MOLT DURING SPRING MIGRATION: A COMPARISON OF FOUR SPECIES OF RAPTORS

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1 J. Field Ornithol., 72(1): MOLT DURING SPRING MIGRATION: A COMPARISON OF FOUR SPECIES OF RAPTORS EDNA GORNEY AND YORAM YOM TOV Dept. of Zoology, Tel Aviv Univ., Tel Aviv 69978, Israel Abstract.We quantified the extent of molt in Steppe Buzzards (Buteo buteo vulpinus), Marsh Harriers (Circus aeruginosus), Levant Sparrowhawks (Accipiter brevipes), and Eurasian Sparrowhawks (A. nisus) caught during spring migration in Elat, southern Israel. Ten percent of yearling Steppe Buzzards (58 of 550) and four percent of yearling Marsh Harriers (3 of 77) were actively molting their remiges while on migration. These findings are contrary to suggestions that birds do not molt their flight feathers during migration when they should avoid extra energy expenditure and maintain flight performance. Active molt of primaries and secondaries, however, was not found among adult buzzards and harriers, or in any individual of the two Accipiter species. Molt strategies may be related to differences in flight mode during migration between species that primarily soar (buzzards and harriers) and species that flap more (sparrowhawks). Body condition was not related to the state of molt. This, and the fact that only yearling birds were in active molt of remiges, suggest that a delay in arrival on the breeding grounds may be a significant cost of molt during spring migration. MUDA DURANTE LA MIGRACIÓN PRIMAVERAL: COMPARACIÓN ENTRE CUATRO ESPECIES DE RAPACES Sinopsis.Cuantificamos, durante la migración primaveral, la extención de la muda de Buteo buteo vulpinus, Circus aeruginosus, Accipiter brevipes y A. nisus. El trabajo se llevó a cabo en Elat, al sur de Israel, entre 1985 y El 10% de los B. b. vulpinus, de primer año (88 de 550) y el 4% de los C. aeruginosus (3 de 77) estaban mudando las remeras durante el período migratorio. Nuestros hallazgos son contrarios a las sugerencias que se han hecho con respecto a que las aves no mudan durante el período migratorio para evitar gastar exceso de energía durante estos vuelos. No se encontró muda activa de las primarias ni de las secundarias en adultos de ninguna de estas cuatro especies. Las diferencias en las estrategias de mudas entre las especies pudiera estar relacionada a diferencias en los patrones de vuelo durante la migración. Las especies un B. b. vulpinus, y C. aeruginosus planean primordialmente mientras que las dos especies de Accipiter tienden a batir las alas. La condición corporal no estuvo relacionada con el estado de muda. Esto, unido al hecho de que solamente los juveniles de primer año se hallaron mudando, sugiere que la llegada tardía al área de reproducción pudieran ser el costo significativo de mudar durante la migración primaveral. Molt in birds is associated with elevated energy costs because of (1) synthesis of new feathers (reviewed by Lindstrom et al. 1993), (2) increased energy costs of thermoregulation due to a decrease in insulation capacity of the plumage (Payne 1972), and (3) increased cost of flight due to reduction of wing area and/or wing span (Ginn and Melville 1983). Another cost associated with molt is a higher risk of predation due to lower mobility (Newton 1966; Haukioja 1969; Vega Rivera et al. 1998). In most temperate bird species, the energetically expensive activities of breeding and migration do not overlap significantly with molt (Stresemann 1966; Payne 1972; King 1974; Kjellen 1994). However, molt of flight feathers during migration has been reported for a small number of species (e.g., Traylor 1972; Hyytia and Vikberg 1973; Sealy 1979; Koopman 1986; Yuri and Rohwer 1997). We quantified molt during in several species of migrating 96

