Influence of mate and nest-site fidelity on a declining, urban avian population

Abstract

As urbanization reduces species' habitats and population sizes, managers need information on whether within-population processes, such as changes in mate and nest-site fidelity and dispersal distances, may be contributing to declines. Few avian studies have examined changes in these behaviors in declining populations or in urban settings. We investigated whether mate fidelity, nest-site fidelity or breeding dispersal distance changed over time in a population of burrowing owls (Athene cunicularia), a short-lived, socially-monogamous species. During the 18-year period of the study, the population declined by 69% in urban Santa Clara County, California, USA--a region of rapid urbanization. We assessed whether these behaviors were influenced by key factors including age, breeding success in the previous year, and years with the same mate, and examined the relationship between mate and nest-site fidelity over time and annual reproductive success. Our analyses showed no change over time in mate fidelity rates, nest-site fidelity rates, dispersal distances or annual reproductive success, indicating these behaviors remained stable even during a severe population decline. Although burrowing owls are a short-lived species, we found that increasing years with the same mate resulted in increased nest-site fidelity and annual reproductive success. To achieve increasing annual reproductive success in this species and others with similar fidelity behaviors, nest sites and pairs must be protected over many years allowing mates to stay together in the same nest territory. Since these fidelity behaviors were maintained during the population decline, other factors require investigation to determine the causes for decreases in this population. Burrowing owls are an urban-adaptable species that can maintain important fidelity behaviors in human-altered habitats. However, even such species are subject to population declines in urban settings.

Fig 1. Study region in Santa Clara County, CA (outlined in white) and the breeding burrowing owl study sites (outlined in black), 1999-2016 (Satellite image courtesy of the U.S. Geological Survey).

Fig 2. For burrowing owls in the study region in Santa Clara County, CA from 1999–2016: A) mate and nest-site fidelity rates (percent) shown with the overall size of the adult population; B) mean (±SE) annual reproductive success (number of chicks per brood) for successful and failed nests combined, shown with the number of birds included in the analysis.

Fig 3. Relationship between years with the same mate and A) nest site fidelity rates (percent) and B) mean (± SE) breeding dispersal distances (m) for male and female burrowing owls during the study period, 1999–2016, in the Santa Clara County, CA study area.

Number of birds of each sex shown at points or bars.

Fig 4. Influence of A) mate fidelity, B) nest-site fidelity, and C) number of years with the same mate on annual reproductive rate, as measured by the mean (± SE) number of chicks per brood for male and female burrowing owls during the study period, 1999–2016, in the Santa Clara County, CA study area.

Numbers of birds per sex shown above bars.