(262) SONG-THRUSH NOBLE ROLLIN

Transcription

1 (262) SONG-THRUSH BY NOBLE ROLLIN SONG SOME aspects of Song-Thrush (Turdus e. ericetorwm) song are dealt with below under the following headings: total output for the day, portion of day used, hour to hour variation, day-time singing, dawn and dusk singing, token singing, singing and roosting, dawn penetration, notes. The observations were made in Northumberland, at Glanton and at Wallsend, between 1927 and 1944, the penetration records mostly inclusive and the remaining' records inclusive. All times, unless stated otherwise, are in Local Apparent Time, i.e. time by the sun at the place of observation. TOTAL OUTPUT FOR THE DAY. In Fig. 1 are given three Glanton examples of the total output of song at three different stages in the breeding cycle. In these diagrams and the subsequent one in Fig. 3, the first full hour of singing is entered from the time of the first song. The first full hour is thus set partially forward from the first normal hour's timing, e.g. the June 13th singer in Fig. 1 began to sing at 1.57 a.m. and its output of song from 1.57 a.m. to 2.57 a.m. is shown as the first full hour. The first normal hour, 2.30 a.m. to 3.30 a.m. is also shown, the two hours overlapping by 27 minutes. The last hour of singing is treated in a similar manner, but in reverse. The diagrams therefore indicate besides hour to hour output, the first and last full hour of output, and an indication of the time of commencement and cessation of the day's singing. The April 9th record shows the total output hour by hour, of an unmated bird in the Bird Station area at Glanton. The full singing of this bird continued day after day and was very noticeable even to casual observation. So far as I was able to ascertain, this bird never obtained a mate, and eventually abandoned the territory. The June 13th graph is the total output of a bird observed in a wood a little over half a mile from Glanton. This bird was apparently at the pairing phase of its breeding cycle, as it was seen the previous day with its (presumed) mate sitting opposite it (see Note 1). During the day it passed slowly through its territory, taking the whole day to go from one end to the other and back. The June 12th graph shows the total output of a bird in the Bird Station area at Glanton, which was feeding a brood of three to five young, three days out of the nest. The total output for the day in the above instances, together with the length of the singing day (measured from the first song in the morning to the last song in the evening), is given in the following

2 VOL. XXXVIII,] SONG-THRUSH SONG. 263 is FIGURE I. Total Output of Song During the Day.

3 264 BRITISH BIRDS. [VOL. XXXVIII. table. Also included in the table is a record of the last singing of the season in the Station area, July 22nd, which amounted to a quarter of a minute for the whole day, and the output of the Wallsend " token " singing bird (Fig. 3), which is described later. April June June July May , Mateless bird in full song. Bird apparently pairing. Bird feeding young. Last singing of season. " Token " singer. Singing Day. 15 hrs. 00 mins. 18 hrs. 55 mins. 18 hrs. 16 mins. 10 hrs. 11 mins. 17 hrs. 47 mins. Output of Song. 377 mins. 217 mins. 33 mins. min. 55 mins. This makes a somewhat direct correspondence between the stage in the breeding cycle and the output of singing in so far as the Glanton birds are concerned. The output for the day of the mateless bird, 377 minutes, is respectively about twice and three and a half times the maximum figures given for the Sky-Lark (Alauda a. arvensis) and the Willow-Warbler (Phylloscopus t. trochilus) (antea, Vol. xxxvii, p. 87). Whilst it is extremely unlikely that any of these outputs are the maximum for the species, they suggest that the Song-Thrush occupies a high place for maximum output of song. PORTION OF DAY USED. The Song-Thrush's song is not continuous in the sense that the Sky-Lark's is, nor cut into more or less set lengths of continuity, as is the Willow-Warbler's. Thus, the unmated Song-Thrush averaged about 38 seconds of output of song for every minute it was occupied in singing and the total amount of time thus occupied in singing was 9 hrs. 55 mins. or 41% of the 24-hour day. This together with the time used in roosting is given in Fig. 2, which.shows graphically the relatively small amount (5 hrs. 5 mins. or 21%) of the day remaining for feeding and other daily activities of this unmated bird. HOUR TO HOUR VARIATION. In regard to the hour to hour variation, in all three graphs in Fig. 1 there is a morning block of song in which the dawn peak is highest (See Note II) and there is a block of song in the evening which reaches into dusk, but whose peak is not necessarily included in dusk. Judging by the all-day records I have made so far, this appears to be the typical construction of a Song-Thrush's daily song. Increased output over this basic pattern appears to be obtained not so much by raising the height of the peaks, though this may be done, as by filling in the space between them by more persistent day-time singing.

