Birds that respond to the addition/removal of eggs by laying fewer/extra eggs are known as indeterminate layers. Species that are insensitive to additions/removals are said to be determinate layers. Hornbills are an interesting group with which to investigate this phenomenon, because females are sealed into a small dark nest cavity and initiate continuous contact with eggs (incubation) with the laying of their first egg. However, because of their nest plug, they generally face neither partial clutch loss as a result of predation nor brood parasitism (factors hypothesised to influence the evolution of indeterminate laying). By providing Monteiro’s Hornbill Tockus monteiri females with three model eggs after they had laid the first egg of their clutch, I tested whether they exhibit ‘addition indeterminacy’. Females in the egg addition group and those in the control group did not differ significantly in the number of eggs laid. This finding suggests that Monteiro’s Hornbills do not exhibit addition indeterminacy.

通过产下更少/额外的卵来对卵的添加/去除做出反应的鸟称为不确定层。对添加/去除不敏感的物种被认为是决定层。犀鸟是研究这一现象的一个有趣的群体，因为雌鸟被密封在一个小的暗巢腔中，并在产下第一个卵后开始与卵持续接触（孵化）。但是，由于它们的巢塞，它们通常既不会因捕食而失去部分离合器，也不会面临育雏寄生（假设这些因素会影响不确定产卵的进化）。通过提供蒙特罗的犀鸟山雀monteiri产下三只卵的雌性卵在产下第一个卵后，我测试了它们是否表现出“不确定性”。鸡蛋添加组和对照组的雌性在产卵数上没有显着差异。这一发现表明，Monteiro的犀鸟没有表现出额外的不确定性。