“Grey Herons (Ardea cinerea) are solitary birds when they are not breeding. Why do they congregate into a colony and nest in a heronry during their breeding season? My guess is that there is safety in numbers. Like birds roosting together, birds in communal nesting will have many pair of eyes, which will have a multiplying effect in alerting against predators. Breeding in heronries will therefore provide Grey Herons and their offspring with a relatively higher chance of survival than breeding alone in an isolated spot.
“Grey Herons’ nests are huge bulky platforms that are constructed using mainly sticks that are found in the nesting area. During initial nest building, the herons will be busy collecting sticks picked up from nearby trees. They can be dead twigs that get lodged in trees, or living stems that will need to be broken off. During repeated visits to the new heronry at Sungei Tampines, the nesting materials collected were observed to be of various shapes and sizes. The delivered materials included twigs with dead leaves, twigs with living leaves, barren twigs, and an extra long twig that was even longer than the heron’s body length (below). At the other extreme, a small piece that consisted of just two green leaves without much stem was delivered.
“Grey Herons’ nest building has evolved to the breeding pair sharing different duties. Normally, one heron will be searching and collecting nesting materials while its mate will be waiting for the material to build the nest. The heron’s long and powerful beak is used as a tool for delivery of nesting materials. The heron will grip the nesting material firmly in its beak, before flying with it back to its nest (below left). At the nest, the material will be transferred beak-to-beak to its mate (below right). The mate will then place it into the nest while its partner flies off in search of more. With one bird always in the nest, it guards and prevents nesting materials from being stolen. An occupied nest will help advertise nest ownership to other herons and prevent them from taking over its ownership.
“On 25the July 2010, a heron was first observed scrutinising for suitable nesting materials in a tree. It found a suitable stick, which was at a higher level than the bird. To reach for the stick, it had to do a full body stretch; stretching its entire body with both legs and long neck completely straightened. It managed to grip onto the lower end of the stick with its beak before proceeding to pull it down. Great effort was exerted as the bird used its strength and body weight. Swaying precariously on its perch because of its effort, it had to spread its wings for balance. The stick remained stuck and refused to be dislodged. A few tugs later, its attempt was still not successful. One final tug ended with the stick slipping from its beak and causing the heron to lunge downwards. The heron almost lost its balance. Only its huge wings that were instinctively spread helped it to overcome its imbalance. Eventually, the heron gave up on this stubborn stick and proceeded to look for an easier target. Collecting of nesting materials can be exhausting.
“On 31 July, the same heron (presumably, as it was from the same nest) discovered another twig in the same tree. This time, due to the twig at lower level than its eyes, the heron had to bend its knees to lower itself before extending its neck forward for the twig. The heron managed to dislodge the twig. With the twig held firmly in its beak, it took flight to deliver it back to its nest, which was about thirty metres away. The heron landed next to the nest where its mate was waiting. On this occasion, instead of transferring the nesting material to its mate, the heron decided to get into the nest (top). With the soft morning sunlight shining through their saturated orange-coloured beaks, the couple lovingly placed the stick into the nest mutually.
“Delivering of nesting materials in the heronry was a common sight and seemed to be never ending. Without doubt, nests will need constant care and maintenance as some material will drop off due to continuous use. Wind and rain are contributing factors that will cause damage. Hence, a continuous supply of nesting materials will be needed to upkeep the nests. With the privilege of observing the entire nesting cycles of some of these Grey Herons, it was interesting to note that delivery of nesting materials was carried out throughout the nesting period; from initial nest building until the chicks had fully fledged.”
Kwong Wai Chong
Singapore
22nd November 2010
