S10.4: Moult strategies of European passerines

Natural History Museum Basel, Box 1048, CH-4001 Basel, Switzerland, fax 41 61 266 55 46, email raffael.winkler@bs.ch

About 80% of the European passerines wintering in Europe or Africa perform their annual complete moult in the breeding area. These are the resident birds and short distance migrants as well as 40% of the long distance migrants. 60% of the long distance migrants undergo their complete moult in Africa, most of them wintering south of 10° N. Early hatching residents and short distance migrants may renew some outer primaries during the postjuvenile moult (eccentric moult). This prevents excessive abrasion of the wing tip. Young long distance migrants wintering north of 10° N may include an eccentric primary moult in their prebreeding moult. Most young long distance migrants wintering south of 10° N undergo a complete winter moult like the adults.

The main function of moult is to ensure a good plumage condition. Feathers are of limited durability. They have an expiry date, as do all commodities, after which they become useless. A feather of a passerine has a shelf life of about one and a half to two years. If a bird delayed its moult until the feathers became useless, it would crash. Therefore, feather replacement has to occur before their expiry date. Hence, looked at closely, moult is the prevention of lethal plumage abrasion and not a response to an already worn plumage. Moult is a sort of a prophylaxis. Durability of the feathers and cycle of feather replacement are nearly, but not quite, synchronised. Feathers last about one and a half years but are replaced annually. Thus, a passerine bird has an emergency time reserve of about half a year.

Moult strategy is the skill of the bird to fit its feather change as conveniently as possible into its annual cycle. In Europe there are about 150 passerine species that winter in Europe or move regularly between Europe and Africa. According to their migratory behaviour, they can be divided into two groups, namely a group of residents and short distance migrants which winter in Europe and north Africa (about two thirds of the species) and a group of long distance migrants which winter in tropical Africa south of the Sahara (about one third of the species). According to their moult strategy they can also be divided into two groups, namely species which perform their complete moult in Europe and others which perform it in Africa. It could be expected that all residents and short distance migrants moult in Europe and all long distance migrants in Africa. However this assumption does not quite hold: Some of the long distance migrants moult in Europe.

Passerines wintering in Europe or in north Africa undergo their complete moult in the period between the end of the breeding season and the beginning of autumn, in most cases between July and October. Depending on the species, the complete moult takes 2-4 months. None of these species moult during winter. Their moult proceeds uniformly without interruption and is therefore unspectacular. In addition to the complete postbreeding moult in summer, one third of the species perform a prebreeding moult in early spring. This prenuptial moult is of modest extent and usually only includes a part of the body plumage.

Passerines wintering south of the Sahara are late breeders and leave the breeding grounds in summer. They have much less time to moult than residents and short distance migrants. Despite that, slightly more than one third of them renew their plumage in Europe before migration. Some compress their moult to only 30-40 days and the feathers are shed so rapidly in succession that for a short period they are almost flightless (Berger 1967; Haukioja 1971; Francis et al. 1991). The advantage of moulting in the breeding region is familiarity with the environment. It apparently outweighs the disadvantage of a short-term near flightlessness. But this home advantage is not the only reason. The second reason to moult at home is provided by the climatic conditions that the birds encounter when they arrive in the winter quarters. Most of the long distance migrants which moult before autumn migration winter in the northern part of tropical Africa, in the dry belt around 10° N latitude. When they arrive there they meet the end of the summer rains, which means that the region will soon dry out and become more and more unfavourable for moulting. Birds undergoing their complete moult in the dry belt have a rapid moult (Bensch et al. 1991, Hedenström et al. 1993). The farther south we move the better the conditions get for moulting and the higher the proportion of long distance migrants undergoing their complete moult in Africa (Jenni & Winkler 1994).

