Breeding behaviour of Mistle Thrushes

158

BIRD REPORT

seen arriving. At Southwold a flock of 17 Fieldfares came in about noon, and later in the day Fieldfares, Redwings and Blackbirds were found to be very numerous in the vicinity of the gasworks.

On the 4th, this great movement was still in progress, but by the 6th daylight migration was reduced to one or two odd flocks, although a few Thrushes and Blackbirds were heard Over Corton at night.

Over this period there was a steady westerly wind, strong at times on the 2nd and 3rd.

On the 14th, between Southwold and Easton, 20 flocks of Starlings came in during about one hour at noon, most were of between 30 and 40 birds, with one of c. 100.

SOME OBSERVATIONS ON T H E BREEDING BEHAVIOUR O F M I S T L E T H R U S H E S .

The

Rev. P. H .

T.

HARTLEY.

It has been said that field ornithology is the most sporting of all the disciplines of science. If the difficulty of coming to terms with the quarry be the criterion of "sport ", then, from the point of view of the bird watcher, the Mistle Thrush ranks high as a sporting bird. Nomadic in habits for much of the year and occupying a breeding territory which is, in comparison with the territories of Song Thrushes and Blackbirds, enormous, the Mistle Thrush presents a real challenge to the naturalist who embarks on a detailed study of its behaviour. There is need for such a survey, to provide material for comparative ethological studies of the thrush family. . The observations which follow were made at Badingham in East Suffolk in 1954 and 1955.

DISPLAYS. The most usual form of low intensity sexual display—that is, display which is self-exhausting and does not lead on to an end-point of attempted or successful mating—seems to be a chase from perch to perch :— February 23rd, 1954. 17.07 hrs. " T w o Mistle Thrushes flew with a bobbing, bucketing, jay-like flight, one behind the other, into the top of a tall elm. At once they took wing again and flew to another tree : it was noticeable that they kept their tails a little raised as they flew. Then one of the two flew to a third tree, while the other went off to a fourth ".

BIRD REPORT

159

February 6th, 1955. 09.30 hrs. " With loud, grating cries, two couples of Mistle Thrushes were bouncing about in an oak.

One couple flew away N.W. One bird of the other couple—X— flew after them, but only when they had a long Start and not making any great speed. X landed in a birch and began to sing. The second bird of this couple—Y— flew from the oak to the birch.

X took wing, and they flew, one close behind and a little above the other, not fast and both notably ' up by the head ' . " " They approached a cherry tree, in which there was a nest last year.

One went on past the tree, but the other landed in it. One—I think the bird which had flown on—sang a little more."

The second of these observations illustrates the difference which is emphasized in the " Handbook " (Witherby et al., 193841) between the noise of territorial encounters and the quiet and leisurely quality of epigamic chases. At times the pursuit is entirely on the wing :— April 12th, 1955. C. 1640 hrs. " A single grating rattle drew attention to two Mistle Thrushes Aying over at a height of perhaps 200 feet, a few yards apart. The flight of both birds was ' abnormal' in a tendency to carry the heads raised, and in curious breaks in level, sometimes up and sometimes down—queer little half-loops in an otherwise more or less straight flight; the whole effect was very different from the usual free, bounding flight.

Once or twice the pursuing bird closed up on the other, but each time it was avoided just before it came within touching distance— both the closing in and the avoiding action being unhurried. Just before they dropped out of sight behind some trees there was a change of role, and the pursuing bird became the pursued."

Cock Blackbirds sometimes use a similar, bobbing, jay-like flight in the spring months, but I have never seen a pair of Blackbirds in display flight nor anything to suggest that the cock's flight is a display to a female in close proximity.

I have once seen the actual mating of Mistle Thrushes :— April 1 Ith, 1954. 06.25 hrs. " A pair of Mistle Thrushes were feeding on a flower bed. They came more or less into line ahead, and about a foot apart, and then began to hop forward— three or four hops and a pause. Suddenly the pursuing bird tried to mount the pursued, but she hopped forward and avoided him.

There was another attempt at mounting in which the hen crouched as though ready to receive the cock, but the attempt was unsuccessful. A Song Thrush rose from the ground a dozen feet away, and flew low over the pair. There were three or more advances by the Mistle Thrushes, the hen foiling the cock's attempts to mount after each group of hops by moving forward.

Again the hen Mistle Thrush crouched, and the cock mounted her ; and again the Song Thrush took wing and flew at them, so

160

BIRD REPORT

that it almost landed on the cock's back. The Mistle Thrushes then moved forward again—three or four foiled attempts, and then the hen crouched to allow the male to mount her : and this behaviour was repeated another half dozen times at least. The display behaviour then ended abruptly. There was no posing of any kind, save that the cock drew himself rather upright just before each mating."

The attempted interference by the Song Thrush is curious, and is reminiscent of the rabbling of mating Rooks by other members of the rookery (cf. Goodwin, 1955). In the same place on the previous morning a Song Thrush had hopped about a yard and tried to catch an earthworm from the beak of one of these Mistle Thrushes, a most unusual piece of behaviour from a member of this species, which is usually the victim of displacing attacks by both Mistle Thrushes and Blackbirds.

