What happened to the Grey Heron nesting colony?

posted in: Heron-Egret-Bittern, Nesting-failed | 0

In November 2008, TC chanced upon an active heronry in the north of Singapore. It was the incessant noise of the birds that attracted his attention. The casuarinas trees (Casuarina equisetifolia) were covered with Grey Herons (Ardea cinerea), many sitting in their nests, others perching on branches nearby (above). The herons were in their breeding colours – their legs and bill were distinctly orange to reddish (below).

TC returned a few times to the site to monitor the breeding. Then one day, a few weeks after he first came across the heronry, he found the trees silent. The nests were still there but the birds were all gone. The Grey Herons had abandoned their nests.

Our bird specialist, R Subaraj, has this to say: “There are a few possibilities as to why the herons may have abandoned their nests but we cannot be certain of the true reason unless we know all that have taken place during the period that they were there.

“Possible scenarios include human disturbance, directly or indirectly or that this was a new site that the herons were trying out but found unsuitable due to unknown reasons. Such temporary colonies have occurred before and human disturbance has been the main reason for past abandonment.”

Our field ornithologist Wang Luan Keng, who did two major surveys of heronries in Singapore some years ago, found that their breeding sites are fragile entities. The birds are very sensitive to human disturbances and these can lead to the breeding birds abandoning their nests.

Once, there were many heronries in Kranji, Khatib Bongsu, Pasir Ris and Sungei Buloh. As the marshes were reclaimed and trees felled, many disappeared. Excessive fogging to control mosquitoes can also cause the birds to abandon their nests. If the trees are still standing, the birds may or may not return to nest the following year.

Images by TC.

References:
1.
Wang, L. K., 2003. Where have the herons gone? Singapore Avifauna 17(2): 32-34.
2. Wang, L. K., 2004. Where have the herons gone? Singapore Avifauna 17(4): 24-26.

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