Egg volume, chick growth and survival across a carrion/hooded crow hybrid zone

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1 Bolletino di zoologia ISSN: (Print) (Online) Journal homepage: Egg volume, chick growth and survival across a carrion/hooded crow hybrid zone Nicola Saino & Anna Maria Bolzern To cite this article: Nicola Saino & Anna Maria Bolzern (1992) Egg volume, chick growth and survival across a carrion/hooded crow hybrid zone, Bolletino di zoologia, 59:4, , DOI: / To link to this article: Copyright Taylor and Francis Group, LLC Published online: 28 Jan Submit your article to this journal Article views: 279 View related articles Citing articles: 8 View citing articles Full Terms & Conditions of access and use can be found at Download by: [ ] Date: 12 January 2018, At: 18:53

2 Boll. Zool. 59: (1992) Egg volume, chick growth and survival across a carrion/hooded crow hybrid zone NICOLA SAINO ANNA MARIA BOLZERN Dipartimento di Biologia, Sezione di Zoologia - Scienze Naturali, Università di Milano, via Celoria 26, Milano (Italy) ABSTRACT The carrion and the hooded crow (Corpus corone corone and Corvus c. comix) are usually considered as morphs of a single species. They have parapatric distributions with narrow belts of overlap where fertile hybrids are produced. This three-year study analyzed the variation of egg volume, and of chick growth and survival among allopatric parental populations and phenotypically parental and hybrid sub-populations in a hybrid zone. Hybrid females laid eggs significantly smaller than those of parental females, living both in areas of sympatry and allppatry. No differences in chick growth were observed between pairs of different composition in the hybrid zone. Nestlings in the hybrid zone were significantly lighter than those in allopatric areas at all ages, and grew to a lower asymptotic weight. Tarsus grew faster and to a higher asymptote in the carrion crow area than in the hooded crow and in the hybrid areas. Chick survival was significantly influenced by female but not by male phenotype. Chicks in hybrid female nests had a comparatively low survival rate. The results indicated that, overall, crows in the hybrid zone are less fit than crows in allopatric areas. In particular, some hybrid females appear to be inferior to parental ones in their laying performance and their chicks are less surviving. The coincidence of the hybrid zone with an ecotone suggests that environmental factors could be effective in determining the observed pattern of variation in the breeding parameters considered. KEY WORDS: Corvus c. corone - Corvus corone comix - Hybrids - Egg volume - Growth and survival. ACKNOWLEDGEMENTS We are grateful to Laura Romagnoli, Simona Villa, Manuela Schlüter, and Laura Scatizzi for help in field work. We would like to thank Prof. Mirella Sari for valuable advice for statistical data analysis. This research was supported by CNR grant No. CT.9O.O32l4l.O (Received 17 September Accepted 9 January 1992) INTRODUCTION The carrion crow (Corvus corone coroné) and the hooded crow (Corvus corone cornix) are usually considered as morphs of a single species. Their ranges are parapatric through continental Europe and Scotland and share relatively narrow zones of overlap (see e.g., Sharrock, 1976; Mingozzi et al., 1989; Fasola, 1989; Saino, 1989) where the parental morphs mate and hybridize. Hybrids are known to be fertile (Picozzi, 1976; Saino & Villa, 1992). In northwestern Italy the contact zone between the parental populations runs at the base of the Alps. It separates the allopatric carrion crow populations mostly inhabiting the alpine valleys and those of the hooded crow inhabiting the intensively cultivated lowlands. Hybrid phenotypes are rather frequent within a strip, a few tens of kilometers wide along the contact zone (Saino, in press; Saino & Villa, 1992). Hybrid zones between distinct species or morphs occur rather frequently in animals (for reviews, see Endler, 1977; Rising, 1983; Barton & Hewitt, 1985). They have often been considered to be due to: 1) secondary contact between populations that have diverged to some extent during a period of geographic isolation or 2) natural selection maintaning steep clines in contiguous populations. Three main hypotheses have been formulated to explain and predict the dynamic of hybrid zones. 1) The 'ephemeral zone' hypothesis states that the hybrid zones will a) become extinct as pre-mating isolating mechanisms evolve as a consequence of selection against hybrids (adaptive speciation model), or b) that they will broaden through introgressive hybridization if divergence between the-populations in contact has not progressed to the extent that it will cause lowered or null hybrid fitness (Dobzhansky, 1940; Sibley, 1957; Remington, 1968). 'This hypothesis is usually hard to test, owing to the lack of long-term historical data on the geography and genetics of most hybrid zones. It has, however, been pointed out that many hybrid zones appear to be geographically stable and persistent,, at least to the relatively short-lived human observer (Remington, 1968). 2) The 'dynamic equilibrium" hypothesis (Bazykin, 1969; Barton, 1979a, b; Barton & Hewitt, 1981) states that persistent and narrow hybrid zones can be maintained by hybrid unfitness, resulting from disruption of coadapted gene complexes, which prevents the hybrid zone from becoming broader through introgressive hybridization. This hypothesis denies any role for ecological selection gradients in the maintenance of the hybrid zones (Moore & Buchanan, 1985). Dispersal from the areas of allopatry, where no selective pressures against hybridization exist, towards the hybrid zone will prevent the evolution of pre-mating isolating mechanisms. 3) In several instances, the hybrid zones have been found to coincide with an ecotone (Moore, 1977). In the

