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Seasonal Variation in the Breeding Success of the Herring Gull: An Experimental Approach to Pre-Fledging Success

Jasper Parsons
Journal of Animal Ecology
Vol. 44, No. 2 (Jun., 1975), pp. 553-573
DOI: 10.2307/3611
Stable URL: http://www.jstor.org/stable/3611
Page Count: 21
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Since scans are not currently available to screen readers, please contact JSTOR User Support for access. We'll provide a PDF copy for your screen reader.
Seasonal Variation in the Breeding Success of the Herring Gull: An Experimental Approach to Pre-Fledging Success
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Abstract

(1) Seasonal variations in the breeding success of herring gulls Larus argentatus were studied on the Isle of May, Scotland, from 1966 to 1969 inclusively. The breeding season was extended by large-scale egg removal resulting in many relayings, so that any effect due to the date of laying would be exaggerated in late laid repeat clutches. (2) The first eggs were laid between 26 and 28 April, and the clutch size decreased significantly after late May. This decrease was maintained in repeat layings, so that the final mean clutch size was 1.7 eggs per nest compared with 2.8 at the beginning of the season. (3) There was a significant decrease in the mean egg volume through the season, though there was a minimal size below which eggs were not laid. (4) Throughout the season clutches of one and two eggs hatched fewer chicks per egg than three egg clutches. This may be evidence of a lower incubative drive in those birds laying fewer than the normal first clutch of three eggs. (5) Within a sub-colony, eggs laid during the peak laying period were the most successful and both early and late laid eggs had a significantly lower hatching success. This occurred in both the control areas, and also with eggs laid in the experimentally delayed areas, so that hatching success was correlated with nest synchronization irrespective of the time of the season. Therefore, at a time when hatching success of control clutches was decreasing, the repeat clutches were significantly more successful. (6) In the control areas, fledging success decreased progressively through the season, but very early hatching chicks also suffered a higher mortality. Chicks hatching from repeat clutches in the delayed areas did not accentuate this seasonal decline, but showed a peak period of fledging success similar to the control areas but delayed in the season. Predation by herring gulls themselves was a major cause of egg and chick loss. (7) Clutch size was, therefore, the most important factor contributing to the lowered production of chicks per pair later in the season. The synchronization of nesting affected both hatching and fledging success, so that late laying was not a disadvantage in terms of pre-fledging success providing that it applied to the sub-colony as a whole.

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