- ERON , CENSUS

THE 1981 GREAT BLUE HERON CENSUS ' by Rosemary Curley, Fish and Wildlife Division

A thorough census of great blue herons in 1981 revealed some startling statistics regarding Prince Edward Island's breeding population. Final results will be presented by Al Smith, the Maritime Heron Census Co-ordinator, in a comprehensive report in 1982, but on request of several interested heron censusers, some preliminary data are presented here. All data are subject to interpretation, and slightly different figures may be used later. Data for Rustico Island have been kindly forwarded by P.E.I. National Park staff.

Natural History Society members and friends spent an estimated 75 man—hours inside nine active colonies ranging in size from 20 to 380 nests (Table I), while P.E.I. National Park staff censused the colony on Rustico Island. Four of five colonies unused in 1980 were also rechecked, but their status remains unchanged (Table II).

In total, 1715 active nests were recorded, far exceeding the approximately 900 nests counted during 1980 surveys. Mean colony size of 172 nests again surpasses the estimated average of 75 nests calculated by Al Smith from pre-l980 datafi(Smith, 1980). It is possible that heron numbers have increased naturally .since the mid—seventies when colonies on P.E.I. were surveyed by Geoff Hogan, but expansions in actual colony size have probably resulted in part from an influx of birds from several colonies now.deserted (Table II). The increase over 1980 estimates can be attributed largely to improved survey techniques, wherein individual trees were marked with flagging tape to facilitate accurate nest counts. ‘After the 1980 experience, censusers allowed adequate time to properly census colonies, and the weather was more conducive to travel to offshore islands. More colonies were well-censused in 1981 than in 1980.

An estimate of 100 nests on Governors Island in 1980 is now replaced by an accurate count of 248 nests, 202 of which were active. Similarly, estimates for Boughton, Herring and Glenfinnan Islands are replaced by counts exceeding the 1980 estimates by 50 to 100 nests. pn Oulton's Island censusers discovered an additional part of the colony which was previously active, and expanded their 1980 count by approximately 100 nests - to 375! A first count of Courtin Island by five enthusiasts indicated that approximately 380 nests were present, but censusing the sprawling colony posed problems for the group, so this remains an estimate.

A rough estimate of the peak population of great blue herons on P.E.I. is now possible. If there are 1715 pairs (3430 adults) of great blue herons, each successfully raising 2.4 young annually (4116 young), the Maritime average last year, this adds up to a post-fledging population of 7546 herons. This does not take into account numbers of non-breeding birds, or the possibility of adult mortality.

The 1981 heron census people have achieved excellent results under often difficult circumstances, and this data would not be available without their perseverance and ready co—operation. Thanks to them, several points of curiosity about great blue herons have been satisfied. An additional article on the Heron Census will be presented in this newsletter in early 1982, with further information on breeding success, including comparative results from New Brunswick and Nova Scotia.

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