The chicks of most passerines enclose their wastes within a flexible bag known as a faecal sac. The parent birds meticulously remove these sacs and dispose them some distance away from the nest. These sacs help keep the nest sanitary and the absence of wastes around the nest will not attract unwelcome attention, especially from potential predators.
In the Scarlet-backed Flowerpecker (Dicaeum cruentatum), the adult birds attempt to remove these sacs as and when they appear from the cloaca of the chicks. But in many instances they are not able to cope with the situation and the chicks simply excrete the sacs out through the opening of the nest.
This is not the case with the Oriental White-eye (Zosterops palpebrosus). During the first two days after hatching, the adults will enter the nest, pick up the faecal sacs and swallow them. It is believed that at this early stage of the chicks’ development they are not able to totally digest the food fed to them. The adults thus recycle what the chicks excrete to benefit from the nutrients that are still present in the faeces.
However, as the chicks get older and their digestive system becomes more efficient, they are able to extract most of the nutrients in the food. The adult birds thus simply remove the sacs and dump them some distance away from the nest.
From observations, it was noted that the white-eye chicks excrete only when an adult was around and that they would do this in an orderly way. The moment a chick stuck its posterior up, the adult would immediately move near and grabbed the faecal sac directly from the cloaca (above). The adult would then fly off and dispose of it some distance away. In the example shown on the right, the adult picked up the sac but somehow dropped it.
Is it a wonder then that the nest of the Oriental White-eye is always clean?
Images by Chan Yoke Meng.
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