Abstract
Willow ptarmigan females show a considerable annual variation in parental behaviour, which may be due to changes in plasma prolactin levels. To study this variation, free-living willow ptarmigan hens were given exogenous prolactin. Their parental investment was compared with that of control hens. Prolactin treatment did not increase incubation constancy, though a slight but non-significant increase in sitting tightness was observed. When hens with chicks were flushed, prolactin-treated hens showed more distraction display, had shorter flushing distances, stayed longer with the brood, and flew or walked less far from the brood than control hens. All these behavioural differences disappeared 5 days after hatching, coinciding with the time when prolactin administration stopped. One week after hatching, experimental hens had more than twice the mean number of chicks per brood as control hens. A prolactin-mediated increase in risk taking may thus form part of the mechanism whereby parental, investment is adjusted to brood value.
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