(144) REACTIONS OF SOME PASSERINE BIRDS TO A STUFFED CUCKOO. II. A DETAILED STUDY OF THE WILLOW-WARBLER. BY GEORGE EDWARDS, ERIC HOSKING AND STUART SMITH IN a previous paper {British Birds, Vol. xlii, pp. 13-19) we described the reactions of various small passerine birds to a stuffed Cuckoo (Cuculus canorus) placed in their nesting territories. These experiments, which were exploratory in nature, raised a number of interesting problems, and it became evident that if we were to obtain answers to certain of them, it would be necessary to give close study to a single species during the whole of the breeding period. We chose for this study the Willow-Warbler (Phylloscopus trochilus) because it is a common bird whose nest is not very difficult to find, and one which reacts strongly to a stuffed Cuckoo. It can be shown mathematically that if one assumes a probability of at least 0.05 (one in twenty) to be desirable, then at least 10 experiments on one species must be carried out to obtain significant results from which sound deductions can be made. Altogether we found, and did experiments at, 14 nests of the Willow-Warbler, whilst in addition, experiments were tried in 6 territories of cock birds of this species, from the day of their arrival in mid-April until the beginning of nest construction. The results relating to the period of territory formation, therefore, are not so significant as those obtained at the nests. During this intensive study we attempted, among other things, to answer the following questions :— (a) What effect has the stage reached in the breeding cycle on the reactions of the birds ? (b) Has any particular part of the Cuckoo a specially high signifi­ cance in evoking attack ? (c) Is the Cuckoo " recognized " as such by the Willow-Warblers, and can they differentiate one dummy from another? These questions, and the experimental data obtained during their elucidation, are dealt with below. (a) Effect of stage reached in breeding cycle. It is convenient to divide the breeding cycle into four periods, namely, (i) from the arrival of the cock birds to the start of nest construction ; (ii) the period including nest-construction, clutch- completion, and incubation ; (iii) the period during which the brood is in the nest; and (iv) the post-fledging period. (i) The first cock Willow-Warblers arrived in their territories in the North Cheshire area in mid-April, and experiments were started on the day of their arrival. There were two cocks singing vigorously on the 13th, three on the 14th, and five on the 18th. The sixth did not VOL. XLIII.] REACTIONS TO A STUFFED CUCKOO. 145 arrive until April 24th. The method of experimentation at this stage was to use stuffed Cuckoos specially prepared for us in which a stout wire protruded through the belly. This wire was thrust down the hollow inside of a 10 foot bamboo cane, and the Cuckoo could then be lifted nearly 20 feet into the tree in which a bird might be singing. Animation could be give to the Cuckoo by slightly rotating the cane in either direction. The birds reacted at once to this, even though an observer held the cane in his hand, but once a bird's attention had been drawn by such methods to the Cuckoo, the cane was normally thrust into the ground and reactions observed from a distance. In general, during this period, the following may be taken to summarize average reaction to the dummy, though there were individual variations between different cocks. The results are based on 42 hours of watching and experimenting in the six territories. Immediately on arrival and for a period afterwards (which is variable and which probably corresponds to that before the hens arrive) the cock Willow-Warblers respond to the Cuckoo by posturing at it with " flicking " wings and very loud and sustained song. Occasonally the chittering " chee-chee " note, which is such a feature of later display, is used against the dummy, but not at all frequently. On no single occasion was an actual attack made on the dummy, the chief features being a close approach and vigorous and sustained singing. As soon as a hen bird arrived and one of the cocks had paired with her, reaction of that cock to the Cuckoo diminished quickly. In two instances, paired cock Willow-Warblers ceased altogether to react to the Cuckoo, although they continued to sing in their territories and attack other Willow-Warblers which ven­ tured in. Reaction to the Cuckoo during territory formation, seems strongest in the region of the song-post, but thereafter it is strongest in the area of the nest. It seems probable that during this stage when the cock birds are forming their territories, the Cuckoo has no very great " valence " for the Willow-Warbler, and that reaction to it may take place more because of the high general level of aggressive potential within the birds, than from specific reaction to the dummy as a Cuckoo. Cock birds which were merely passing through on migration did not react to the Cuckoo at all, although they sang whilst resting in areas as yet untenanted by other Willow-Warblers. Thus in one case, the behaviour of a cock bird which was singing in one area puzzled us at first because, whenever the Cuckoo was presented to it, the bird flew away to a distance. Next day, however, this bird had disappeared and was evidently a passing migrant. (ii) Experiments were conducted at three nests during the build­ ing stage, at four nests during laying and incubation of the eggs, and at seven nests with young in various stages of development. In only one case were the three stages of building, laying and incu­ bation, and rearing of the young, investigated consecutively at 146 BRITISH BIRDS. [VOL. XLIII. one and the same nest, since at two of the three nests found in the building stage, the experiments resulted in the birds abandoning the nest before it was completed. This desertion of nests at the building stage, if experiments are attempted, was also noted by Lack to occur with three Robins (Erithacus rubecula), (Life of the Robin, Witherby, London, 1943). It should be noted, however, that in the case of the nest at which experiments were carried out through all three stages, the reaction at any one stage duplicated exactly that obtained at other nests at which only one, or sometimes two stages were investigated. The general picture, allowing for small individual variations from pair to pair, is fairly constantly as follows. During the nest- building period the Willow-Warbler reacts to the Cuckoo with chittering and wing-flicking, but not with an attack. At one nest, completed externally but not lined, a stuffed Cuckoo placed in a bush a yard above the nest evoked loud chittering and wing-shaking from the pair. This brought in other Willow-Warblers until there were six birds of this species in the small bush, all demonstrating against the Cuckoo. There was no attack, however, although occa­ sionally two of the cocks would sing a snatch of song and then attack one another. While the six birds were in the bush, a live Cuckoo came to a tree 20 yards away and called. Two of the Willow-Warblers then left the bush and flew to the tree, where they chittered and demon­ strated against the live Cuckoo. Later, this nest was deserted, as was one other at which experiments were done during the building stage. As soon as egg-laying starts, reaction to the Cuckoo becomes more violent. At a nest with four fresh eggs of an incomplete clutch, the hen bird returned soon after a dummy Cuckoo had been placed just above the nest. She demonstrated by chittering and flicking her wings, but did not attack. When the cock arrived three minutes later, he attacked at once and the hen joined in, and the combined attack was violent enough to knock over the dummy. After incubation has started, violent attack by both birds of the pair is general, and accompanied by the usual chittering and wing- flicking. (iii) With young in the nest, reaction to the Cuckoo is immediate, violent, and sustained, and does not seem to vary with the degree of development of the young. (iv) In the post-fledging period, experimental work is difficult and uncertain, especially after the young have scattered, but at two nests at which we were able to experiment on the day the young were leaving, or had just left, the nest, the adult birds persisted in attacking the Cuckoo so long as the young remained near to the site of the nest. On the following day, however, when the young were more scattered, the adults no longer attacked the Cuckoo, even though it was placed in its previous position immediately over the VOL. XLIII.] REACTIONS TO A STUFFED CUCKOO. 147 nest. The young were by this time scattered over a radius of about 20 yards from the nest-site, and the adult birds merely chittered at the Cuckoo and flicked their wings. This marked diminution in aggressive behaviour after the young are well clear of the nest duplicates what we observed at a Nightin­ gale's {Luscinia megarhyncha) nest, where violent attack on a dummy ceased quickly a few minutes after the young had scattered into the undergrowth around the nest. Although violent attacks on the Cuckoo occur in the area imme­ diately surrounding the nest during the incubation and fledging periods, the cock Willow-Warblers will react to the Cuckoo by clutter­ ing and wing-flicking at any point within their territories.