Resource suitability drives low use of avian-excavated tree cavities: A multi-state occupancy dynamics approach

Avian tree-cavity excavators are widely held to maintain diversity of forest vertebrate faunas through the facilitation of nesting resources, and yet in many systems they are absent or redundant. Why do avian excavators sometimes supply only a small proportion of cavities used by non-excavating species? Researchers hypothesized that low re-use of excavated cavities could be driven by high rates of excavated cavity loss (low availability) or by low suitability of excavated cavities. The two hypotheses imply different cavity use dynamics. The availability hypothesis predicts high rates of excavated-cavity transition from excavators to secondary cavity-nesters. The suitability hypothesis predicts high transition rates from excavators to empty (unused), and high reuse rates for cavities previously used by secondary cavity-nesters. From 2006 to 2021, we studied 438 excavated and non-excavated bird nest cavities in the Atlantic Forest of Argentina, where excavators provide ~20% of cavities used by secondary cavity-nesters. We fit our data with a multi-state occupancy dynamics model that accounts for observation errors and estimates transition probabilities among cavity states 'Empty', 'Occupied by an excavator', 'Occupied by a secondary cavity-nester' and 'Lost'. We complemented the modelling results with a numerical simulation of cavity use dynamics. As predicted by the suitability hypothesis, the estimated probability of transition was high from excavator to empty (~0.75), low from excavator to secondary cavity-nester (~0.05) and high for reuse by secondary cavity-nester. Transition from 'Empty' to use by secondary cavity-nester was much more probable among non-excavated (~0.2) than excavated cavities (~0.05), which-our simulation shows-is compatible with secondary cavity-nesters using excavated cavities in proportion to their availability. Excavated cavities remained available for several years after their last use, suggesting suitability declines with age. We conclude that the marginal role of excavators as cavity producers is driven primarily by low suitability of their excavated cavities for other birds. Statistical support for the suitability hypothesis relied on the quantification of uncertainty about cavity states afforded by multi-state occupancy dynamics models. We encourage further exploration of state-transition probabilities among tree cavities and other multi-use resources to test ecological hypotheses and inform resource conservation policy.