2 Vol. 72, No. 1 Raptor Molt [97 raptors in southern Israel in order to determine (1) the pattern and stage of molt in adults and yearlings, (2) the relationship between molt and body condition, and (3) the relationship between molt and mode of flight during migration. METHODS Migrating raptors were trapped during spring migration (March May) in the agricultural areas just north of Elat (29 33 N, E), Israel, 20 m above sea level. From 1985 to 1988 we captured 766 Steppe Buzzards (Buteo buteo vulpinus) (Shirihai 1996) and 109 Marsh Harriers (Circus aeruginosus), mainly in bal-chatri traps (Berger and Mueller 1959) and bow nets (Bloom 1987), and 262 Levant Sparrowhawks (Accipiter brevipes) and 59 Eurasian Sparrowhawks (A. nisus) mainly in mist nets and also in bal-chatri traps and bow nets (see Gorney et al. 1999). All birds were measured, weighed to the nearest 1 g and banded before release. Wing chord (straight line from wrist to wing tip, without stretching or flattening the wing) was measured with a ruler, to the nearest 1 mm. Culmen length was measured to the nearest 0.1 mm using Manostat calipers. Age and sex were determined by plumage, eye color and size (wing chord and body length; Cramp and Simmons 1980). Age was determined as either adult (2 yr old or more) or yearling (approximately 1 yr old; birds in their second calendar year). In some cases, based on plumage and feather retainment, birds were determined to be in their third calendar year. We classified molt as (1) no molt, (2) body moltonly body feathers were new and/or growing, (3) interrupted moltrecently replaced fully grown remiges and/or rectrices but not all feathers replaced, and (4) activeremiges and/or rectrices missing or part-grown. Birds in interrupted and active molt were usually also molting body feathers. Molt was examined in 77 yearlings and 33 adult Steppe Buzzards, randomly chosen throughout the entire migration season. Molt was scored and averaged for both wings according to Ginn and Melville (1983): old feathers were scored 0; missing and growing feathers were scored between 1 and 4; and new, fully grown feathers were scored 5. Because buzzards have 10 primaries on each wing, the maximum mean primary molt score was 50 (all primaries new and fully grown). Body condition was evaluated using a condition index. The index was calculated as body mass divided by a multiplication of wing chord by culmen. This index highly correlated with total body fat extracted from a sample of nine Steppe Buzzards (Gorney and Yom Tov 1994). Sample sizes from the condition index may be lower than the total number of birds in the different molt categories because sometimes not all measurements were taken, and for these individuals it was impossible to calculate the index. Results are reported as means and standard deviations (SE). We used nonparametric Kruskal-Wallis and two-tailed Mann-Whitney tests (Abstat

3 98] E. Gorney and Y. Yom Tov J. Field Ornithol. Winter 2001 rel. 5.09, 1987, AndersonBell, Parker, Colorado USA), and two-tailed 2 tests (Siegel and Castellan 1988). RESULTS Steppe Buzzard.Of 216 adult buzzards, 199 were in interrupted molt of remiges and rectrices. The primary molt was asymmetrical. None of the adults was in active primary molt. Eleven birds retained juvenile remiges and rectrices and were probably in their third calendar year (approximately 2 yr old). Of these individuals, 10 showed symmetrical primary molt. Interruption of primary molt usually occurred after completion of 7 8 new primaries. None of the third year birds were in active primary molt. Molt in adults (mean primary molt score SE, , n 33) was generally less advanced than that of birds in their third year ( , n 11, Mann-Whitney U 110, P 0.05). Out of 550 yearling Steppe Buzzards, all but eight individuals were in various stages of body molt, and 109 (19.8%) were in interrupted or active molt of the primaries. The proportion of yearlings in active primary molt (58 of 550) was significantly larger than among adults (0 of 216; , df 1, P 0.001). No yearlings of the two species of sparrowhawks were molting primaries, a significant difference from yearling buzzards ( , df 1, P 0.001). The proportion of yearling buzzards in active molt of remiges was not different than in Marsh Harriers ( , df 1, P 0.1). The proportion of yearling buzzards in active primary molt increased from 7.7% (28/363) in March April to 16.1% (30/186) in May ( , df 1, P 0.01). However, we found no difference in proportion of birds in interrupted molt in March April (32 of 363) and in May (19 of 186; , df 1, P 0.7). Fewer yearlings (51 of 550) than adults (199 of 216) were in interrupted molt ( 2 480, df 1, P 0.001). Fifty three of 77 yearlings whose primary molt was scored had a score equal to or smaller than 5 (maximum one new completed primary on each wing) and 18 individuals had a score equal to or smaller than 10 (maximum two new completed primaries on each wing). Only six yearlings were in more advanced stages, with a primary molt score higher than 10. We found no significant difference in mean primary molt score between March and April ( , n 43) and May ( , n 34; Mann-Whitney U 659, P 0.46). In general, primary molt in buzzards started at the first (innermost) primary and proceeded outward. Primary molt in migrating yearlings was symmetrical in 38% of birds showing active molt (n 50) and in 59% of birds showing interrupted molt (n 27). Thus, it appears that in many cases one wing was more advanced than the other wing during active molt, whereas molt was often interrupted in a symmetrical situation. There was no significant relationship between molt category and mean condition index in Steppe Buzzards (Kruskal-Wallis , df 3, P 0.59; Table 1). Marsh Harrier.We found no significant differences in the proportion