BEHAVIOUR AT THE NEST. In May, 1 9 5 4 , I was able to spend a few hours in the Observation of a Mistle Thrushes' nest 7 feet up in a cherry tree. Three young birds were hatched from four eggs on April 29th. The activities of the parent birds are summarized in the table :—

Date.

3.v.

5.v.

10.v.

11.v.

12.v.

14.v.

Age of Young.

5 days 7 „ 12 „ 13 „ 14 „ 16 „

Time of beginning of watch Period.

09.47 hrs. 131 mins.

12.47 75 „ 09.59 66 „ 16.02 69 „ 14.41 50 „ 13.18 92 „

Feeds / Number young I of feeds. 100 minutes.

9 (all by <J) 2.3 9 4 4 2.02 5 2.4 4 2.67 7 2.54

Düring most of the watch on May 3rd the rain was falling heavily. The hen brooded closely, including one period of 106 minutes, and when she did leave the nest brought no food back at her returns. In the course of the 106 minute period the hen ate two cargoes of the food brought to the nest by the cock.

(These are not included in the nine cargoes delivered to the young).

On May 5th, six of the nine observed feeds were administered by the cock, and three by the hen who was still doing some brooding.

Most of the faecal pellets produced by the young were swallowed by the parents : one was seen to be carried away on May 12th, and on May 15th an adult was seen to carry a pellet for a distance of at least 100 yards.

All three young were still in the nest at 18.15 hrs. on May 15th, 1954.

There was still one in the nest at 06.35 hrs. next morning, but all were fledged by 07.45 hrs.

At 11.20 hrs. one of the chicks was found in an apple tree about T H E FLEDGED YOUNG.

BIRD REPORT

161

80 yards from the nest perched at a height of 5 feet from the ground ; on being approached it flew a distance of between 30 and 40 yards to a perch more than 20 feet up in a maple.

This bird was in its 18th day. (The fledging period quoted in the " Handbook " (op. cit.) is 14 to 16 days). At 18.00 hrs. on the same day, one fledgling was found 4 feet up in a birch some 60 yards from the nest. It was observed that on this day the parent birds fed frequently in the garden where the nesting tree stood, whereas, while the young were in the nest, the adults went often a fĂźll ÂŁ mile to feed, and fed in the garden only occasionally in the early mornings. On May 17th one chick was 5 feet up in an apple tree, and another more than 30 feet up in a maple. On the 18th, fledglings were observed four times, always on perches 30 feet or more above the ground. T w o of the three were certainly still alive. On May 20th the whole family had vanished.

This entry on a nomadic life within a few days of quitting the nest seems to be not unusual in Mistle Thrushes : on July 3rd, 1953, two adult Mistle Thrushes appeared in a large garden in Berkshire accompanied by a fledgling so callow that it allowed itself to be caught in the hand. T h e young bird weighed only 80 g.and was still almost tailless.

Within a few hours this family party had moved away. The vagrant existence of newly fledged Mistle Thrushes is in sharp contrast to the early life of Song Thrushes and Blackbirds : in these two species, although the first flight is usually to a distance of 20 to 50 yards from the nest, the fledglings remain within a circumscribed area for about three weeks. T h e early resort of fledgling Mistle Thrushes to elevated perches is also different from the habit of the other two common breeding thrushes. Although I have seen a young Blackbird 30 feet up in a yew tree within a few minutes of its leaving a nest 7 feet above the ground it is usual for fledgling Song Thrushes and Blackbirds to spend the first fortnight or so of their free ranging existence on, or within 6 feet of the ground.

PAIRS IN WINTER. Blackbirds certainly pair for life, but Jackson (1954) has shewn that in winter the paired birds pay little attention to one another, though they continue in the same territory. In the nomadic and far-ranging Mistle Thrush winter territories are either non-existent or so large as to do little to keep a pair of birds together.

In the winter of 1954-55 I gained the impression (and let it be at once admitted that a general impression is not evidence !) that it was more usual to see Mistle Thrushes in couples than in any other grouping. The Solution of this problem in Mistle Thrush sociology, and also the problem of pair formation would be a fascinating study for the application of Charles Darwin's dictum, " It's dogged does it " !

162

BIRD REPORT

Referertces.

Goodwin, D. (1955). " Some observations on the reproductive behaviour of Rooks " Brit. Birds, xlviii: 97 - 105.

Jackson, R. D. (1954). " Territory and pair formation in the Blackbird " Brit. Birds, xlvii: 123 - 131.

Witherby, H. F. et al., (1938-41). The Handbook of Briti Birds, London.

Scientific names of birds mentioned in the text.

Rook Corvus frugilegus L. Mistle Thrush Turdus viseivorus L. Song Thrush Turdus ericetorum Turton.

Blackbird Turdus merula L.