Where have the Black Swans gone in Singapore Botanical Garden ? 新加坡植物园内的黑天鹅到底去了那儿？

There were Black Swans in the Singapore Botanical Garden, But where are those Swans now ? As seen from the postcard above, I can count, there are 5 swans on the picture.
新加坡植物园内的黑天鹅到底去了那儿？以上图片可清楚看到5只天鹅徘徊在植物园内的湖边。