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Races of Phylloscopus trochilus

Breeding of Willow warbler

Mating system varies between monogamy and polygyny.

Polygyny is sometimes associated with the acquisition of a second territory, but this is not a pre-requisite.

Factors favouring polygyny are:-

Ability to replace a lost brood or clutch.

Long breeding season.

High density

In ASSR polygyny involved on average 9% of nests. At Witley Common (Surrey) c13% were polygynous. In some cases both females resided in the same territory. Breeding cycles may be concurrent or successive. In years where May is exceptionally cold territoriality becomes reduced (birds have to spend more time feeding) and the incidence of polygyny may rise.


British Birds 71: - 592-593


Bigamous Willow Warbler Since 1976, at Witley Common, Surrey, I have colour-ringed Willow Warblers Phylloscopus trochilus to enable individual recognition. During spring 1977, a total of 20 males established firm territories; of these, 19 were ringed, and the other held territory at the very edge of my study area. Several females were also ringed, most having been trapped away from the males’ territories, but their origin was not always certain; eight, however, were eventually assigned to particular males, and several males were known to have unringed mates.

On 2nd May 1977, one male (A) was ringed at a site (i) where he seemed to have established territory; on 6th May, a female (a), ringed as a juvenile on 3rd July 5976, was seen nearby apparently carrying nesting material. At another site (ii) about 200 m away, a male (B) was ringed on 27th April and seen again briefly on 29th and 30th; this bird took up a new territory about 500 m away, but finally disappeared altogether. Another male (C), ringed as a juvenile in 1976, apparently took over male B’s new territory.

     1982. Ecological significance of morphometric variation in three sympatric Phylloscopus warblers. Ann. Zool. Fennici 19: 285-295.

     1983. Dynamics of a local population of the Willow Warbler Phylloscopus trochilus in southern Finland. Ornis.Scand. 14:1-15.

VERNER, J. 1964. Evolution of polygamy in the Long-billed Marsh Wren. Evolution 18:


VON HAARTMAN, L. 1951. Successive polygamy. Behaviour 3: 256-274.

Since the disappearance of male B, I observed male A several times from 8th May singing at site ii, and at first thought that he had changed his territory from site i. On 3rd June, however, I located male A back at site i with female a; both were carrying food, and the next day I found a nest with six very recently hatched young (which I ringed on 8th June); both A and a frequently fed the nestlings until at least 11th June, but just before they fledged only the female (a) was seen doing so.

On 8th June, I discovered an unringed female (b) carrying food at site ii; during several hours of intermittent observations to locate the nest, I saw only this female with food, although once or twice I glimpsed a male nearby, occasionally singing. I could not see this male’s rings clearly, but believed him to be A. The nest held five young nine or ten days old (about two days older than those at site i), which I ringed.

    I kept record cards of both nests until the young successfully fledged from each. At site ii I saw only the unringed female (b) feeding the young; at site i only female a was seen doing so during the latter stages and after fledging. At site ii, however, shortly after the young fledged, male A was frequently present, often singing, and close to female b and the young. On s5th June, he was calling there; he carried food into a dense hawthorn Crataegus monogyna, where I saw and heard the young, and a moment later emerged without the food. This was my only obser­vation of male A apparently feeding the young at site ii; but, during the few days after this brood fledged, he was frequently at the site, often singing. He was only rarely at site i, where he was never seen to pay any interest in female a and her young. In effect, male A was defending two territories some 200 m apart: at least twice he was seen to fly directly to site i, where he gave a few song phrases, before returning to site ii.

There can be little doubt that male A was bigamous, although I could not confirm that he fertilised both clutches. The only other male which I believe could have fertilised the eggs at site ii was male B, but he had apparently moved to a different area very early in the season before disappearing completely; it is hard to believe that, if pair formation had already taken place, this male would desert his mate (b) and allow an intruding male (A) to take over the territory. There was no evidence of second broods, and male A was not seen again until 3oth July, in com­pletely fresh plumage and singing at site ii.

The likely sequence of events was as follows: male A arrived and established territory at site i; male B took up territory at site ii; male A paired with female a, who chose the nest site and started building; male B moved from site ii, leaving a vacant territory; male A changed territory to site ii, maintaining a pair bond with female a and limited hold over territory at site i; new female (b) arrived and mated with male A at site ii, which was then defended more actively than site i; male A directed main territorial behaviour towards site ii, but parental behaviour to site i.


20 Croft Road, Godalming, Surrey GU7 I BY


There is no mention of polygamy in D. J. May’s two major papers on territory and breeding behaviour of Willow Warbler (Brit. Birds 40:2-11; Ibis 91: 24-54), M. Philips Price’s studies of this species and Chiffchaff P. collybita (Brit. Birds 29:158- 166) or The Handbook. Eds


     1982. Ecological significance of morphometric variation in three sympatric Phylloscopus warblers. Ann. Zool. Fennici 19: 285-295.

