Abel, Daniel C., 67 Aronson, Richard B., 303 Atchison, Gary J., 11 Berkman, Hilary E., 285 Bodkin, James L., 73 Britton, Robert H., 109 Bugaev, Victor F., 241 Cole, Kathleen S., 235 Coutant, Charles C., 161 Crivelli, Alain J., 109 Danzmann, Roy G., 249 Davis, Will iam P., 67 Degani, Gad, 149 Dewees, Christopher M., 313 Duthie, Garry G., 309 Ferguson, Moira M., 249 Fischer, Eric A., 143 Fletcher, Garth L., 295 Golovatjuk, Galina Ju., 241 Grover, Jill, 313 Harris, Margaret A., 173 Hart, Thomas F. Jr., 41 Henry, Mary G., 11 Hlohowskyj, Ihor, 277 Hofer, Rudolf, 209 Jameson, Ronald J., 73 Jastrzebski, Zbigniew T., 66 Kao, Ming H., 295 Klimley, A. Peter, 27 Koenig, Christopher C., 67 Kramer, Donald L., 81 L’AbCe-Lund, Jan Henning, 219 Lejeune, Pierre, 135 Levanon, Dan, 149 Lyons, John, 93 Margraf, F. Joseph, 77 McDonald, Michael E., 229 McNeely, David L., 195 Meffe, Gary K., 3 Meyer, Axel, 127 Moses, Boniface S., 51 Nelson, Joseph S., 173 Quinn, Thomas P., 155 Rabeni, Charles F., 285 Reisman, Howard M., 295 Rheinberger, Veronika, 209 Sanderson, S. Laurie, 303 Sandheinrich, Mark B., 11 Savvaitova, Ksenia A., 241 Schaeffer, Jeffrey S., 77 Shapiro, Douglas Y., 183 Shears, Margaret A., 295 Smith, R. Jan F., 235 Tallman, Ross F., 155 VanBlaricom, Glenn R., 73 Vollestad, Leif Asbjorn, 219 Walsh, Will iam J., 257 Werner, Robert G., 41 Wieser, Wolfgang, 209 Wissing, Thomas E., 277 Ziuganov, Valery V., 241

Alevizon, William, 149 Balon, Eugene, K., 301 Baltz, Donald, M., 101,159 Barnes, Marvin, A., 297 Barwick, D., Hugh, 271 Benz, George, W., 111 Chilton, Glen, 215 Colgan, Patrick, 209, 261 Congleton, James, L., 15 Cumaranatunge, Ruchira, T., 77 De Silva, Sena, S., 77 Foster, George, R., 129 Gee, John, H., 215 Giorgi, Albert, E., 15 Graham, John, H., 137 Grant, James, W., A., 261 Gupta, Awadh, B., 221 Hastings, Robert, W., 137 Jones, Geoffrey, P., 43 Kirschbaum, Frank, 3 Landmeier, Dennis, 149 Lassuy, Dennis, R., 183 Lorenzen, William, E., 271 Luna, Adriana, 243 Maitipe, Parakum, 77 Martin, Kathleen, A., 215 McNicol, Richard, E., 29 Moyle, Peter, B., 101 Mwebaza-Ndawula, Lucas, 69 Neil, Susan, J., 59 Noakes, David, L., G., 29 Persson, Lennart, 305 Pfeiler, Edward, 117, 243 Portt, Cameron, B., 301 Power, Geoffrey, 297 Power, Mary, E., 173 Prejs, Andrzej, 281 Romero, Aldemaro, 203 Ross, Robert, M., 253 Silburt, Barbara, 209 Snelson, Franklin, F., Jr., 89 Srivastava, Anil, K., 221 Subramanian, Annamalai, 195 Takamura, Kenzi, 225 Turner, Bruce, J., 123 Turner, James, S., 89 White, Matthew, M., 123

--Two closely related Old World flycatchers, the Pied Flycatcher (Ficedula hypoleuca) and the Collared Flycatcher (F. albicollis), are allopatric on most of the European mainland but have overlapping ranges in central and eastern Europe and on the island of Gotland in the Baltic. On Gotland, the Collared Flycatcher is approximately 10 times as abundant as the Pied Flycatcher. The two species hybridize (4% of all matings) at frequencies less than those predicted for random mating (13%). Mixed pairs produce as many offspring as pure Pied Flycatcher pairs and more offspring than Collared Flycatchers. The competence at courtship and/or viability of hybrid offspring, however, is probably reduced, because fewer hybrids breed than would be expected from the proportion of hybrid fledglings. SPECIES are defined as intrafertile but noninterhybridizing populations, and interspecific hybrids are rarely encountered in most samples of animals under natural conditions. Among birds, interspecific hybrids occur at a frequency of one in about 50,000 (Mayr 1970). We assume that numerous isolating mechanisms owe their existence to selection against interspecific hybridization, which wastes genes. Yet, some bird species pairs remain distinct over much of their common range despite hybridization in certain regions, perhaps especially those regions in which habitats have recently become greatly modified and/or where sympatry is recent (e.g. West 1962, Short 1969, Mayr 1970, Gill 1980). Similarly, largely allopatric species may hybridize in narrow zones of overlap, in which case it is sometimes a matter of taste whether the two are best regarded as species or subspecies. While situations of this kind are commonly described in the literature, quantitative information about the rate of hybridization and the reproductive output of mixed, as opposed to pure, pairs is almost always wanting. Here we report such details for a case of two European allospecies interbreeding in a zone of sympatry. The species involved are the Pied Flycatcher (Ficedula hypoleuca) and the Collared Flycatcher (F. albicollis). These Old World flycatchers occur as allospecies (semispecies), with ranges overlapping in central and eastern Europe and on the Baltic islands of Gotland and Oland (Fig. 1). Sympatric populations hybridize to some extent (L6hrl 1955, Alerstam et al. 1978). Collared and Pied flycatchers are closely related. Both are highly sex-dimorphic in plumage. Males of the two species are easily distinguished from one another, the male Collared Flycatcher differing from the Pied Flycatcher in having a white collar and rump. On the other hand, females are almost indistinguishable in the field by plumage characters. The songs and alarm calls, however, are highly species-specific. On the island of Gotland, where this study was performed, the total populations of Collared and Pied flycatchers have recently been estimated at 4,000 and 500 pairs, respectively (Gustafsson and H6gstr6m in press). The nearest Collared Flycatcher population is found approximately 600 km southeast of Gotland, while the Pied Flycatcher breeds abundantly on the mainland of northern, central, and eastern Europe (see Fig. 1). Collared Flycatchers thus predominate in this peripheral, isolated area within the range of the more northerly