Most birders know that the female hornbill seals herself inside a tree cavity when she is ready to lay her eggs. But how many have actually witnessed the hornbill breaking out of its cavity when the chicks inside are ready to leave the nest?
Let alone document the stages?
It has to be left to a photographer to undertake the assignment. Dr Eric Tan, an avid nature (bird) photographer, was at Pulau Ubin at the right time when the female Oriental Pied Hornbill (Anthracoceros albirostris) was breaking out of her nest. He meticulously documented the various stages and is here sharing them with everyone.
When the female is ready to lay her eggs, she enters the nest cavity, usually in a tree trunk. The male then brings her mud that she mixes with her faeces and mashed fruits to seal the entrance until only a narrow opening is left (above).
The female then begins to pull out her wing and tail feathers and then lays her eggs. The male will, in the meantime, delivers food for her and her chicks (above).
Once the chicks are ready to fledge, she will start breaking out of the cavity. This seal is then brick-hard and generally impenetrable from the outside. She needs to use her bill like a pickaxe to slowly break down the seal. As the seal slowly breaks down and the entrance enlarges, she needs to squeeze herself out. First her large bill and casque appear, then her head pops out (above, top row). Next, one of her shoulders pushes out, followed by the wing. Once one of her wings is free, she emerges partially before becoming totally free to immediately fly off (above, bottom row).
The chicks will then be enticed to leave the nest by the adults not bringing them food. As the chicks are not of uniform age when the female breaks out, the questions that need to be answered are: 1. Does the female re-seals the nest? 2. Do the chicks take over the job? 3. Is the entrance left unsealed? 4. Who returns to feed the remaining chick/s, the male or the female? There is obviously a need for further observations.
The sealing of the female inside the cavity provides security from predators, prevents the nesting cavity from being flooded and keeps off competitors – other hornbills that may otherwise try to evict the occupants for their own use. However, Kinnarid & O’Brien (2007), “…believe that nest-sealing evolved as a female strategy to ensure male fidelity.” With the male kept busy foraging for himself as well as for his mate and later the chicks, he would have no time to indulge in extra-pair copulations or maintain another female sealed in another cavity.
Kinnarid, M. F. & O’Brien, T. G. (2007). The ecology and conservation of Asian hornbills: Farmers of the forest. University of Chicago Press.
This post is a cooperative effort between www.naturepixels.org and BESG to bring the study of bird behaviour through photography to a wider audience.