3 408 N. SAINO, A. M. BOLZERN 'hybrid superiority' model (Moore, 1977), ecological selection gradients are such that hybrids are at least as fit as the parental taxa within the hybrid zone but they are selected against outside the hybrid zone. The hybrid 'superiority' is therefore 'bounded' to the ecotone. The Eurasian crow (Corvus corone) has become a paradigmatic example of a species whose morphs have parapatric ranges and narrow zones of hybridization between them. No detailed analysis of these hybrid zones has been carried out until now (Meise, 1928; Cook, 1975; Picozzi, 1976). In this paper, we focus the attention on the variation of some breeding variables across a crow hybrid zone in the northwestern Alps. In particular, we analyze the variation of egg volume, chick growth and chick survival among populations of the parental morphs and hybrids. This study is part of a multi-disciplinary analysis of the carrion/hooded crow hybrid zone in northwestern Italy. STUDY AREAS AND METHODS Study areas The study was carried out in four areas chosen to correspond to the areas of allopatry of the two parental morphs and to two areas in a hybrid zone (Fig. 1). Data on egg size and chick survival were', collected during spring of 1988, 1989 and 1990 in the areas of allopatry, and during spring of 1989 and 1990 in the hybrid areas. Chick growth data were collected in spring of 1989 and All the data from the two areas in the hybrid zone were pooled for statistical analyses. 7-E NW ITALY 9 E Fig. 1 - Study area locations. C all: area of allopatry of the carrion crow; H all.: area of allopatry of the hooded crow. The dotted line represents the estimated position of the axis of the hybrid zone. Elevation contours are indicated. A brief description of the habitat features and composition in the study areas is given here below (see also Saino & Villa, 1992). Area of allopatry of the carrion crow This was an area of 65 km"' covering all the bottom of the Stura di Demonte River valley from 750 to 950 m a.s.l. and a few small lateral valleys of the Stura valley. All the area was in meadows and pastures but a few small maize fields (<1 %) and rye fields or vegetable gardens (<0.5%) also existed. 'Pure' carrion crow phenotypes (see Melde, 1984 and below for a description of the parental and hybrid phenotypes) accounted for more than 99% of the crow population in this area. Area of 'overlap and hybridization' between the parental phenotypes (for simplicity, "hybrid zone') This was an area of about 250 km*' located within the Cuneo-Turin flatland, ranging in altitude from 350 to 650 m a.s.l. The location of hybrid nests indicated that all the study area lay within the hybrid zone. In this area meadows and pastures constituted 43% while the remainder was formed by wheat, barley and rye (25%), maize (22%), seeded grassfields (5%) and poplar plantations. Just a few kilometers outside this study area towards the area of allopatry of the carrion crow, the crow population was composed almost entirely of 'pure' carrion crow phenotypes. Some hybrids or carrion crow phenotypes occurred outside the study area towards the area of allopatry of the hooded crow. Area of allopatry of the hooded crow This was an area of 150 km : located near Milan and Pavia. The area was an intensively cultivated farmland ranging in altitude from 40 to 100 m above sea level. The main crops were maize (39%), rice (36%), wheat and barley (7%); meadows constituted 6% and a smaller percentage consisted of soy bean and poplar plantations. This area was situated at least 50 km from the nearest known carrion crow population in the central Alps. Methods Nests were visited at least six days after their location to be sure that the clutch was complete. Egg length and breadth were measured using Vernier's calipers to the nearest 0.1 mm. A measure of egg volume was obtained applying Hoyt's (1979) formula: Volume = length x (breadth)" x The parents of each nest were scored into carrion crow, hybrid and hooded crow phenotypes according to the characters of their plumage. The carrion crow phenotype has completely black plumage, while the hooded crow phenotype is black on the head, throat, breast, wings and tail. Phenotypically hybrid individuals show all the degrees of intermediacy in plumage between the parental phenotypes with various amounts and patterns of dispersion of black pigmentation on the body feathers (see Melde, 1984 for an accurate description and photographic report of the patterns of variation in pigmentation). Females in the hybrid zone were scored during the incubation period, while males were scored mostly by observing individuals bringing food to the nest after egg-hatch (Saino & Villa, 1992). Chicks were weighed by Pesóla spring balances, and the tarsus was measured at intervals of 3-5 days, from egg hatch to 23 days of age. Tarsus length was measured by bending the foot and the toes and recording the distance between the notch of the intertarsal joint and the bending point of the toes. Crow chicks in the study areas left the nest usually at about days after hatch, but metric data for the last few days of presence in the nest were not collected; nests were just inspected, often by a mirror mounted at the top of a long pole, due to the tendency of some older chicks to leave prematurely the nest if severely disturbed. Adult crows in the study areas are sexually dimorphic, males being

