Monteiro’s Hornbills Tockus monteiri do not exhibit addition indeterminacy

Abstract

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.