4 VOL. XXXVIII.] SONG-THRUSH SONG. 265 DAY-TIME SINGING. A rather different type of observation was made on a bird at Wallsend which was kept under observation for a whole season. The first singing began in November and continued into December, when it ceased with the onset of wintry weather. The song was resumed again about mid-february and the bird continued to occupy its territory for the rest of the season. Systematic observations were begun in March. Observations were made on the territory in the morning from approximately 8.50 a.m. to 8.55 a.m., British Summer Time, and later double Summer Time. Out of ninety-one day-time observations from March to the beginning of August inclusive, this bird was heard singing only ten times, once in March, four times in May and five times in June. Some observations were also made near noon and in the afternoon in March and April, but no song was heard in twenty-six such observations. This shows that, for this bird at least, day-time singing was unusual. Similar observations were made on two other territories in the vicinity where the birds also began their singing season in November. A hundred and twenty observations were made on each and the results corresponded closely with those of the first bird. One of them was never heard in the day-time at all and the other was heard only six times. FIGURE 2, Portion of Day used in Various Activities. Another Song-Thrush, which took up its territory for the first time in the first half of March, sang more freely in the day-time as it became established, and was heard singing on eight out of seventeen day-time observations during this month. However, in April, May and June, it too lapsed into virtual day-time silence, being heard only four times out of fifty-four observations. In July and early August what I presumed was the same bird came into full day-time singing again, being heard ten times out of twenty-four observations. The loud end-of-season singing of this bird became quite a feature in the area as the " silent month " of August approached, and on July 31st I found that it was singing sufficiently loudly for me to hear it at a distance of three quarters of a mile (over land). The bird continued in full song till August 8th, when it ceased for the season.

5 266 BRITISH BIRDS. [VOL. XXXVIII. DAWN AND DUSK SINGING. Dawn and dusk records were also made on the first-mentioned bird, which was so silent in the day-time. It is worthy of note that during eight dawn records and seven dusk records made at intervals from the beginning of March to the beginning of August, not &i a single occasion did this bird fail to sing, thus showing it to be a systematic twilight singer which was relatively quiet in the day-time. Similar, though less systematic, observations were made on the other three mentioned birds, and they revealed substantially the same state of affairs. Finally, three all-day records were made on the first mentioned birds, in April, May and early July. Fig. 3 is an example of these and gives a good idea of the type of singing indulged in by this bird. All three records agreed in conforming to the basic pattern of Song-Thrush song already outlined under Hour to Hour Variation. The total outputs for the day for these all-day records FIGURE 3. Total Output of Song During the Day. were 27, 55 and 42 minutes respectively, thus conforming to the type of output found in the Glanton bird feeding young out of the nest. TOKEN SINGING. It is clear, then, that these Wallsend birds, after they have established themselves in their territories with more or less day-time singing, normally relapse for most of the rest of the season into an abbreviated morning and evening reassertion by song of their territorial claim. Three samples of this type of singing show a mean of a little under three quarters of. an hour of output per 24-hour day, which is but one ninth of the output of the mateless bird at Glanton. I have ventured to call this " token " singing, and suggest that this three quarters of an hour of song may be in the region of the lowest mean amount of song which a Song-Thrush finds suitable for maintaining its ascendency in its territory. This

6 VOL. XXXVIII.] SONG-THRUSH SONG. 267 token singing conforms to the basic pattern of song, and suggestions have already been given as to how song may be built up from the basic pattern for other biological purposes, i.e. the advertising of the mateless bird and the song during probable pair formation. It is no doubt the token singing or restatement of territorial right at dawn by nearly every male Song-Thrush, regardless of the stage it has reached in its biological cycle, which makes so effective the wonderful dawn chorus of the Song-Thrush ; a performance which adds the necessary volume to the general outburst of song from other species and together with them makes the impressive dawn chorus of the British woodlands. It is not a very wild guess to suggest that token singing may be the basis of the dawn and dusk singing of other species. SINGING AND ROOSTING. One of the reasons, no doubt, why token singing is a prominent feature amongst Song-Thrushes is that they do not necessarily roost in their own territories. The Station area at Glanton, for instance, accommodates one breeding pair of Song-Thrushes, but is also used by other adults for roosting. Thus I noticed one evening at a season when the two occupying birds were feeding a brood which had left the nest, that two birds (one may have been the owner of the territory) sang against each other within the inner limits of the territory, both apparently intent on roosting in an archway. In the subsequent season the Station bird lost its brood in April and moved to a new territory, the Station area being unoccupied as a breeding territory for the rest of the season. Towards the end of evening singing in May, two singers came to roost in the Station area. Both sang at the same time, and as I stood under the archway mentioned above, both came and fought and buffeted each other over my head. Eventually each retired to different parts of the Station area, long used as Song-Thrush roosts, sang there and then became silent. When the mateless bird mentioned in these notes was singing his evening song, two other birds arrived quietly in his territory from beyond its limits, and perched some ten yards from a bay tree which is a favourite roost. A minute or two afterwards one sang some sub-song and finally a very few snatches of full song and then was silent again. Subsequently one, possibly both, retired to roost in the bay tree, being joined later by the mateless bird, which also roosted there. On several occasions I heard the Wallsend bird, whose singing was described earlier, stopping and singing at two or three points on its way from its territory to its roosting place, and vice.versa. It appears, then, that a bird may roost in its own territory, in the territory of another, or presumably, though no instance is given, in unoccupied ground. The desirability of a statement of ownership prior to retiring in the evening, and particularly of a restatement of ownership after leaving the roosting place in the morning, is obvious.