Between the two basic strategies of moult in Europe and moult in Africa we find a lot of transitions. There are species that consistently start their moult in Europe and finish it in Africa, e.g. Barred Warbler Sylvia nisoria (Hasselquist et al. 1988). There are species that normally only moult in Africa, but may occasionally perform their complete moult in Europe, e.g. Great Reed Warbler Acrocephalus arundinaceus (Spina 1990; King 1994; de la Puente & Mejias 1996) and Garden Warbler Sylvia borin (Jenni & Winkler 1994). There are species that may undergo their complete moult in two temporally separated phases within Africa, e.g. Sedge Warbler Acrocephalus schoenobaenus (Aidley &Wilkinson 1987) and Marsh Warbler Acrocephalus palustris (Pearson 1982; own observations) and finally there is the Willow Warbler Phylloscopus trochilus which makes a full moult in Europe and another full moult in Africa (Underhill et al. 1992).

However, not only the great flexibility of the complete moult distinguishes the short and long distance migrants from each other but also the ability of the long distance migrants to include the replacement of some flight feathers in the prebreeding moult. A good example for that is the Pied Flycatcher Ficedula hypoleuca. For some of these birds, especially Scandinavian ones, there is not enough time to accomplish the moult before autumn migration, and they retain 1-4 old inner secondaries. The replacement of these old secondaries is not undertaken when arriving in the winter quarters. Hence, it would theoretically be possible that a Pied Flycatcher twice or more in succession leaves its inner secondaries unmoulted. Therewith the shelf life of these feathers would be overdrawn. To overcome this, many birds renew some inner secondaries during their prebreeding moult in February in Africa. It must be emphasised that this renewal of inner secondaries during the prebreeding moult is not a resumption of an incomplete postbreeding moult but a strategy to counteract a possible future incomplete postbreeding moult. This is demonstrated by the fact that there are more birds (48%) moulting inner secondaries during the prebreeding moult than birds leaving them unmoulted during the postbreeding moult (15%). The prophylactic role of moult is especially well demonstrated in this case.

We have seen that the complete summer moult of the adult residents and short distance migrants passes uniformly and undramatically. This is different in the first year birds of this group. Their moult is of great flexibility because the weak structure of the juvenile plumage and the variable duration of wear of the juvenile wing feathers results in a lot of adaptations. 20% of the juveniles of the species in this group perform a complete moult: that is, they renew their whole plumage in summer like the adults. The remaining species only change the body feathers but may, depending on the duration of wear, include some of the flight feathers, too. A juvenile bird which fledges in June and renews its wing feathers only in August of the next year for the first time, keeps these feathers during 14 months, i.e. two months more than the adults. Herewith it is already approaching the expiry date of its feathers. The longer the time-lag between hatching and first wing moult is, the more imminent is the danger of reaching the limits of the durability of the wing feathers; but also the more time is available for moulting. This time is fully exploited for instance by European Finches. Early-hatching individuals perform an especially extensive postjuvenile moult. They use the postjuvenile moult to strengthen the most exposed parts of the plumage. This happens in the form of a partial moult that is called eccentric (Evans 1986) or sectoral (Mester & Prünte 1982) primary moult. This moult includes, as well as the usually replaced body feathers and wing coverts, also the tail feathers, tertials and a variable number of outer primaries in the sector of primary 5-9. Eccentric primary moult has been described for numerous passerine species (summarised in Jenni & Winkler 1994) and also occurs in Albatrosses (Prince et al. 1993; Langston & Rohwer 1995), Waders (Tree 1974; Prater et al. 1977) and Honeyguides (Stresemann & Stresemann 1966).

The young long distance migrants have, like the adults, only a little time available to moult before autumn migration. Their juvenile moult is therefore not of the same flexibility as that of the short distance migrants. It is generally of little extent or is even completely suppressed. All young long distance migrants defer the essential parts of the moult process to the winter quarters. Their plumage is stressed during the long migration and then exposed to the tropical sun in Africa. It is therefore encumbered as heavily as the plumage of early-hatching short distance migrants that is worn for an exceedingly long period. Some of the young long distance migrants prevent this strain by undergoing a complete moult in the winter quarters. These are the same species where the adults perform a complete winter moult too, thus the species that winter south of 10° N. They have enough time for moulting. Compared with most of the short distance migrants and resident birds that keep their juvenile wings for at least 14 months, these long distance migrants only keep them for 6 months. Those of the long distance migrants that winter in the dry belt north of 10° N and that do not perform a complete winter moult, partially take advantage of the prebreeding moult to prevent an excessive abrasion of wing feathers, e.g. Whitethroat Sylvia communis (Jenni & Winkler 1994) and Subalpine Warbler Sylvia cantillans (own observations). They do this in the same way as the early-fledging short distance migrants do with their postjuvenile moult, by undergoing an eccentric primary moult. Thus, in these species, an extensive prebreeding moult supplements a scanty postjuvenile moult.