4 TABLE 1. Mean physical condition index (body mass/wing chord culmen) of yearling Steppe Buzzards and Marsh Harriers in four molt categories: interrupted molt of remiges and/or rectrices (I), active molt of remiges and/or rectrices (A), molt of body feathers only (B) and no molt found (N). Data are presented as mean SE (sample size in parentheses). Species I A B N Steppe Buzzard (49) Marsh Harrier Adult (5) Adult (6) (12) (20) (58) (422) (8) (1) (3) (6) (12) (23) (12) (4) (1) Vol. 72, No. 1 Raptor Molt [99

5 100] E. Gorney and Y. Yom Tov J. Field Ornithol. Winter 2001 of Marsh Harriers in interrupted molt between adults (37.5%; 12 of 32) and yearlings (40.2%; 31 of 77; , df 1, P 0.95) or between s (38.5%; 25 of 65) and s (40.9%; 18 of 44; , df 1, P 0.95). Active molt occurred in one of 32 adults (3%) and in 10 of 77 yearlings (13%; 2 1.5, df 1, P 0.2). The only adult in active molt was growing one rectrix, probably lost by accident. Six yearling harriers in active molt were also growing only rectrices. Two yearlings were growing one secondary each, and one yearling had new left and right rectrices #1, new left and right primaries #1 and 2, and was growing primary #3 in both wings. We found no differences in body condition indices within age and sex groups between individuals in different molt categories (Table 1; Kruskal-Wallis for yearling s: , df 2, P 0.92; for yearling s: , df 3, P 0.35; for adult s: , df 3, P 0.41; adult s were all in interrupted molt). Levant Sparrowhawk.Approximately a third of Levant Sparrowhawks were in interrupted molt: 26.7% of yearling s (20/75); 22.2% of yearling s (14/63); 35.9% of adult s (28/78); and 67.4% of adult s (31/46). We found no significant differences between the sexes in proportion of birds in interrupted molt ( 2 2.6, df 1, P 0.05). However, a larger proportion of adults than yearlings were in interrupted molt ( , df 1, P 0.001). Out of three adults in active molt, two molted only one rectrix each. We checked details of interrupted molt of remiges and rectrices in 30 adults. Primary flight feathers were of the same age in all adults (all new in 26 of 30 and all old in four). Secondaries were usually of mixed ages (21) but also all new (9). Tail feathers were of mixed ages in all 30 adults. In contrast to yearling Steppe Buzzards, no yearling Levant Sparrowhawk was in active molt of primaries. Molt included either body feathers only or was interrupted molt that involved mostly rectrices. Eleven yearling s molted only rectices, and one had one new secondary (#4 on the right wing). Tail molt began often (7 of 11) with rectrix #1, however, four s began tail molt with rectrices number 2,3,3, and 5, respectively. Similarly in yearling s, all new feathers were tail feathers (16 of 16), and only one also had a new secondary (#6 on left wing). In 11 of 16 s, rectrice molt began with rectrix #1. Exceptions included four s with new rectrix #2 and one with #3. We found no differences in mean body condition indices between individuals in interrupted molt and others (Table 2). s and s were in either body or interrupted molt, and no differences were found within the sexes (Mann-Whitney U 321, P 0.8 for s; U 397.5, P 0.54 for s). We also found no difference in condition index between adult s in interrupted, active, body and no molt categories (Kruskal-Wallis , df 3, P 0.9). Adult s that showed no signs of molt or were in interrupted molt did not have a higher condition index than adult s in active and body molt (Kruskal- Wallis , df 3, P 0.08). Eurasian Sparrowhawk.Only adults were in interrupted molt (7 of 16

6 TABLE 2. Mean physical condition index (body mass/wing chord culmen) of Levant Sparrowhawks and Eurasian Sparrowhawks in four molt categories: interrupted molt of remiges and/or rectrices (I), active molt of remiges and/or rectrices (A), molt of body feathers only (B) and no molt found (N). Data are presented as mean SE (sample size in parentheses). Species I A B N Levant Sparrowhawk Eurasian Sparrowhawk Adult Adult Adult Adult (29) (26) (14) (18) (7) (5) (1) (2) (1) (2) (11) (19) (48) (49) (5) (3) (22) (7) (3) (29) (2) (4) (1) Vol. 72, No. 1 Raptor Molt [101

7 102] E. Gorney and Y. Yom Tov J. Field Ornithol. Winter 2001 s and 5 of 12 s). Detailed molt pattern was checked in a few individuals only. As found in Levant Sparrowhawks, primaries were all new, secondaries were all new or a mixture of new and old feathers, and rectrices were always a mixture of old and new feathers. Two adult s were actively molting one rectrix each, five were molting only body feathers, and two showed no signs of molt. Three adult s were molting body feathers only, four showed no signs of molt, and none was in active molt. All seven yearling s and 23 of 25 yearling s were molting body feathers only. One yearling showed no signs of molt, and one had one new rectrix. Among adult Eurasian Sparrowhawks, we found no differences in condition index between individuals in different molt categories (Kruskal-Wallis for s: , df 3, P 0.36; for s: , df 2, P 0.47; Table 2). DISCUSSION Ten percent of yearling Steppe Buzzards were actively molting flight feathers during spring migration in Elat. In contrast, yearlings of two species of sparrowhawks investigated during this study did not molt remiges while on migration. This difference in molt between buzzards and sparrowhawks may be related to a difference in flight mode during migration. Levant Sparrowhawks spend 39% of their flight flapping (combined with gliding or circling) compared to only 4% in Steppe Buzzards (Spaar 1997). Because soaring flight requires less energy than flapping flight (Kerlinger 1989), and because growing or missing feathers may have a greater impact on flapping than on soaring flight, it is possible that molt is less costly for yearling buzzards (and harriers) than for yearling accipiters. Marsh Harriers flap during 12% of their flight. The fact that no difference was found in proportion of molting individuals between buzzards and Marsh Harriers is probably because both of these species behave like typical soaring migrants compared to accipiters and other species of harriers (Spaar and Bruderer 1997a). The energetic cost of growing a few feathers may be relatively low compared to the cost of migration and may not involve mass loss (Payne 1972). Indeed we found no difference in physical condition indices between individuals in molt and not in molt. However, no adult buzzard and harrier actively molted remiges during spring migration. This suggests that a more serious cost may be a decrease in migratory speed as a result of extra energy expenditure and flight impairment (Verbeek and Morgan 1980; Tucker 1991). If flight speed is reduced, molting birds may arrive later on the breeding grounds. This may result in reduced breeding success, which typically declines in the temperate zone as the breeding season progresses (Price et al. 1988). This may be one reason why in most raptor species studied, adults migrate before yearlings during spring migration (Newton 1979; Kerlinger 1989; Gorney and Yom Tov 1994; Gorney et al. 1999). In agreement with this, time minimization during migration was found to be important in migrating harriers (Spaar and Bruderer 1997b).

8 Vol. 72, No. 1 Raptor Molt [103 Since birds typically avoid molting during migration (Payne 1972; King 1974; Berthold 1975; but see Traylor 1972; Hyytia and Vikberg 1973; Sealy 1979; Koopman 1986; Yuri and Rohwer 1997), what could be the benefit of molting during spring migration? In general, large birds have larger wing loading and thus must replace their flight feathers more slowly in order to maintain flight ability (Kjellen 1994). Therefore, young buzzards and harriers may be under stronger selection to begin molt earlier than the smaller accipiters. A recent model suggested that birds will start the period of the annual cycle that is most crucial for overall fitness with a set of fresh feathers (Holmgren and Hedenstrom 1995). For large raptors that season may be the breeding season and having a set of fresh feathers during breeding may result in a greater increase in fitness than at other times of year. Several studies, including ours, have found that Steppe Buzzards do not complete their post-breeding molt in the breeding grounds and resume molt in the wintering grounds (Broekhuysen and Siegfried 1970; Cramp and Simmons 1980; Schmitt et al. 1980). Most adult buzzards examined in Elat were in interrupted molt of remiges and rectrices. The primary molt was asymmetrical, in agreement with data from the African wintering grounds (Schmitt et al. 1980). Birds in their third calendar year showed symmetrical primary molt, as was reported previously for this species (Schmitt et al. 1980). We found that mean primary molt score in adult buzzards trapped during spring migration was lower than in buzzards in their third calendar year, and the same was found for buzzards in their wintering grounds in southern Africa. Birds in their third calendar years are mostly non-breeders, and they are probably able to spend more energy on molt and begin primary molt earlier than adults. As a result they are able to grow more primaries and thus have a higher primary molt score than older individuals. Most second-year birds arrive on the wintering grounds with five to eight new primaries, and majority of these birds complete primary molt in the wintering grounds (Schmitt et al. 1980). Mean primary molt score of migrating adults in Elat ( , n 33) was higher than on the wintering grounds in South Africa ( , n 10). This is expected since new fully grown feathers (during migration in Elat) get a higher score than growing feathers (on the wintering grounds). In contrast, the mean primary molt score of buzzards in their third calendar year in Elat was lower than in South Africa. It is possible that the primaries that were replaced in spring were scored as new while in southern Africa and as old by us. It is also possible that we were sampling birds that wintered in a different location. The most likely explanation, however, is that third year buzzards with a complete new wing and no retained juvenile feathers were aged by us as adult birds. We could identify as third year birds only the subsample of individuals that were behind in their molt and retained some juvenile feathers. Migrating with a new set of flight feathers is advantageous (Kjellen 1994). Adult Levant Sparrowhawks and European Sparrowhawks com-

