Motivationally controlled stimulus preferences in chicks of the black-headed gull ( Larus ridibundus L.)

Abstract

The main aim of this study was to investigate the effect of motivation on the stimulus selectivity of the black-headed gull ( Larus ridibundus L.) towards a constant stimulus situation. Soon after hatching the young of this species respond to the parent bird in either of two ways; when ‘hungry’ they beg for food by pecking at its billtip, when satiated but ‘cold’ they seek warmth and shelter by snuggling beneath the parent. From the second day after hatching both activities can be preceded by approach from a distance. At the same time the chicks start responding to the adults' alarm calls by running into cover and crouching there. The present experiments were carried out to see (1) whether ‘hungry’ chicks when approaching objects would show similar or different stimulus preferences than when pecking; (2) whether ‘satiated cold’ chicks would show similar or different preferences than ‘hungry’ chicks when approaching the same stimulus objects. In addition stimulus preferences when running into cover as response to alarm-calls were studied. Hand-raised chicks (24 to 36 hr posthatching) were therefore tested (a) several hours since being fed, i.e. ‘hungry’, (b) just after feeding, i.e. ‘satiated but chilled’ and (c) in a situation where alarm-calls were played to them from a tape recorder, i.e. ‘alarmed’. For approach ‘hungry’ and ‘cold’ chicks were tested in standard apparatus with flat cardboard models resembling an adult gull in profile presented two at a time which differed in either of the following dimensions: colour, brightness, background relations or pattern. ‘Hungry’ chicks, upon reaching a model, would usually peck at it, ‘cold’ chicks would snuggle beneath it. In the alarm situation the objects presented were either small rectangular cardboard boxes or gull models and the chicks approached them and then crouched. A subjective brightness order of the stimuli was assessed by computation from the spectral sensitivity curve of the pigeon and the reflectances of the colour papers used. With uniformly coloured models, ‘hungry’ chicks showed similar colour preferences whether pecking at model beaks or approaching entire model gulls. It was concluded that in both situations the colours were discriminated by hue and not merely by their brightness. The preferences of ‘satiated cold’ and ‘alarmed’ chicks differed from those of ‘hungry’ chicks. Thus the results clearly show a shift in preference with changing motivational state. With uniformly grey models, ‘hungry’ chicks showed similar grey preferences whether pecking at model beaks or approaching entire model gulls. ‘Hungry’ and ‘cold’ chicks showed the same preference order when approaching models. With white and black models on backgrounds in different shades of grey, ‘hungry’ chicks showed roughly the same scores of pecking and approach preferences. ‘Cold’ chicks chose the black model more often than did ‘hungry’ chicks suggesting that they responded less to the contrast between stimulus-object and background than ‘hungry’ chicks and to a greater extent to the absolute shade of the object. Tests designed to discover whether food or temperature contributed most to the change in preferences between ‘hungry warm’ and ‘satiated cold’ chicks showed that the change in preference was more affected by the degree of satiation than chilling. The possible adaptiveness of the preferences was discussed on the basis of some previously described tests and some other tests specially designed to find out about the mutual adaptiveness between the chicks' stimulus selectivity and the characteristics of the parent bird. It is suggested that the change in stimulus selectivity found was possibly not so much response or motivation specific but due to a general change of activity.