Ardeola 48(1), 2001, 81-84
HOW DOES THE EXTENT OF A PARTIAL MOULT VARY? SOME DATA FOR MELODIOUS WARBLERS HIPPOLAIS POLYGLOTTA IN CENTRAL IBERIA ¿CÓMO VARÍA LA EXTENSIÓN DE UNA MUDA PARCIAL? ALGUNOS DATOS DEL ZARCERO COMÚN HIPPOLAIS POLYGLOTTA EN EL CENTRO DE LA PENÍNSULA IBÉRICA
Jesús PINILLA*
Partial moults are known to be affected in their timing, progress and extent by many factors ranging from sex, age or fitness to season or time of breeding (Dhondt, 1973; Baillie & Swann, 1980; Aidley & Wilkinson, 1987; Espie et al., 1996; Merilä, 1998). These moults, though mainly regarded as adjustments in response to age and seasonal changes (see, e.g., Ginn & Melville, 1983; Middleton, 1986), may also play an important role in the replacement of feathers especially exposed or worn, particularly in long-distance migrants that undertake a complete moult soon after arrival at the wintering grounds (Pearson, 1973; Jenni & Winkler, 1994). The Melodious Warbler Hippolais polyglotta is one of the few European passerine species known to undergo three moults subsequently: a partial post-breeding moult in the breeding grounds involving body feathers; a complete moult in early winter in Western Africa; and an additional partial moult in late winter affecting body feathers, greater coverts and/or tertials and secondaries (Cramp, 1992; Jenni & Winkler, 1994; Gargallo, 1995). However, it is still discussed whether these moults actually represent separate processes or different stages of the same process (Jenni & Winkler, 1994; Norman, 1997; W. Salewski, pers. com.). The extent of the additional/complementary partial pre-breeding moult in the Melodious Warbler has been described by Gargallo (1995) for a pooled group of birds comprising different years and localities. It is suggested that there may exist a geographical variation in the extent of this moult, related to the latitude of the bird’s breeding grounds.
In order to know more details about these moult processes, in the present study I have tried to (1) characterise the partial pre-nuptial moult in birds from Central Iberian populations and (2) explore the factors that could affect its extent. Birds were trapped for ringing purposes in four localities of Central Iberia during the springs of 1997 to 1999 (Rivas-Vaciamadrid, Madrid —30TVK 561631—; Santa Olalla, Toledo —30TUK 790306—; Colmenarejo, Madrid —30TVK 139912—; and Barajas, Madrid —30TVK 547797—). In these places, 86 birds that could be assigned to local populations were ringed. Assignment to local populations was established by means of (1) further recaptures at the site during the breeding season and/or (2) sexual characters (incubation patch and/or cloacal shape), that were also used to sex them (Svensson, 1996). The following measurements were also taken: third primary length, to the nearest 0.5 mm (Jenni & Winkler, 1989); tarsus length with toes bent, to the nearest 0.05 mm (Svensson, 1996); and weight to the nearest 0.25 g. To obtain an index of body condition, the bird weight was regressed on its tarsus length and the residuals of this relationship were used (Lindén et al., 1992; Gosler, 1994). Replacement of feathers later than the complete moult (recognised by the degree of wear) was recorded for all birds, all flight feathers having been considered for this (primaries —PP— numbered descendantly; secondaries —SS—, including tertials —TT—, ascendantly; rectrices —RR— centrifugally) and greater coverts —GC— 1 to 9, numbered to-
* Centro de Migración de Aves (SEO/BirdLife). C/ Melquiades Biencinto, 34. E-28053 Madrid, Spain. E-mail:
[email protected]
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wards the body. Both wings and both sides of the tail were checked in 25 birds, whereas only the right wings and tail sides were checked in the remaining 61, so that only data for the right side feathers were used for analysis. In 87.2% (75 out of 86) of the studied birds, at least one of the considered feathers had been replaced, and in 14 out of 25 (61%) this partial moult had been symmetric. These results match with those obtained by Gargallo (1995) in a pooled group of birds from different populations. It seems, thus, that the occurrence and the symmetry of the partial moult studied is not conditioned by the origin of the birds. Among the sample, no primary or rectrix replacement was observed, so the feathers replaced fell within the following tracts, ordered by the frequency of occurrence: TT (81.3%; 70/86), GC (38.3%; 33/86) and SS (9.3%; 8/86). Details on the number of feathers replaced in each tract are shown in Table 1. Again, compared with the data from Gargallo (1995) the proportions of birds that had renewed some of those feathers are similar. Considering feathers individually (see Fig. 1), the extent of GC and SS replacement takes place from the body outwards, whereas in TT it follows the typical sequence of partial moult seen in small passerines: 8-9-7 (Jenni & Winkler, 1994). In order to explore the factors that could affect the extent of this partial moult, I have used three different variables (sex, body condition and season) against three measurements of the moult extent: number of moulted GC, TT and SS.
