Retrospective study on admission trends of Californian hummingbirds found in urban habitats (1991–2016)

Abstract

Background Hummingbirds are frequently presented to California wildlife rehabilitation centers for medical care, accounting for approximately 5% of overall admissions. Age, sex, and reason for admission could impact hummingbird survivability, therefore identification of these factors could help maximize rehabilitation efforts. Methods Mixed-effects logistic regression models were used to identify specific threats to the survival of 6908 hummingbirds (1645 nestlings and 5263 non-nestlings) consisting of five species (Calypte anna, Calypte costa, Selasphorus rufus, Selasphorus sasin, Archilochus alexandri), found in urban settings, and admitted to California wildlife rehabilitation centers over 26 years. Results In total, 36% of birds survived and were transferred to flight cage facilities for further rehabilitation and/or release. Nestlings were more likely to be transferred and/or released compared to adult hummingbirds. After accounting for age, birds rescued in spring and summer were twice as likely to be released compared to birds rescued in the fall. A high number of nestlings were presented to the rehabilitation centers during spring, which coincides with the nesting season for hummingbirds in California, with the lowest number of nestlings presented in fall. Reasons for presentation to rehabilitation centers included several anthropogenic factors such as window collisions (9.6%) and interactions with domesticated animals (12.9%). Survival odds were lower if a hummingbird was rescued in a “torpor-like state” and were higher if rescued for “nest-related” reasons. Evaluation of treatment regimens administered at wildlife rehabilitation centers identified supportive care, including providing commercial nutrient-rich nectar plus solution, to significantly increase hummingbird survivability. Discussion Our results provide evidence of threats to hummingbirds in urban habitats, based on reasons for rescue and presentation to rehabilitation centers. Reasons for hummingbird admissions to three California wildlife rehabilitation centers were anthropogenic in nature (i.e., being associated with domestic animals, window collisions, and found inside a man-made structure) and constituted 25% of total admissions. There was a clear indication that supportive care, such as feeding a commercial nectar solution, and medical treatment significantly increased the odds of survival for rescued hummingbirds.

Keywords

• Anthropogenic threats
• Archilochus alexandri
• