Eggshell removal in Black-headed Gull, Colour Silent Print, Tinbergen
op filmhouder: Cyldon, Made in England en STH Pacific
titels in film:
The breeding behaviour of the Black-headed Gull, Larus ridibundus L. -
IV: the removal of the egg shell - an anti-predator response. A film of the Department of Zoology and comparative Anatomy, University of Oxford, based upon the investigations of: G.J. Broekhuysen, C. Feekes, J.C.W. Houghton, H. Kruuk, E. Szulc, N. Tinbergen. Camera: N. Tinbergen. - The Black-headed Gull removes the empty egg shell once the chick has hatched.
Three circumstances prompted a closer investigation: 1. The response is not confined to hatching time, but can be elicited from well before the laying of the first egg until about a week after hatching (C. Beer) - The Gulls remove a great variety of objects, in fact any strange object provided it is not too large (C. Beer) (1:40) - The Kittiwake [drieteenmeeuw] does not remove the egg shells, nor do Sandwich Terns. [Grote Stern] Neither species relies much on camouflage (E. and J.M. Cullen) - The film deals with an investigation of two questions: A. What is the survival value of the response? B Which stimuli elicit it? - The work was carried out in the sand dunes near Ravenglass, Cumberland, which harbour a large colony of Black-headed Gulls. Our thanks are due to Sir William Pennington Ramsden and to the Cumberland County Council for permission to establish our field station in the dunes (2:27) A rabbit "stop". (4:05) the same, threee weeks later. (4:31) Our field station (4:40) an essential commodity..... (4:58) ...... and a scarcely less essential one (5:26)
Experiments on the survival value of egg shell removal. The available evidence suggested that the response serves to remove objects which would make the brood more conspicuous to predators hunting by sight. This presupposes that the brood is protected by camouflage. This was studied first. Equal numbers of naturally coloured eggs and of white eggs were laid out in the dunes, and a watch was kept for predators (5:39) Of a total of 82 eggs of each colour, 14 naturally coloured eggs and 41 white eggs were taken. The predators concerned were: - 1. Carrion Crows - (6:22)v2. Herring Gulls (7:21) 3. Black-headed Gulls (7:41) Conclusion: Naturally coloured eggs are less vulnerable than white eggs. The eggs of many other ground nesting birds are similarly camouflaged (8:03) Would the brood be endangered if the shells were not carried away? 80 eggs were now laid out, of which half had an egg shell at approximately 10 cm distance. In all tests the eggs were covered with a few straws (8:21) Crows and Herring Gulls found 39 eggs-plus-shell and only 13 single eggs. (8:42) The nearer the shell is to the egg, the more the egg is endangered. In tests in which the distance egg: shell was varied, the following nmbers of eggs were taken: shell at 15 cm: 63, shell at 100 cm: 48, shell at 200 cm: 32 - conclusion: These predators would find broods more readily if the egg shells were not removed (8:56) Experiments to discover the stimuli which make the gulls remove the shell (or by which they "recognise"an egg shell) [uitleg van de test] (9:09) - A positive result - A negative result - Results: White and egg-coloured dummies were removed most; all other colours, however bright, were carried less often, and green models were least stimulating of all (10:47) [Test with shape and size] (11:05) Real egg shells were still better than small rings, even when they had the same colour (11:42) How do the gulls distinguish between an egg and an egg-shell? an agg-shell filled with plaster is rolled in; therefore, the absence of the egg's "lid" is of little importance.(11:58) Empty (blown) eggs are treated like eggs, i.e. rolled in, but any thin edge elicits removal. this bird, when offered a "flanged egg", hesitated between removing annd rolling. (12:38) An egg-shell with a piece of lead inside is nibbled but not removed; this response to its weight prevents a gull from removing the chick when it is still in the shell. (13:25) The next scene summarises how the gulls respond to: a normal egg shell, a shell with lead, a plaster-gilled shell, a blown egg with "flange", a blown egg (13:51) The stimulating value decreased rapidly with distance from nest (14:40) Conclusion: [reaction on variety of objects] (15:15) [test with different coloured eggs] (15:26) One other aspect was studied: The response, while obviously under considerable selection pressure, is less prompt than one would expect. The opposite selection pressure, forcing gulls to sit on the chicks until they are dry, is exerted by neighbouring gulls, which prey selectively on wet chicks. (16:02) Once a chick is dry, it is much safer (16:51) Older chicks are sometimes attacked, but they come to little harm (17:03) Usually they even stand their ground and chase intruders away (17:17) thus this study shows: [4 conclusions] (17:30)
Beschrijving
16 mm Film van Niko Tinbergen over de kokmeeuw op rol in blik. kleurenfilm stom.
Formaat
doos Hoogte: 2.8 cm doos Diameter: 25 cm spoel Diameter: 23.7 cm film Filmformaat: 16 mm film Speelduur: 18 min
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