Use of a New Reproductive Index to Evaluate Relationship between Habitat Quality and Breeding Success

Abstract

--A new method of measuring reproductive success is proposed that uses a composite of breeding-behavior observations (for behaviors that reflect different stages in the reproductive cycle) as an index of fitness. This reproductive index does not rely on discovery of nests, but is comprehensive in that it includes information on all monitored territories. The reproductive index was applied to three co-occurring grassland emberizine sparrows, two of which required special care because of their regional rarity. Ranks derived from this reproductive index were used to distinguish territories of birds of known high success (i.e. those that fledged young in at least one brood) from territories of birds with known low success (unpaired males), and were compared with findings for "spot-mapped" territories. Principal-components analyses of habitat measurements for these territory types revealed a similar pattern for all three species: spot-mapped territories overlapped broadly with nonterritory (unoccupied) plots, whereas high-success territories formed a discrete, isolated cluster within the spot-map matrix. Univariate analyses revealed that high-success territories were described by 15 vegetation features that differed (P < 0.01) from nonterritory values, whereas in spot-mapped territories only 8 vegetation measures differed and in low-success territories only 2 differed. The ability to distinguish high-success territories allowed us to identify a greater number of habitat features that were correlated with reproductive success. If we had relied on the spot-mapping method, we would have been unable to identify many of these important habitat features. Yet, the ability to make such discriminations is likely to be critical in the management of threatened species.