4 CHICK GROWTH AND SURVIVAL ACROSS A CROW HYBRID ZONE 409 on average bigger than females (Saino, unpublished data). Male and female nestlings are also different in size and weight (Richner, 1989a). However, it is difficult in the field to determine the sex of each chick. We pooled the data for all the chicks assuming, as was likely, that the sex ratios in the three areas were similar. In the analyses of egg volume variation and chick survival, five area-phenotype categories (APCs; carrion crow in allopatry, carrion crow in the hybrid zone, hybrid, hooded crow in the hybrid zone, hooded crow in allopatry) and three area categories (area of allopatry of the carrion crow, hybrid zone, area of allopatry of the hooded crow) were adopted. Nests definitely destroyed by humans or by late snow falls (the latter only in the carrion crow allopatric area and in the hybrid zone) were discarded from the analysis of chick growth and survival CARRION CROW ALLOPATRY CARRION CROW HYBRID ZONE Statistical analysis Analysis of variance by a general linear model approach was used to study the variation of egg volume, total clutch volume (the total volume of the eggs in a nest), chick weight and tarsus length among the APCs and area categories. The Bonferroni t test was used to compare egg volumes and chick measurements between APCs and areas. Logistic regression analysis was performed to fit logistic curves to the entire data sets of body weights and tarsus lengths of each area. The logistic (or autocatalytic) growth function adopted was: y = B 0 /[l + B,xEXP(-kxage)] (Draper & Smith, 1981) where B is the horizontal asymptote of the logistic curve and k is the growth rate constant. B, is linked to starting growth value by the relation: B, = (B,,-y,,)/y,,, where y 0 is size at birth. The regression analysis was performed adopting an iterative least-square method. All the analyses were carried out by the SAS computer package (SAS Institute Inc. Version 5 Edition, 1985). RESULTS Egg volume The frequency distribution of egg volumes (Fig. 2) is bimodal for hybrid females whereas parental females of both phenotypes in the hybrid zone and in allopatry had associated unimodal distributions. Intra-APC variances were heteroscedastic (Bartlett's test for homogeneity; F = 1.0; P < 0.001). To correctly perform the analysis of egg volume variation, egg volumes of each APC were weighed for the reciprocal of the square root of the intra- APC variance (Snedecor & Cochran, 1967). The variation of egg volume among the APCs was highly significant (F, 5(U = 23.3; P < 0.001). The pairwise comparisons showed that in the hybrid zone the parental females of both phenotypes laid eggs significantly bigger than those laid by hybrid females and that carrion crows laid eggs significantly bigger than those of hooded crows (Table I). Outside the hybrid zone an asymmetry existed since carrion crow eggs were significantly bigger in allopatry than in the hybrid zone, whereas no such difference between the two populations of hooded crows was observed (Table I). Egg volume significantly varied among the three areas (F, 1V,, = 37.1; P < 0.001). However, a significant difference in egg volume was found only between the area of allopatry of the carrion crow and the other two areas (P < in both cases: Bonferroni t test). Egg volume variation among clutches was highly o LU Ö LÜ er u. Lü LÜ * r HYBRIDS \hr HOODED CROW HYBRID ZONE J hr-tv HOODED CROW ALLOPATRY I I I ' i ' I EGG VOLUME (cm 3 ) Fig. 2 - Frequency distribution of egg volumes for the five APCs. (Sample sizes are in Table I). F.ach bar is 0.67 cm' wide. Hybrid eggs are bimodally distributed. One small egg (9.4 cm') of hooded crow in allopatry and one large egg (26.9 cm') of carrion crow in allopatry are not represented. significant for all APCs. In particular, variation among hybrid female nests (F,. = 11.5; P < 0.001) was such that only 22% of the total egg volume variability was attributable to intra-clutch variation. The total clutch volume showed a borderline nonsignificant variation (F,,, = 2.38; P = 0.052). However, mean clutch volume of hybrids differed significantly from that of carrion crows in the hybrid zone and i

