Demography of two species and one genus of hummingbirds with contrasting population trends in California, USA

Abstract

Hummingbirds in North America are currently experiencing contrasting population changes, and little is known about the factors contributing to these changes. We examined the demography of two species and one genus of hummingbirds in western North America, including Anna's Hummingbirds (Calypte anna), Black-chinned Hummingbirds (Archilochus alexandri), and hummingbirds in the genus Selasphorus, to investigate the mechanism underlying these contrasting trends. We analyzed mark-recapture data collected over periods ranging from 6 to 11 years in California, USA, to quantify demographics, including sex ratios, the proportion of transients, and age-dependent, sex-specific, and species-specific apparent annual survival. Transience was estimated at sites where parameterization of annual survival allowed the inclusion of time-dependency. We estimated that 34% of hummingbirds were transient at one site, but only 5% at another site. Estimates of annual survival followed a negative trend after birds reached their first year. Evaluation of the short-term (4 yr) effect of subcutaneous implantation of radio-frequency identification transponders on survival estimates of Anna’s Hummingbirds revealed no difference in apparent annual survival. Robust estimates of demographic parameters are essential for conserving birds with changing populations. As such, our results contribute important context for the contrasting population trends among hummingbirds in western North America.