Ardea
Official journal of the Netherlands Ornithologists' Union

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Milstein P., Prestt I. & Bell A.A. (1970) The breeding cycle of the Grey Heron. ARDEA 58 (3-4): 171-257
In 1967 a study was made of the breeding cycle of a sample of Grey Herons in a Lincolnshire heronry. Particular attention was given to the possibility that the sub-lethal residues of certain organochlorine insecticides, known to be present, might be causing aberrant behaviour. An attempt has been made to consolidate the results from this study with data in the literature to provide a comprehensive basis for current long-term research. In the study the breeding pairs were divided into three age classes. Two pairs were in adult plumage, five pairs were in sub-adult plumage, two comprised an adult mated with a sub-adult, and in four pairs both parents were yearlings. These appear to be the first definite records of yearling pairs breeding. The mean clutch-size was 4.1, the mean number of chicks hatched per pair 3.1 and fledged per pair 2.5. There was no significant difference in the clutch-size or breeding success for the different age classes of parents. A higher percentage of chicks survived from clutches with three eggs than four eggs, and from those with four eggs than with five eggs, but the mean success per pair was lowest from the clutches of three. The greatest losses were as a result of eggs failing to hatch and the death of very young chicks. Apparent aberrant behaviour was noted in two pairs: in one of these a live chick was apparently dropped from the nest after the behaviour of the parents suggested they had restarted the breeding cycle, and in the other the male destroyed in succession the twenty-one eggs laid by the female. Possible correlation with pesticidal contamination is discussed. The typical breeding cycle of the Grey Heron starts in January to March, varying in different years. Occupation of the colony is gradual and begins with the herons roosting there. Previous suggestions of a 'dance' on the gathering grounds are not supported. The restricted nesting territory, usually an old nest, is chosen by the male. Early in the season it is only occupied for a short period in the morning and evening. Advertising for a mate commences with loud yelps, with the 'bitterning', 'bow-snap' and 'threat' displays interspersed. The females select the males and older herons appear to enjoy preferential treatment during pair formation. Copulation takes place from pair formation until the clutch is complete. During this early period the bill and legs become reddish under hormonal control. Incubation is by both sexes and included periods of at least twelve hours. Intensive incubation begins the day after the first egg is laid and apparently lasts 27-28 days. Apart from disturbance, one or other parent constantly guards the nest until the chicks are four weeks old. Double broods are rare, but replacement clutches common, possibly due to corvid predation of the first clutch. The hatched egg-shells are dropped over the side of the nest and there is a second peak of stick-bringing at hatching, the first occurring soon after pair formation. Most pairs utilise old nests, but new (often frail) nests are frequently built in the course of a few days, even when unoccupied old nests are available. Many of these may be built by yearling pairs and may be an important contributory factor to sudden increases noted in some colonies. The post-hatching period is characterized by brooding and guardian periods when the chicks are constantly attended, and post-guardian and fledgling periods when the chicks are usually unattended. Both nocturnal and diurnal feeding of chicks occurs. Feeding is by both parents and the intervals between feeds become increasingly longer as the young develop. Feeding is by regurgitation into the nest and small chicks are provided with tiny fish in this way. Chicks solicit by bill-grasping their parents and this should not be confused with direct feeding. The first flight of the chicks is usually at about seven weeks of age, but they may still return to the nest for a further three weeks.


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