5 410 N. SAINO, A. M. BOLZERN TABLE I - Mean, standard error (SE), coefficient of variation (CV), and sample size (n) of the egg volume (cm 3 ) of the female area-phenotype categories (APCs). Significance of the pairwise comparisons considered (Bonferroni t test). APC* Mean SE CV CY HY Hall Call CY Y HY Hall P<0.005 P<0.001 P<0.01 P<0.01 NS * C all, carrion crow in allopatry; CY, carrion crow in the hybrid zone; Y, hybrids; HY, hooded crow in the hybrid zone; H all, hooded crow in allopatry. The mean egg volume in the hybrid zone was 17.9 (SE = 0.08) cm 3. was markedly lower than that of the parental females (Table II). No significant variation in mean clutch size among the APCs, nor among the areas, was observed. The frequency distribution of clutch sizes was not statistically different among the APCs or the areas (Table III; one clutch of eight eggs for the carrion crow in allopatry and one of seven eggs for the carrion crow in the hybrid zone are not reported in the Table). For all the APCs the modal clutch size was five eggs and the second most frequent clutch size was four eggs. Chick growth We first analyzed variation of chick weight and tarsus length at the different stages of the nestling period among nests of four types of pairs: 1) pairs of carrion crows in allopatry, 2) homogamic pairs containing only parental hooded crow individuals in the hybrid zone, 3) pairs containing at least one hybrid and 4) hooded crows in allopatry. Due to nest destruction, not enough data from homogamic carrion crow nests in the hybrid zone were collected to be analyzed. Since no significant differences were observed for any of the stages of development between group 2 and 3 for tarsus length or for TABLE II - Mean (X), standard error (SE), and sample size (n) oftbe total clutch volume (cm 3 ) of the female area-phenotype categories (APCs). Significance of the pairwise comparisons considered. Abbreviations as in Table I. APC SE CY HY Hall weight, these groups were pooled and the analysis performed again considering only the three geographic areas as distinct groups. In Figures 5 and 6, however, the logistic curves of weight and tarsus length increase are reported for groups 2 and 3 separately, together with those for the pooled group. Chick weight varied significantly among the three areas at all stages of development (Fig. 3). At 2 days chicks in the carrion crow and in the hooded crow allopatry areas were significantly heavier than those in the hybrid zone. At 6 days a significant difference existed only between the hooded crow allopatric area and the hybrid zone, the chicks in the former being heavier than those in the latter. At 9 days carrion and hooded crow chicks in allopatry were significantly heavier than those in the hybrid zone. From 13 days up to 23 days the variation among the three areas was highly significant and all the comparisons between areas revealed significant differences with the following ranking of decreasing weights: carrion crow area > hooded crow area > hybrid zone. The analysis of variance performed for tarsus length disclosed a similar pattern of variation among the areas TABLE III - Relative frequency (%) of clutches of different sizes for the five female area-phenotype categories (APCs). The relative frequencies (%) of the clutches of different size in the hybrid zone are shown in parentheses. Abbreviations as in Table I. Clutch size Call CY APC Y HY Hall Call CY Y HY Hall NS P<0.05 NS NS NS Sample size (n) (2.5) 17.2 (9.5) 27.6 (34.8) 41.4(43.2) 6.9 (8.9)

6 CHICK GROWTH AND SURVIVAL ACROSS A CROW HYBRID ZONE ~ 300 o B37 ess O4S SS7 ± ï,. 53» 10 i 400 O V* Z40 CARRION C D ios HYBRID A A HOODED C. Cf o" AGE (days) J Fig. 3 - Mean body weight (bars represent SE) of the chicks in the area of allopatry of the carrion crow (carrion a), of the hooded crow (hooded c.) and in the hybrid zone (hybrid) at different stages of development. Numbers indicate the sample size; those on the left of the symbols refer to the carrion crow chicks. Below the figure: single lines indicate differences at P < 0.05 level; double lines indicate difference at P < level (Bonferroni f test) g 40 lu z> in 20 0 CARRION C 2B n 34 x «HVBRID \ HOODED C- 5 Q3B D OY nil TS2 \ 10 AGE 04S 15 (days) K o i > 20 nit Fig. 4 - Mean tarsus length (bars represent SE) of the chicks in the three study areas (see Fig. 3) at different stages of development. Numbers and symbols as in Figure 3. up to stage 13 days (Fig. 4). Chicks in the carrion crow and in the hooded crow allopatric areas had significantly longer tarsi than chicks in the hybrid zone at 2 days. At 6 days a significant difference persisted only between hooded crow chicks and chicks in the hybrid zone. From 9 days onwards the ranking of tarsus length was similar to that of chick weight. Significant differences were detected between the areas at all stages except between carrion crow and hooded crow allopatry areas at stages 17 and 23 days. The logistic curves fitted to weight data differed in their parameters (Table IV, Fig. 5). In particular, chicks in o AGE (days) Fig. 5 - Logistic curves fitted to chick body weight data by logistic regression analysis. C: carrion crow chick in allopatry; H, hooded crow chicks in allopatry; P, chicks of homogamic hooded crow pairs in the hybrid zone; Y, chicks of pairs containing at least one hybrid parent; P + Y, chicks 'P' and chicks 'Y' pooled AGE (days) Fig. 6 - Logistic curves fitted to chick tarsus length data by logistic regression analysis. Legends as in Figure 5. the carrion crow area and those in the hybrid zone had very similar k values (i.e., they took similar times to reach their weght asymptote). However, carrion crow chicks gained weight much more quickly than hooded crow chicks. By comparison, the asymptote (B o ) for chicks in the hybrid zone was much lower than that of either the chicks in the hooded crow or carrion crow allopatric areas. The asymptotic weight of fledglings in the hybrid zone was just 74% of the asymptotic weight of allopatric carrion crow chicks and 83% of hooded crow chicks. Tarsus growth was similar to body weight results with carrion crow chicks having the highest k value and reaching the highest asymptote (Table IV, Fig. 6). Tarsi of chicks in the hooded crow area grew faster than those of chicks in the hybrid zone, but reached a slightly lower asymptote. Weight of adult crows in the three areas Body weights from a sample of crows killed in the allopatry and in the hybrid areas were measured im-

