Commentary: Subadult Nest Occupancy Rates and Floater-To-Breeder Ratios in Raptor Population Assessment

Abstract

Subadult (immature) raptors of a variety of species are capable of holding breeding territories and even reproducing (Newton 1979, Steenhof et al. 1983). Robust populations, however, typically contain few, if any, pair members in pre-definitive plumage, and the general explanation is that adults tend to outcompete younger individuals for territory ownership where space is limiting. Their rarity as territory-holders can therefore be a useful indicator of breeding habitat saturation, whereas their increase has been considered ‘‘early warning’’ that vital rates (survival and reproduction) are insufficient to fill territorial space (Ferrer and Donazar 1996, Ferrer et al. 2003). Territory saturation also constitutes a threshold beyond which floaters (nonterritorial adults) can be expected to accumulate. The size of such a population and the ratio of floaters to territory-holders will stabilize if vital rates remain high enough to maintain saturation, a mode of population limitation known as Moffat’s equilibrium (Hunt 1998). The equilibrium floater-tobreeder ratio is useful in broadly indexing the durability of territory saturation as well as the degree of expected feedback of floater intrusions upon nest success (see Haller 1996). A stochastic population matrix model developed by Monzón and Friedenberg (2018) explored the floater dynamics of Moffat’s equilibrium (Hunt 1998, Hunt et al. 2017). Importantly, the rate of floater transition to the breeder stage was modeled dynamically so that transitions were determined by the availability of territories. Their model projected life-stage-structure and allowed computation of instantaneous floater-to-breeder ratios and rates of subadult nest occupancy in a hypothetical Golden Eagle (Aquila chrysaetos) population. The authors ran simulations of population decline and growth, habitat expansion and contraction, and valuably, the 10-yr cycling of a hypothetical prey population. Monzón and Friedenberg’s (2018) report emphasized that floater-to-breeder ratios and the incidence of subadult nest occupancy responded to these scenarios in ways indicating that neither, as a ‘‘snapshot metric,’’ can diagnose the status of a population in the absence of other information. For example, high instantaneous floater-to-breeder ratios characterized not only robust populations, but also those where territory occupancy was shrinking because of habitat loss. High rates of subadult occupancy manifested in both increasing and declining populations. In all, we found Monzón and Friedenberg’s (2018) modeling results consistent with Moffat’s equilibrium dynamics as described by Hunt (1998), Hunt and Law (2000), and Hunt et al. (2017). We believe that variations upon the authors’ algorithm can find useful application in studies of