PROCEEDINGS OF THE ASSOCIATION FOR THE STUDY OF ANIMAL BEHAVIOUR

ised light . The importance of such responses inthe field is uncertain .

REFERENCES(1) Evans, F . G . C . (1951) . An analysis of the behaviour

of Lepidochitona cinereus in response to certainphysical features of the environment . J. Anim .Ecol., 20, 1 .

(2) Morton, J . E . (1960) . The responses and orientationof the bivalve Lasaea rubra Montagu . J. mar. biol .Ass. U.K ., 39, 5 .

2 3 5

(3) Brafield, A. E . & Newell, G . E . (1961) . The behaviourof Macoma balthica (L .) . J. mar . biol., Ass . U.K .,41, 81 .

(4) Newell, G . E. (1958) . The behaviour of Littorinalittorea (L .) under natural conditions and itsrelation to position on the s hore . J. mar. biol.Ass . U.K., 37, 229 .

(5) Charles, G. H. (1961) . The orientation of Littorinaspecies to polarised light . J . exp . Biol ., 38, 189 .

ORIENTATION AND LEAPING BEHAVIOUR OF MIGRATING SALMONIDS AT FALLS AND OBSTRUCTIONSBy T. A. STUART, Freshwater Fisheries Laboratory, Pitlochry.

Laboratory and field observations and ex-periments have shown that the behaviour ofmigrating salmonids, adult and juvenile, whenfaced with a barrier or obstruction, can be cor-related directly with the hydraulic conditionsobtaining in the river channel.

It appears likely that the signal that stimulatesthe fish to move forward preparatory to the leapover an obstacle is related to the impact of fallingwater on the surface of a pool, As the force of theimpact varied with changing discharge so alsodid the relative size of the fish that responded .The response also varied with the amount of airentrained in the water .

The stimulus to leap was found to be closelyrelated to the presence of a standing wave (orhydraulic jump) and the location of the standingwave distant to the obstacle influenced thesuccess of the leap . Thus in shallow fall poolsand pools below sloping weirs the standing wavebecomes located too far downstream for thefish to strike the crest of the fall on the upwardarc of its trajectory. If the fish strikes the fallingwater on the downward arc it is immediatelyswept downstream . Optimum leaping conditions

THE SEASONAL MOVEMENT OF TOADS (Bufo bufo) . By H . J . MOORE, Clayesmore School, Blandford.

The spring movement of toads to a lake inDorset was described . The movement was foundto be a radial congregation, rather than a linearmigration, though more of the animals camefrom off the chalk lying higher to the East .

The movement spread over a period of from afortnight to two months according to weatherconditions .

The possible directive stimuli were discussed .It was not thought that the sound of callingmales could carry far enough . They can be heardagainst a light breeze only up to 40 yards andwere audible downwind up to 200 yards . What-ever the primary directive stimulus is, sound

were obtained when the depth of the fall poolwas increased so that the standing wave becamesubmerged at the `toe' of the fall, the ideal ratioof height of fall to depth of pool being 1 :1 . 25 .

All leaps in the laboratory and in the fieldwere observed to be initiated at the surface of thepool (on the neutral point of the standing wave)and no fish was observed to approach in a runfrom a distance, laterally or vertically and per-form a leap without a halt near the surface ofthe pool .

The great amplitude of the flexions of thefishes' body as it leaves the surface of the poolsuggests powerful strokes of the tail near thesurface as the main source of propulsion throughthe air. This action can be compared to the effortexpended by the nest-building female whosepowerful body flexions, by displacement cur-rents, excavate the gravel in the river bed . Inthe latter case, in contrast to the disposition of thefins in the leaping fish, the pectorals and ventralsare rigidly distended and extended forwardsthus acting as brakes to forward movement anddirecting the forces downwards and to the rear .

would play its part when the animals were closeto the lake and probably be of importance in thefinal stages of congregation . Olfactory stimuliappear to offer the best hope but the sense ofsmell of Bufo bufo is not well deveoped as in theNatterjack (B . calamita) . The difficulty of theodour of algal flora reaching toads against thewind was emphasized .