7 412 N. SAINO, A. M. BOLZERN TABLE IV - Estimated parameters of the logistic functions fitted to body weight and tarsus length growth data in the three study areas (see 'Statistical analysis' for details). TABLE VI - Mean (X). standard error (SE), and sample size (n) of the chick survival rate (chicks fledged/clutch size) for the five female area-pbenotype categories (A PCs) and for the three areas. Significance of variation (Kruskalt-Wallis test). Abbreviations as in Table I. B, k APCs Three areas Weight Carrion crow allopatry Hybrid zone Hooded crow allopatry Tarsus length Carrion crow allopatry Hybrid zone Hooded crow allopatry i Call CY Y HY Hall X SE n X SE n Significance of variation H = 28.6; d.f. P < = 4 H = = 22.6; d.f. - 2 P < TA DI 17 V. Mon** fv\ etsivt/isivri weights (in grams) of female and male crows killed in the three areas at the beginning of the breeding season. In parentheses the size of the samples. Significance of the variation among areas. Area Female Male Carrion crow allopatry Hybrid zone Ó.O Hooded crow allopatry Significance of variation SE = SE = NS mediately after death. Weights of the females varied significantly among the areas (Table V).; Females in the hybrid area were heaviest and allopatric carrion crow females were lightest. Male mean weights were very similar in the three areas, and despite the small sample, it seems clear that no trends of variation existed. Chick survival and parents'phenotype Chick survival rate, expressed as the number of chicks fledged/clutch size, are binomial proportions, and normality of errors cannot be assumed a priori. To analyze the variation of survival rate among APCs and among areas, a non-parametric test was used. There was significant variation in chick survival among the female APCs (Table VI). Chicks in hybrid female nests had a survival rate lower than chicks in parental female nests in both allopatric areas and in the hybrid zone. Hooded crows in allopatry and carrion and hooded crows in the hybrid zone had very similar survival rates. Carrion crow females in allopatry had comparatively high survival rates. On the contrary, no significant effect of male APCs on chick survival was found. Chick survival significantly varied among the three areas (P < 0.001; Kruskall-Wallis test). However, it was highest for the carrion crow area while no marked differences occurred between the hybrid zone and the hooded crow allopatric area. DISCUSSION In birds, several sources of variation are known to affect the volume of the eggs laid by an individual female. The ratio of additive genetic to the total phenotypic variance ('heritability') of egg volume determined in field and aviary studies is high (Ojanen et al., 1979; van Noordwijk et al, 1980; Moss & Watson, 1982). On the other hand, an intricate network of environmental factors and maternal phenotypic traits correlated with egg volume. Egg volume positively correlates with female size and weight in hooded crows (Loman, 1984). The size of a bird is also an inherited character (Smith & Zach, 1979; Garnett, 1981; but see also Ricklefs & Peters, 1981); yet it is under strong ontogenetical control since rearing conditions can markedly affect the size of the adult (Richner, 1989a). Weight at breeding can be causally related to size. Bigger females are expected to be heavier but they can also be better nourished since, being dominant (Richner, 1989b), they have easier access to food sources during shortage periods or can hold better territories than smaller ones. Therefore, the volume of the eggs is affected by the size of the female and by its weight which, in turn, possibly reflects territory quality and dominance relationships. Furthermore, differences in egg volume may reflect different breeding strategies. Females can, with similar efforts, either produce a large clutch of