The analysis showed no differences between sexes in the extent of the moult in any of the considered feather tracts: GC (Z = –0.37, P = 0.70); TT (Z = –1.49, P = 0.13); and SS (Z = –1.03, P = 0.30; Mann-Whitney U-tests; Zar, 1999). These results were somehow expected as the species has no sexual dimorphism in plumage characters. Likewise, there was no significant correlation between the number of renewed feathers in each tract and the body condition of birds: GC (rs= –0.04, P = 0.78); TT (rs = 0.07, P = 0.63); and SS (rs = –0.02, P = 0.89; Spearman rank correlations; Zar, 1999). It is probable that body condition in the breeding grounds may not be related to a physiological process that takes place some time (one to three months) earlier. Among years, however, I have found statistically significant differences in the number of replaced feathers. Not so in GC (H = 3.50, P = 0.17; Kruskal-Wallis test; Zar, 1999), but in TT (H = 6.65, P < 0.05), and especially in SS (H = 11.82, P < 0.01). These differences among years (see Table 2) suggest that the extension of the partial moult could be affected by the environmental conditions that the birds face in their winter quarters. Better environmental conditions may allow more extensive moults, as is suggested by the fact that longer and, thus, more energy-demanding feathers are those whose replacement extent varies most. Finally, it would also be interesting to consider other variables, like age or body condition at the time of moulting (both impossible to determine in the breeding grounds), that could also play an important role in the extent of the partial moult.
TABLE 1 Descriptive statistics of the number of feathers replaced in the partial pre-breeding moult of the Melodious Warbler (n = 86) within each feather tract. GC: Greater Coverts; TT: Tertials; SS: Secondaries. [Estadísticos descriptivos del número de plumas reemplazadas en la muda parcial prenupcial del Zarcero Común para cada grupo de plumas. GC: Coberteras mayores; TT: Terciarias; SS: Secundarias.]
Mean [Media] Mode [Moda] Freq. of mode [Frecuencia de la moda] Minimum [Mínimo] Maximum [Máximo]
GC
TT
SS
TOTAL
0.47 0 53 0 3
1.55 2 47 0 3
0.19 0 78 0 3
2.20 2 33 0 6
HOW DOES THE EXTENT OF A PARTIAL MOULT VARY? SOME DATA FOR MELODIOUS WARBLERS
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FIG. 1.–Frequency of moult of the different flight feathers and greater coverts during the partial pre-breeding moult among those birds that replaced some (n = 75). All feather tracts have been numbered ascendantly: secondaries (S), including tertials (T), and greater coverts (GC). [Frecuencia de muda de las diferentes plumas de vuelo y coberteras mayores en la muda parcial prenupcial en aves que mudaron alguna de estas plumas (n = 75). Todos los grupos de plumas han sido numerados de forma ascendente: secundarias (S), incluídas las terciarias (T), y las coberteras mayores (GC).]
TABLE 2 Descriptive statistics of the number of feathers replaced in the partial pre-breeding moult of the Melodious Warbler each year. [Estadísticos descriptivos del número de plumas reemplazadas en la muda parcial prenupcial del Zarcero Común según años.] Mean [Media]
1997 (n = 16) 1998 (n = 42) 1999 (n = 28)
Mode [Moda]
Freq. Mode [Frecuencia de la moda]
GC
TT
SS
GC
TT
SS
GC
TT
SS
0.19 0.55 0.50
1.13 1.55 1.79
0.75 0.05 0.07
0 0 0
1 2 2
0 0 0
13 25 15
6 23 18
11 40 27
ACKNOWLEDGEMENTS.—Javier Pérez-Tris, Pascual Campos and Sol Sotillos helped with the field work. Óscar Frías, John Muddeman and Javier de la Puente kindly provided some of their ringing data. Juan Carlos Atienza, Jesús Fernández, John Muddeman and Lukas Jenni made useful comments that improved the original manuscript. I am also grateful to
ARIPRESA for allowing us ringing in its property «El Porcal». Most of the data used were obtained within the project «Seguimiento del impacto real de la Línea de Alta Velocidad Madrid-Barcelona-Frontera francesa» carried out by SEO/BirdLife and financed by the Ente Gestor de Infraestructuras Ferroviarias (GIF).
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