Aidley, D.J. & Wilkinson, A. 1987. Moult of some Palaearctic warblers in northern Nigeria. Bird Study 34: 219-225.

Bensch, S., Hasselquist, D., Hedenström, A. & Ottoson, U. 1991. Rapid moult among palaearctic passerines in West Africa - an adaptation to the oncoming dry season? Ibis 133: 47-52.

Berger, W. 1967. Die Mauser des Sprossers Luscinia luscinia. Journal für Ornithologie 108: 320-327.

Evans, P.G.H. 1986. Ecological aspects of wing moult in the European Starling Sturnus vulgaris. Ibis 128: 558-561.

Francis, I.S., Fox, A.D., McCarthy, J.P. & McKay, C.R. 1991. Measurements and moult of the Lapland Bunting Calcarius lapponicus in West Greenland. Ringing & Migration 12: 28-37.

Hasselquist, D., Hedenström, A., Lindström, A. & Bensch, S. 1988. The seasonally divided flight feather moult in the Barred Warbler Sylvia nisoria - a new moult pattern for European passerines. Ornis Scandinavica 19: 280-286.

Haukioja, E. 1971. Fligthlessness in some moulting passerines in Northern Europe. Ornis Fennica 48: 101-116.

Hedenström, A., Bensch, S. Hasselquist, D. Lockwood, M. & Ottosson, U. 1993. Migration, stopover and moult of the Great Reed Warbler Acrocephalus arundinaceus in Ghana, West Africa. Ibis 135: 177-180.

Jenni, L. & Winkler, R. 1994. Moult and ageing of European passerines. London; Academic Press: 224 pp.

King, J.R. 1994. Initiation of remige moult by first-year Great Reed Warblers Acrocephalus arundinaceus arundinaceus in Europe. Ringing & Migration 15:123-126.

Langston, N.E. & Rohwer, S. 1995. Unusual patterns of incomplete primary molt in Laysan and Black-footed Albatrosses. Condor 97: 1-19.

Pearson, D.J. 1982. The migration and wintering of Palaearctic Acrocephalus Warblers in Kenya and Uganda. Scopus 6: 49-59.

Prater, A.J., Marchant, J.H. & Vuorinen, J. 1977. Guide to the identification and ageing of Holarctic Waders. Tring; British Trust for Ornithology, Field Guide 17: 168 pp.

Prince, P.A., Rodwell, S., Jones, M. & Rothery, P. 1993. Moult in Black-browed and Grey-headed Albatrosses Diomedea melanophrys and D. chrysostoma. Ibis 135: 121-131.

de la Puente, J. & Mejias, J.F. 1996. Captura de Carricero Tordal Acrocephalus arundinaceus realizando una muda completa en el sur de Espana. Butlleti del Group Catala d’Anellament 13: 41-43.

Spina, F. 1990. First data on complete moult in the Great Reed Warbler Acrocephalus arundinaceus in northern Italy. Journal für Ornithologie 131: 177-178.

Stresemann, E. & Stresemann, V. 1966. Die Mauser der Vögel. Journal für Ornithologie 107; Sonderheft.

Tree, A.J. 1974. The use of primary moult in ageing 6-15 month age class of some Palaearctic Waders. Safring News 3: 21-24.

Underhill, L.G., Prys-Jones, R.P., Dowsett, R.J., Herroelen, P., Johnson, D.N., Lawn, M.R., Norman, S.C., Pearson, D.J. & Tree, A.J. 1992. The biannual primary moult of Willow Warblers Phylloscopus trochilus in Europe and Africa. Ibis 134: 286-297.