8 CHICK GROWTH AND SURVIVAL ACROSS A CROW HYBRID ZONE 413 relatively small eggs or invest more per egg and produce a small clutch of big eggs. Thus, three aspects of the pattern of variation of egg volume observed in this study deserve close consideration. 1) The overall variation across the hybrid zone. The possibility that such variation in egg volume is related to female size is not yet properly assessable since an extensive morphological and osteometrical study of the crow populations across the northwestern alpine hybrid zone is only now in progress. 'However, tarsus length, which is strictly related to other linear body measures did not vary significantly among females from the three study areas (Saino, unpublished data). No statistical significance nor tendency for female weight to vary across the hybrid zone has been observed in this study. 2) The asymmetry existing on the two sides of the hybrid zone in the variation of egg volume. Carrion crows in allopatry laid eggs significantly larger than those in the hybrid zone whereas hooded crows did not so. Overall, carrion crows laid eggs significantly larger than those laid by hooded crows. Under the hypothesis that egg size is under strong genetic control, the lower mean volume of eggs laid by female carrion crows in the hybrid zone than in allopatry could be explained by introgression from the hooded crow population. Introgression should be mirrored by a higher variability in egg volume in the carrion crow population living in the hybrid zone than in allopatry. However, the coefficients of variation of egg volume had exactly the same value for the two area-phenotype categories. Differences in egg volume could also be due to differences in habitat characteristics or to different reproductive tactics. Between the two populations of the carrion crow considered the existence of a sharp change in habitat composition is striking. Carrion crows in allopatry inhabited Alpine valleys where meadows and pastures were the prevailing foraging habitat available, whereas the carrion crows in the hybrid zone live in finely patched agricultural land. However, the effect of habitat should result in carrion crows in the hybrid zone having also lower total clutch volumes than those in the allopatry area. When the mean total clutch volumes of the two APCs were considered comparatively, no statistically significant difference was observed, and carrion crows in the hybrid zone performed, on average, a higher laying effort. Therefore, it could be argued that the difference observed is the result of different reproductive tactics adopted by the two populations. 3) The comparatively low mean egg volume and the bimodal egg volume frequency distribution for hybrid females. It must be emphasized that, in condition of coexistence (i.e., in the hybrid zone), macrohabitat characteristics exert an even influence on the laying performance of the phenotypes. The possibility that differences in egg volume between parental and hybrid phenotypes, within the hybrid zone, are due to microhabitat factors cannot be ruled out. However, this should indicate that parental phenotypes have easier access to preferred resources (favourable breeding habitats) partly as a consequence of the dominance relationships among the phenotypes (Saino & Scatizzi, 1991). The observed bimodality and the within-clutch egg volume variation showed that some hybrid females laid very small eggs as compared to the parental females in the hybrid zone. Chances of chick survival to the fledging age are known to correlate positively to the volume of the egg from which they hatch in many bird species (e.g., Parsons, 1975; Lundberg & Väisänen, 1979; Ankney, 1980). Large eggs have a comparatively large amount of yolk reserves that can help prevent chick starvation in the days after hatching, if food is scarce. The low mean volume of the eggs laid by hybrid females could partly contribute to determine low survival (measured as number of chicks fledged/number of eggs laid) of their chicks, as well as other factors, such as egg shell frailty observed in some hybrid female nests (Saino & Villa, 1992). Chicks in the hybrid zone were lighter and had shorter tarsi than chicks in the allopatry areas soon after birth (age 2 days). This was probably due to variation in egg volume across the hybrid zone and to the contribution, to the sample, of some small chicks hatched from hybrid female eggs. This evidence is consistent with Rofstad & Sandvik (1987) who found that hooded crow chicks hatched from bigger eggs are bigger and heavier. However, no influence of egg volume or weight on survival, size, and weight of older chicks has been observed in other bird species (e.g., Galbraith, 1988; Greig-Smith et al., 1988). The effect of egg size on chick size, weight, and survival, therefore, must be restricted to the days immediately after hatching and subsequent differences between areas could be imputed to habitat factors. Body weight and size at fledgling age can have relevant, independent, consequences on parents' fitness. Natural and sexual selection will act on these variables in different ways. Body weight at fledging probably affects the short-term chances of young to survive after fledging (Perrins, 1965, 1966; Westmoreland & Best, 1987; Krementz et al., 1989; but see also Leikoinen 1986; Newton & Moss, 1986). Chicks in our hybrid zone reached a lower asymptote for body weight than those in the parental allopatric areas. Their constant rate of weight increase was, however, very similar to the growth rate of the carrion crows and higher than that of hooded crows in allopatry thus suggesting that only the fledgling final weight noticeably varied among allopatric and hybrid zone populations. Tarsus length at fledging remains constant for the rest of life in many passerines and also in carrion crows (Richner, 1989a). We confidently assume that this holds true also for the hooded crows and hybrids we studied.