9 104] E. Gorney and Y. Yom Tov J. Field Ornithol. Winter 2001 plete their molt prior to autumn migration (Cramp and Simmons 1980; Newton and Marquiss 1982; Newton 1986). We found that the primaries of adults of these two species are always of the same age. However, secondaries and rectrices are of mixed ages and show interrupted molt. This molt pattern could be due to three processes. The first is that older birds may not molt a complete set of secondaries and rectrices each year. The second is that they do molt a complete set, but the appearance of feathers of different ages is a result of a long molt period starting in early spring and ending just before autumn migration. Thus, early spring feathers would look older than feathers molted just before autumn migration. Finally, individuals may also molt in the wintering grounds, as is known for long-distance migratory populations of Peregrine Falcons (Falco peregrinus) and Steppe Buzzards (Alerstam 1990). In support of the latter possibility, 25% of yearling Levant Sparrowhawks molt rectrices in the wintering grounds, and two individuals had also replaced one secondary each. In contrast, most yearling Eurasian Sparrowhawks were molting body feathers only. These different molt patterns may represent differences between a short- and a long-distance migratory species. However, some Levant Sparrowhawks do complete molt, as we found in the only we trapped in Elat during autumn migration. This had all new adult remiges and rectrices but retained a few juvenile body feathers. Different populations of Levant Sparrowhawks may have different molt schedules. Molt data from the wintering grounds is needed in order to resolve these questions. ACKNOWLEDGMENTS We thank William S. Clark and all the other tireless field workers that helped us trap migrating hawks. We also thank Kibbutz Elot for allowing the research to be conducted in their fields. This study was funded by the Israel Raptor Information Center of the Society for the Protection of Nature in Israel, the Inter-University Ecological Fund of the Jewish National Fund, and the Elat Ornithological Center. LITERATURE CITED ALERSTAM, T Bird migration. Cambridge University Press, Cambridge, United Kingdom. BERGER, D.D.,AND H. C. MUELLER The bal-chatri: a trap for the birds of prey. Bird Banding 30: BERTHOLD, P Migration: control and metabolic physiology. Pp in D. S. Farner and J. R. King, eds. Avian biology, vol. 5. Academic, New York. BLOOM, P. H Capturing and handling raptors. Pp , in B. A. Giron-Pendelton, B. A. Millsap, K. W. Cline, and D. M. Bird, eds. Raptor management techniques manual. National Wildlife Federation, Washington D.C. BROEKHUYSEN, G.J., AND W. R. SIEGFRIED Age and molt in the Steppe Buzzard in southern Africa. Ostrich Suppl. 8: CRAMP, S., AND K. E. L. SIMMONS The birds of the western Palearctic, vol. 2. Oxford University Press, Oxford, United Kingdom. GINN, H. B., AND D. S. MELVILLE Molt in birds. BTO Guide No. 19. British Trust for Ornithology, Tring, United Kingdom. GORNEY, E., AND Y. YOM TOV Fat, hydration condition, and molt of Steppe Buzzards Buteo buteo vulpinus on spring migration. Ibis 136:

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