     1983. Dynamics of a local population of the Willow Warbler Phylloscopus trochilus in southern Finland. Ornis.Scand. 14:1-15.

VERNER, J. 1964. Evolution of polygamy in the Long-billed Marsh Wren. Evolution 18:


VON HAARTMAN, L. 1951. Successive polygamy. Behaviour 3: 256-274.




Polygamy and double-brooding  of Willow Warbler

Da Prato (1982) discussed polygamy and double-brooding of Willow Warblers Phylloscopus trochilus in southeast Scotland. I am able to comment further on both subjects, using data from my own studies, on colour-ringed individuals, at Witley Common, Surrey, during 1976-82.

Before my first record (Lawn 1978), single cases of bigamy by Willow Warblers were reported by Howard (1907-14) and Trahair Hartley (1934). In 197 7-82, 18 (about 130/o) of my 136 territorial males were bigamous, the most in any one year being seven (25%) out of 28 (in 1982). I have discussed bigamy and territorial behaviour elsewhere (Lawn 1982), and several cases of bigamy have also been recorded among Willow Warblers in southern Finland (Tiainen 1982, 1983; Tiainen, Hanski & Mehtälä unpublished). Bigamy is clearly not unusual among Willow Warblers, but da Prato’s record of trigamy appears to be unique.

I do not agree with da Prato that the occurrence of polygamy would influence estimates of double-brooding to· any appreciable extent; the nesting attempts of most females mated to bigamous males overlap so much (Lawn 1982) that they could not be mistaken for second broods of single pairs. The third mate (G) of Da Prato’s polygamous male in 1981, however, nested late enough to have been confused with either one of the male’s two previous females attempting a replacement clutch; but she had already raised one brood, supposedly with a different male outside the study area. I am unhappy with da Prato’s records concerning this female. If her fledglings were near independence when seen in the study area on 11th June, she must have already started incubation when she was seen there between 9th and 11th May (allowing 13 days each for incubation and fledging, plus egg-laying and post-fledging periods). It seems more likely that the first brood of this female was also fathered by the polygamous male, but that the female’s presence there was overlooked until after the young had fledged. In any case, time obviously did not prevent this female from double-brooding.

Estimates of the frequency of double-brooding are not easy to obtain. There may, in fact, be no clear distinction between genuine second broods and replacements for earlier failures. Fledging of at least some of the young is the usual criterion for a successful nest. If the young die shortly after fledging, the adults may make a second attempt; if they survive to indepen­dence, however, time may not permit a second brood. When part of a brood is lost before fledging, male Willow Warblers sometimes care for the survivors on their own: leaving the female free to start a second clutch immediately, a time-saving of 10-12 days (the time taken for fledglings to reach independence). I know of three such cases at Witley Common, the most extreme of which involved a female that started laying a second clutch only five or six days after the young had fledged from the first nest. In my experience, however, when all (six or seven) young of a normal-sized brood fledge, both parents usually care for them until they reach independence. Second broods are, therefore, probably more frequent after partial losses of first broods.

I agree with da Prato that time limits the frequency of double-brooding (especially in Scotland, where the breeding season is shorter than in southern England), but I would certainly not dismiss earlier references to higher proportions of second broods than he or I found. I have already shown (Lawn 1980) that my study population has been arriving later than May’s (1949) did during the 1940s: my first egg-dates averaged about ten days later than May’s. Later arrivals in recent years may greatly influence the frequency of double-brooding. The earlier studies of Brock (1910),

however, clearly indicate that double-brooding is possible in Scotland, even though Brock’s first egg-dates averaged a little later than da Prato’s.

Although double-brooding may now be less frequent than it once was, owing to later arrival times, there are several time-saving factors which allow some late broods. In addition to male care of fledglings, late cla Prato’s.

Although double-brooding may now be less frequent than it once was, owing to later arrival times, there are several time-saving factors which allow some late broods. In addition to male care of fledglings, late clutches are smaller, incubation may commence before the clutch is complete, and the young may develop faster than in earlier broods (personal observa­tions). Also of importance is overlap between breeding and moult. This is not unusual for Willow Warblers with late broods, whether these be replacements for earlier failures or genuine second broods. I have four records of Willow Warblers trapped, while in active moult, which were known to be rearing late broods. My sight records include one of a male, tail-less and with about half of its primaries missing or growing, feeding nestlings on 18th July 1979. In southern Finland, Tiainen (1981) found much individual variation, but considerable overlap between breeding and moult. Suspended moult has also been recorded in Willow Warblers with late broods (Ginn & Melville 1983).

I do not support da Prato’s speculative interpretation that polygamy allows Willow Warblers to increase reproductive output without adversely affecting moult or migration. He implied that females may actually select already-mated males. This is true of several species in which polygamy is well developed (e.g. Orians 1980, Verner 1964). The system in the Willow Warbler (Lawn 1982), however, is such that the second female is probably usually unaware of the first until she is already committed to breeding with the male. Any ‘choice’ made by females, involving male or territory quality, will be made in ignorance of the male’s marital status. Males, therefore, may ‘cheat’ females so as to increase their own reproductive output at the latters’ expense (e.g. Alatalo et at. 1981, 1982; Dawkins 1976). This male strategy would be advantageous only in those species in which the female can raise at least some of the young with little or no help from the male. I have shown this to be so in the case of the Willow Warbler (Lawn 1982), but this could be dependent on habitat. Further studies are required on the extent of polygamy in relation to habitat and range. M. R. LAWN

20 Croft Road, Godalming, Surrey. 0U7 JBY



ALATALO, R. V., CARLSON, A., LUNDBERG, A., & ULFSTRAND, S. 1981. The conflict between male polygamy and female monogamy: the case of the Pied Flycatcher Ficedula hypoleuca. Amer. Nat. 117: 738-753.

—,      LUNDBERG, A., & ST~~HLBRANDT, K. 1982. Why do Pied Flycatcher females mate with already-mated males? Anim. Behaviour 30:585-593.

BROCK, S. E. 1910. The Willow Wrens of a Lothian wood. Zoologist 4:401-417.

DA PRATO, S. R. D. 1982. Polygamy by Willow Warblers. Brit. Birds 75: 406-411. DAWKINS, R. 1976. The Selfish Gene. Oxford.

GINN, H. B., & MELVILLE, D. S. 1983. Moult in Birds. BTO guide 19.

HARTLEY, P. H. T. 1934. Apparent polygamy of the Willow Warbler. Brit. Birds 28: 78. HOWARD, H. E. 1907-14. The British Warblers. vol.2. London.

LAWN, M. R. 1978. Bigamous Willow Warbler. Brit. Birds 71: 592-593.

     1980. Late arrivals of Willow Warblers in recent years. Brit. Birds 73: 357-358.

     1982. Pairing systems and site tenacity of the Willow Warbler Phylloscopus trochilus in southern England. Ornis.Scand. 13: 193-199.

MAY, D. J. 1949. Studies on a community of Willow Warblers. Ibis 91: 24-54.

ORIANS, G. H. 1980. Some Adaptations of Marsh-nesting Blackbirds. Princeton Univ. Press.

TIANNEN, J. 1981. Timing of the Onset of postnuptial moult in the Willow Warbler Phylloscopus trochilus in southern Finland. Ornis Fenn. 58: 56-63.

Pair formation occurs on the breeding grounds, males arriving ahead of females (c 2 weeks). Older males appear to pair up first. Nest failure tends to result in the females leaving the territory, after which they re-pair in a different territory. The pair bond does not usually persist for more than one season. Nest may be situated on the edge of the territory (Derbyshire).

Lawn recorded nests as being dispersed by 40-100m (Witley Common), although nests may be clustered and as close as 6-9m. Territory established by male alone, and defended by him throughout the breeding season.

Both sexes feed young but male often less than female. Usually 6 -7 young. Fledglings are fed for 10-12 days by both parents and become independent at about 2 weeks leaving the nest.

Uncertain as to whether second broods actually occur ( may be polygyny) - if it does then 8 -15 days after first brood fledged.

Territorial size between 0.2ha and 0.6ha with 0.2ha the average.


Normally on the ground amongst grass in hedge bottoms or open glades, occasionally in a bush. The nest is built by the female alone, is domed with a side entrance. consists of moss, woven stalks and grasses. Nest is freely lined with feathers.

Clutch size 4 - 8 with 6 the average, clutch size decreasing through the season. The eggs are laid daily, beginning 1 - 3 days after nest completion. Incubation beginning with the last or the penultimate egg.


Incubation lasts 10 - 16 days with 13 days average (Britain), 14 days Yamal peninsula; and 13.2 days Finland. In Finland incubated for 86.5% of each 24hr period, and continuously from 2200 -0200hrs. Food breaks were for about 5 -10 minutes. Least incubation around 1200hrs. Fledging occurs between 11 -15 days with an average of 13 days.

Breeding success c 70%. In Scotland (Prato) over 3 yrs 2.9 - 3.9 fledged per breeding female.

Nests susceptible to loss from cold (12.8% ASSR)


3 days.

Breeding success c 70%. In Scotland (Prato) over 3 yrs 2.9 - 3.9 fledged per breeding female.

Nests susceptible to loss from cold (12.8% ASSR)