9 414 N. SAINO, A. M. BOLZERN Body size - as indexed by tarsus length - will affect the chances an individual has to obtain and successfully hold a territory (Richner, 1989a). Individuals with long tarsi are more likely to get a territory than are those with short ones, because they are dominant. Tarsus length of fledglings is therefore a long-term factor affecting parents' fitness. The equations of the logistic curves fitted to tarsus measurements of the three populations had similar growth constant rates and no relevant differences in their horizontal asymptote was detected, except for a small difference between the area of allopatry of the carrion crow and the other areas that could however reflect geographic variation in crow morphology across the hybrid zone. This study provided evidence that some hybrid females are reproductively inferior, with respect to the parental females living in the areas of allopatry or in the hybrid zone, because they lay comparatively small eggs which are likely to result in fewer surviving chicks. In fact, chicks in hybrid female nests were more likely to die before fledging than chicks in parental female nests. However, no significant effect of male's phenotype on chick survival was observed. Hybrid reproductive disadvantage in crows was, therefore, sex-related. Similar evidences have been reported for other avian and for non-avian taxa (e.g., Ashburner et al., 1982; Moore & Koenig, 1986; Alatalo et al., 1990). Carrion crows in allopatry had an associated chick survival rate higher than that in the hybrid zone. Since the chicks in hybrid female nests and those in 'pure' hooded crow pair nests grew in a similar way, and to similar asymptotes, it is presumable that ecological factors in the hybrid zone did not act differentially on the two kinds of chicks during the nest period. The fact that hooded crows and hybrids in the hybrid zone (not enough data were available on chick growth in 'pure' carrion crow nests in the hybrid zone) produced lighter chicks that reached a lower weight asymptote, than chicks in allopatric areas suggests that crows within the hybrid zone were at a reproductive disadvantage in respect to those in the areas of allopatry. Carrion crows in allopatry had an associated chick survival rate much higher than that of carrion crows in the hybrid zone; this evidence could reflect the effect of environmental features during breeding period in the hybrid zone and in the carrion crow allopatric area. In conclusion, our study supports the idea that in the hybrid zone the ecological conditions are such that the parental phenotypes are less adapted, in various respects, than the parental populations in the allopatry areas. Evidences from the present and other studies carried out on the same hybrid zone (e.g., Saino & Villa, 1992) also suggest that hybrids tend to experience a somewhat reduced breeding success with respect to parental phenotypes within the hybrid zone. However, these evidences are not conclusive with respect to reduced hybrid fitness. Breeding success is just one of the components of fitness and hybrids could be at a selective advantage in other phases of the life cycle (Saino, 1992). REFERENCES Alatalo R. V., Eriksson D., Gustafsson L., Lundberg A., Hybridization between Pied and Collared Flycatchers - sexual selection and speciation theory. J. evol. Biol., 3: Ankney D. C., Egg weight, survival and growth of Lesser Snow Goose goslings. J. Wildl. Manage., 44: Ashburner M., Carson H. L., Thompson J. N. Jr, (eds.), The genetics and biology of Drosophila. Vol. 3b. Academic Press, London. Barton N. H., 1979a - Gene flow past a cline. Heredity, 43: Barton N. H., 1979b - The dynamic of hybrid zones. Heredity, 43: Barton N. H., Hewitt G. H., Hybrid zones and speciation. In: W. R. Atchley & D. S. Woodruff (eds.), Essays on evolution and speciation in honor of M. J. D. White. Cambridge Univ. Press., Cambridge, pp Barton N. H., Hewitt G. H., Analysis of hybrid zones. Ann. Rev. Ecol. Syst., 16: Bazykin A. D., A hypothetical mechanism of speciation. Evolution, 23: Cook A., Changes in the carrion/hooded crow hybrid zone and the possible importance of climate. Bird Study, 22: Dobzhansky T., Speciation as a stage in evolutionary divergence. Am. Nat., 74: Draper N. R., Smith H., Applied regression analysis. John Wiley & Sons, New York. Endler J. A., Geographic variation, speciation, and clines. Princeton Univ. Press, Princeton. Fasola M., Cornacchia grigia - Corvus corone comix. In: M. Fasola & P. Brichetti, (eds), Atlante degli uccelli nidificanti in Lombardia. Editoriale Ramperto, Brescia, pp Galbraith H., Effects of egg size and composition on the