7 268 BRITISH BIRDS. [VOL. XXXVIII. DAWN PENETRATION. The unbroken. line in Fig. 4 is based on the mean time of the first singing at Glanton in the morning, through the singing season from March to July. The broken lines show the position of night, relative progress of dawn, and sunrise. It will be seen that up to mid-june the Song-Thrush commences to sing regularly, when the sun is somewhat over 6 below the horizon (actually over a range of means from y 45' to 6 15' below the horizon). The first song varied from a mean of 52 minutes before sunrise in March to a mean of 71 minutes in the first half of June. Owing to the fact that the time of sunrise and the length of dawn both vary throughout the season and at different latitudes, the time of the first song or the number of minutes before sunrise, though specific statements FIGURE 4. Penetration of Dawn. of the bird's behaviour, have limitations for certain comparative studies of dawn behaviour. They are, for instance, not readily directly comparable at different times of the year and at different latitudes, and they do not indicate the position of the song in the dawn period nor any direct relationship with the amount of light. The exact statement of the number of degrees and minutes the sun is beneath the horizon at the time the bird first sings is, however, a means of overcoming these difficulties. It gives the position of the bird's first song in the dawn and is in direct relationship to the mean amount of light. It can be directly compared at any season, at any latitude with any other, and, expressed thus in degrees and minutes, can be used as a measure of the amount that a bird's singing penetrates into the dawn (or dusk).

8 VOL. xxxvin.] SONG-THRUSH SONG. 269 Fig. 5 is a comparison between the time of first singing, expressed in minutes before sunrise, and dawn penetration of first singing, expressed in degrees and minutes. As the penetration gives the actual position of first singing in the dawn, it will be seen that the disparity between this position and the minutes before sunrise is marked between late April and early June. In regard to penetration and light, if the birds' dawn penetration was constant at different seasons it would show that they were commencing to sing at a constant mean light intensity. If their penetration became less it would indicate that they were waiting until it became lighter before singing. From the penetration line in Fig. 5 it will be seen that as the season advanced from mid-may onwards, the Song-Thrushes waited for successively more and more light before commencing to sing. PENETRATION MGREES MINUTES BEFORE SUNRISE MINUTES 7S APL MAY TUNE J~LY FIGURE 5. Comparison oi Dawn Penetration and Minutes of Song before Sunrise. NOTE I. SONG BEHAVIOUR. A Glanton Song-Thrush (Fig. 1, June 13th) has already been mentioned as having been seen singing with its (presumed) mate sitting opposite it, the silent bird looking as if it were entranced by the singing. I.have noticed similar behaviour amongst Wallsend birds. In one instance where a singer was taking up its territory in November, the two birds sat facing each other about two yards apart, one silent and apparently listening, the other singing excitedly. In the other instance, which was later in the season,

9 270 BRITISH BIRDS. [VOL. XXXVIII. the two were closer together on the strut of a telegraph pole. Subsequently the silent bird flew to the ground and began to feed, and was later joined by the singer, which continued to sing on the ground beside it. This behaviour does not conform to the Song- Thrush courtship display described in The Handbook, and it seems most likely that it is connected with pair formation, though my data do not prove this. NOTE II. OUTPUT IN FIRST HOUR OF SINGING. Of nine all-day records of the Song-Thrush which I have, all have the largest output of song for the whole day in the first hour of singing. That the output for the first hour should be the highest might seem natural, but it is not necessarily the usual practice amongst birds. For example, of the six all-day records of the Sky- Lark and Willow-Warbler already described (antea, Vol. xxxvi, p. 148, and Vol. xxxvii, pp ), only one has the first hour of output the greatest. Actually in some two dozen all-day records of various species examined, a little under half had the first hour the greatest.