Nesting Grey Herons: 4. Incubation

“After copulation, a clutch of eggs will be laid by the female Grey Heron (Ardea cinerea). The eggs must be incubated before they can hatch. According to Wikipedia: ‘incubation refers to the process by which birds hatch their eggs, and to the development of the embryo within the egg. The most vital factor of incubation is the constant temperature required for its development over a specific period.’ Being warm blooded animals, the body temperature of birds is constant. Hence, direct physical contact of the adult birds sitting on their eggs will provide a constant temperature that is required for incubation. The embryos in fertilised eggs will develop during incubation. When the embryos are fully developed at the end of the incubation period, the eggs will hatch; leading to the emergence of chicks.

“For Grey Herons, incubation is undertaken by both parents. At any one time, one of the parents will be in the nest with the eggs while its mate could be away foraging for food. When the foraging parent returns, it relieves its mate from incubation duty. ‘Shift changing’ can be fairly swift; with most completed within 20 seconds. There may be a brief exchange of greetings with raised crests when the returning parent lands beside the nest. The heron that is to-be-relieved will step out of the nest for the returning heron. After stepping into the nest, the returning heron assumes incubation duty. Without ceremony, the relieved mate will fly off for a few hours of well-deserved freedom before returning to take its turn again.

“What will Grey Herons do during incubation? From observations, Grey Herons will usually crouch low while sitting on the eggs during incubation (above left). This could possibly be a defensive strategy to stay inconspicuous and be out-of-sight to potential predators. However, it is not possible for the herons to sit on the eggs all the time. The herons will need to check, adjust and re-position the eggs (above right). Other activities observed during incubation are comfort behaviour such as preening, sunning, and stretching.

“The endurance of Grey Herons is quite remarkable. The herons on incubation duties will remain in the nest under all adverse weather conditions. Rain or shine, without fail, all incubating herons will remain in their nests to protect their eggs. Getting drenched in the rain, and sheltering their eggs from the hot sun to prevent them from being baked are sacrifices that the parents will have to endure for their young (above left). Steadfastly, they will remain with the eggs until they are relieved. Shift changing is infrequent and relief could be hours away.

“When an incubating heron feels the urge, how does it ease itself? Observations showed that it will remain in the nest. It will adjust its position, stick its posterior outside the nest and discharge its crap (above right).

Kwong Wai Chong
Singapore
11th December 2010