The impact of soil moisture, nectar production, and spectral reflectance on hummingbird visitation patterns to the Scarlet Gilia (Ipomopsis aggregata)

Abstract

Climate change has brought intense disruptions to the natural patterns of growth and reproduction in sub-alpine ecosystems, generating changes to the food sources that Broad-tailed hummingbirds (Selasphorus platycerus), which breed in the Rocky Mountains, depend on. A hotter, dryer climate results in changing levels of soil moisture, which may impact floral traits such as color and sugar concentration in nectar. In order to find out how variations in these traits impact hummingbird visitation rates to certain flowers, we used data from HummerFlowerWatch, a long-term study gathering camera trap data (documenting every time a hummingbird visits a chosen flower) and transect data (measuring flower abundance), we collected specimens of the native Ipomopsis aggregata flower in Gothic, CO, and we noted the trail and measured soil moisture, glucose concentration in nectar, and spectral reflectance to determine correlations between these variables. We predicted higher soil moisture in Stupid Falls, causing higher sugar concentration in nectar, high general nectar output, and higher intensity of reflected red and UV wavelengths. We then expected current trends in visitation data to reflect higher visitation in Stupid Falls. In contrast, we predicted lower soil moisture in Judd Falls, causing lower sugar concentration in nectar, lower general nectar output, and lower intensity of reflected red and UV wavelengths. We also expected current trends in visitation data to reflect lower visitation in Judd Falls. After a 20-day research period for nectar studies, the data supports our hypothesis about Stupid Falls having higher soil moisture and nectar output, indicates a significant positive correlation between soil moisture and nectar output, and indicates a significant negative correlation between soil moisture and nectar concentration. No significant conclusions can be drawn about nectar concentration differences by trail. After a net data set of seven days of spectroscopy, our results show that reflectance does vary significantly by trail (Stupid Falls flowers reflected with greater intensity), but with a small effect size; there is still a lot of overlap between the spectral shape of flowers from both sites. Analysis also showed that Ipomopsis tubes reflect UV and red wavelengths at a higher intensity that the petals do. Overall, hummingbird visitation rates from 2022-2024 indicate a significant difference between trails, favoring Stupid Falls as the more highly visited sight possibly caused by greater nectar output and reflectance.

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