Visual memory by emigrating toadlets presentcertain difficulties and no emigration across theroad of such young toads has been observed .Finally the existence of a well-marked, but

smaller-scale migration towards the lake inOctober and November was established . It had

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been studied at Clayesmore and since reportedfrom other areas. It is less efficient than thespring movement and differed in that the toadsfed enroute, no pairing took place except onreaching water, the sex ratio was more even, andthe orientation was less accurate, males tendingespecially to wander. Toads are physiologicallyin a condition to breed by the end of September

ORIENTATION IN DUCKS . By G. V. T. MATTHEWS, The Wildfowl Trust, Slinibridge .

Mallard caught at Slimbridge, Gloucestershire,and released at many points in various directionsconsistently fly off between North and Westregardless of time of release, season of capture,sex, age, wind direction or topography of therelease point. The direction is not maintained,subsequent recoveries being scattered at random .Orientation at release is stastically clear within30 seconds and can therefore give a false im-pression of immediate position fixing when thehome and "nonsense" directions coincide . Thework on pigeons in Germany and North Carol-ina is open to criticism on this score .

The thickness of overcast needed to preventsun-orientation is being assessed by photo-metry. At night the use of small `sputnik' lights

THE NAVIGATION OF LAND-BIRDS : OVER THE SEA THE RADAR EVIDENCE. By D. LACK, Edward Gre yInstitute of Field Ornithology, Oxford.

Observations by high-power radar of migrantsover the sea show that

l . Migrants normally maintain steady tracks,both by day and by night .

2. They do not normally change directionwith the change from daylight to dark or fromdark to daylight, though presumably switchingbetween solar and stellar navigation .

3. Their tracks are normally the resultantbetween a determined heading and the wind atthe time, without allowance for lateral drift by across wind . However, migrants do not usuallyset out in a strong cross-wind .4. Tracks were random for 6 per cent . of the

periods analysed, the area of dis-orientationbeing local, and always associated with fog orrain. Visual observations show that low-flying

ANIMAL BEHAVIOUR, IX, 3-4

and the accumulation of prolactin is thoughtlikely to initiate their wandering . The move-ment was regarded as an abortive breeding cycle,that was interrupted by winter, and in particularby the low temperature of the water which wasthought to be the explanation for no spawnhaving been found .

attached to the legs of the Mallard have fullyconfirmed star orientation and possibly moonorientation .

Mallard caught at Borough Fen, Northamp-tonshire, show a N .W . orientation in September,but later in the season no particular orientationis apparent . It is possible that this may be due tothe influx of migrants with a different orientat-tion. Birds caught at St . James's Park, London,fly predominantly between East and South . Thepart played by learning, possibly in relation tothe topography of the `home' area, is beinginvestigated by rearing experiments .

Preliminary tests with Teal have indicated aS.W. `nonsense' orientation in Britain, Hollandand France .

migrants may maintain orientation in fullovercast .

5. Migrants may head in at right angles onsighting the coast, and sometimes continue in arather different direction over the land from thatover the sea, or they may turn up-wind along acoast. More complex gradual changes in direc-tion have sometimes been recorded by day in thesouthern North Sea . When land-birds aredrifted far out over the Atlantic during thenight, they may head back into the wind atdaylight, in a direction nearly opposite to thatin which they were previously flying .

6. Especially in autumn, migrants normallyset out with a very light wind or a more-or-lessfollowing wind. Migration in a direction oppo-site to that normal for the season of year some-times occurs with a head-wind .

MIGRATION AND ORIENTATION IN PINNIPEDIA AND CETACEA . By K . M. BACKHOUSE, Charing CrossHospital Medical School, Chandos Place, London, W.C.2 .

The Pinniedia are commonly considered to be out the North Pacific from its northern breedingmigratory animals. Callorhinus ursinus, for grounds. This constitutes a wide feeding dis-instance, may travel thousands of miles through-

persal with little or no tendency for maintenance