introgressive hybridization or becoming more restricted as premating reproductive isolation is reinforced by selection against hybrids (Wilson 1965, Remington 1968). Whether introgression or reinforcement is expected would depend on whether hybrid phenotypes are at least as fit as the parental phenotypes or less fit, respectively. The three alternatives provide radically different explanations of hybrid zones, each with its own implications for speciation theory. Furthermore, the alternatives appear to be testable, as the dynamic-equilibrium and reinforcement models predict some measure of hybrid breakdown, whereas the hybrid-superiority and introgression models do not. More precisely, according to the dynamic-equilibrium and reinforcement models, one would expect increases in infertility and development aberrations in hybrid phenotypes. The pairs of alternatives can be tested further by studying the historical stability of the hybrid zone. The idea that hybridization between distinctly coadapted gene pools results in hybrid breakdown is an old idea in evolutionary biology that is of particular importance to speciation theory. Collectively, the assorted maladies that are expected to result are termed postmating reproductive isolating mechanisms. Although hybrid breakdown could manifest at any stage in the life cycle, the maladies most often mentioned affect the early development or fertility of hybrid phenotypes. Thus, where natural hybridization has disrupted coadapted gene complexes, one would expect higher frequencies of developmental anomalies and reduced fertility. In birds this should appear as reduced brood and clutch sizes, respectively. We report clutchand brood-size data for four populations of the Northern Flicker (Colaptes auratus) along a transect across the hybrid zone between the Yellowand Red-shafted flickers (C. a. auratus and C. a. cafer) in western Nebraska and eastern Wyoming. The two central locales support hybrid populations, whereas the distal locales represent parental populations (see Fig. 1). Our purpose is to determine whether hybridization between these wellmarked subspecies has disrupted coadapted gene complexes and whether hybrid breakdown is essential to understanding this hybrid zone. Data pertaining to historical stability were reported earlier (Moore and Buchanan 1985). The structure of the hybrid zone has been reported by Short (1965) and Moore and Buchanan (1985). To summarize briefly, the Yellow-shafted Flicker is broadly distributed in diverse woodland types across eastern North America, whereas the Red-shafted Flicker is the western North American counterpart. The hybrid zone occurs on the western Great Plains, primarily in riparian woodlands (cottonwood, peach-leaved willow, and green ash) but also in the coniferous forests of the Black Hills, the Pine Ridge region of Nebraska and South Dakota, and in the Cypress Hills of Saskatchewan. The hybrid zone may be continuous in the solidly forested montane regions of Alberta, British Columbia, and southern Alaska (Short 1965, Moore pers. obs.). The two subspecies exhibit contrasting conditions for 6 (5 in females) plumage characters; e.g. red-shafted males have a red malar stripe, whereas that of the yellowshafted is black. Hybrids exhibit intermediate variations. These characters are easily scored, even with binoculars. Mating is random with regard to the plumage characters (Short 1965, Bock 1971, Moore and Buchanan 1985). The hybrid zone varies in width and is asymmetrical, with introgression evident farther west of center than east (see Fig. 1). Population densities are uniformly high along the study transect (Moore and Buchanan 1985). MATERIALS AND METHODS Active nests were located throughout May and June in 1981-1984 at four study sites on a transect across the hybrid zone (Fig. 1). The study sites are: Sutherland (James Haugland farm on the south bank of the South Platte River between the river and interstate highway I-80, 1.75 km south-southeast of Southerland, Lincoln Co., Nebraska, R33W, T14N, secs. 32, 33); Bridgeport (Blanchard-Lindgren ranch on the This content downloaded from 157.55.39.28 on Thu, 15 Dec 2016 04:53:04 UTC All use subject to http://about.jstor.org/terms 44 MOORE AND KOENIG [Auk, Vol. 103 CONTOUR MAP OF FLICKER HYBRID ZONE