The egg-laying seasons of various captive waterfowl in the Wildfowl Trust collections at Slimbridge and Peakirk are compared with the situation in the wild. Special attention is focussed on the shelduck and sheldgeese (Tadornini), perching duck (Cairinini), diving duck (Aythyini), sea duck (Mergini), eiders (Somateriini) and stifftails (Oxyurini).
Two kinds of response could be distinguished at Slimbridge. In one, egg-laying began sometime in spring, when daylengths reached a stimulatory level, and continued until the corresponding daylengths in summer or autumn were reached, that is, the egg-laying season extended symmetrically on either side of the summer solstice. This is regarded as a “primitive-type” response and is characteristic of phylogenetic lines which are thought to have evolved in, and radiated from, the tropics; the comb duck, muscovy duck, ringed teal and southern pochard are examples. The second kind of response was characterized by the cessation of egg-laying when daylengths still remained stimulatory, that is, a characteristic photo-refractoriness to long days developed, and egg-laying occurred during the first half of the year at Slimbridge. We term this a “temperate-type” response, and examples are provided by nearly all the sheldgeese and shelduck, by the mandarin, Carolina, white-winged wood duck and all Aythya and the sea duck. Only a few examples of apparently intermediate breeding response were noted and these transitional species are discussed, for example the Andean goose, Abyssinian blue-winged goose and Radjah shelduck.
The egg-laying cycles can be described as entrained rhythms and their phase angles measured in relation to seasonal daylength changes at Slimbridge. In Aythya, the onset of reproductive activity was strongly correlated with the latitude of origin whereas the end of breeding was not. This was because refractoriness developed at about the same time in all species, even though they began breeding in response to different photoperiods. For this reason, the length of breeding season expressed at Slimbridge was correlated with the midlatitude of the breeding range in the wild but even more strongly with the photoperiod under which egg-laying was initiated. The onset of breeding in the sheldgeese and perching duck was not related to latitude, perhaps because species have had time to modify their ranges. However, the end of breeding in perching duck was related to latitude. Cessation of breeding corresponds to the onset of moult and the secretion of thyroid hormones.
We suggest that as waterfowl radiated to higher latitudes their “biological clocks” became entrained to the new light regimes and a host of physiological functions were ordered in an adaptive rather than competitive sequence. It is because so many biological functions are liable to compete for scarce environmental energy supplies that some kind of endogenous partitioning is necessary. This endogenous component is seen when different species are kept under standard lighting conditions. It follows that the photo-response of a species reflects its evolutionary history and so the discussion broaches zoogeographical and taxonomic topics. For example, it seems unlikely that the white-winged wood duck evolved in the tropics, and likely that the sheldgeese and shelduck radiated from a post-