size, quality and survival of lapwing Vanellus vanellus chicks. J. Zool., Lond., 214: Garnett M. C., Body size, its heritability and influence on juvenile survival among great tits, Parus major. Ibis, 123: Greig-Smith P. W., Feare C. J., Freeman E. M., Spencer P. L., Causes and consequences of egg size variation in the European Starling Sturnus vulgaris. Ibis, 130: Hoyt D. F., Practical method of estimating volume and fresh weight of bird eggs. Auk, 96: Krementz D. G., Nichols J. D., Hines J. E., Postfledging survival of European starlings. Ecology, 10: Lehikoinen E., Is fat fit? - a field study of survival and fatness in the great tit, Parus major L. Ornis Fenn., 63: Loman J., Breeding success in relation to parent size and experience in a population of the hooded crow. Ornis. Scand., 15: Lundberg C. A., Väisänen R. A., Selective coloration of egg size with chick mortality in the Black-headed Gull (Larus ridibundus). Condor, 81: Meise W., Die Verbreitung der Aaskrähe (Formenkreis Corvus corone L.). J. Ornitol., 76: Melde M., Raben- und Nebelkrähe. Die neue Brehm-Bücherei, Wittenberg Lutherstadt. Mingozzi T., Boano G., Pulcher G., Atlante degli uccelli nidificanti in Piemonte e Val d'aosta. Monog. VIII Mus. Reg. Sci. Nat., Torino. Moore W. S., An evaluation of narrow hybrid zones in vertebrates. Q. Rev. Biol., 52: Moore W. S., Buchanan D. B., Stability of the northern flicker hybrid zone in historical times: Implications for adaptive speciation theory. Evolution, 39: Moore W. S., Koenig W. D., Comparative reproductive success of yellow-shafted, red-shafted, and hybrid flickers across a hybrid zone. Auk, 103: Moss R., Watson A., Heritability of egg size, hatch weight, body weight, and viability in red grouse (Lagopus lagopus scoticus). Auk, 99:

10 CHICK GROWTH AND SURVIVAL ACROSS A CROW HYBRID ZONE 415 Newton I., Moss D., Post-fledging survival of sparrowhawks Accipiter nisus in relation to mass, brood size and brood composition at fledging. Ibis, 128: Noordwijk A. J. van, Balen J. H. van, Scharloo W., Heritability of ecologically important traits in the Great Tit, Parus major. Ardea, 68: Ojanen M., Orell M., Väisänen R. A., Role of heredity in egg size variation in the Great Tit Parus major and the Pied Flycatcher Ficedula hypoleuca. Ornis. Scand., 10: Parsons J., Asynchronous hatching and chick mortality int he herring gull Larus argentatus. Ibis, 117: Perrins C. M., Population fluctuations and clutch size in the great tit, Parus major. J. anim. Ecol., 34: Perrins C. M., Survival of young manx shearwaters Puffinus puffinus in relation to their presumed date of hatching. Ibis, 108: Picozzi N., Hybridization of Carrion and Hooded crows Corvus c. corone and Corvus c. cornix in northeastern Scoltand. Ibis, 118: Remington C. L., Suture zones of hybrid interaction between recently joined biotas. In: T. Dobzhansky, M. K. Hecht & W. C. Steere (eds.), Evolutionary biology, Vol. 2. Appleton-Century- Crofts, New York, pp Richner H., 1989a - Habitat specific growth and fitness in carrion crows (Corvus corone corone). J. anim. Ecol., 58: Richner H., 1989b - Phenotypic correlates of dominance in carrion crows and their effects on access to food. Anim. Behav., 38: Ricklefs R. E., Peters S., Parental components of variance in growth rate and body size of nestling European Starlings (Sturnus vulgaris) in Eastern Pennsylvania. Auk, 98: Rising J. D., The Great Plains hybrid zones. In: R. F. Johnston (ed.). Current ornithology. Vol. I. Plenum Publ. Corp., New York, pp Rofstad G., Sandvik J., Morphology of hatchling hooded crows and its relation to egg volume. Condor, 89: Saino N., Cornacchia nera - Corvus corone corone. In: M. Fasola & P. Brichetti (eds.), Atlante degli Uccelli Nidificanti in Lombardia. Editoriale Ramperto, Brescia, 205 pp. Saino N., Selection of foraging habitat and flocking by crow (Corvus corone) phenotypes in a hybrid zone. Ornis. Scand., 23: (in press). Saino N., Scatizzi L., Selective aggressiveness and dominance among carrion crows, hooded crows and hybrids. Boll. Zool., 58: Saino N., Villa S., Pair composition and breeding success across a hybrid zone between carrion crows and hooded crows. Auk, 109: Sharrock J. T. R., The atlas of breeding birds in Britain and Ireland. T. Calton & H. D. Poyser, London. Sibley G. F., The evolutionary and taxonomic significance of sexual dimorphism and hybridization in birds. Condor, 59: Smith J. N. M., Zach R., Heritability of some morphological characters in a Song Sparrow population. Evolution, 33: Snedecor C. W., Cochran W. G., Statistical methods. Iowa Univ. Press, Iowa. Westmoreland D., Best L. B., What limits mourning doves to a clutch of two eggs? Condor, 89: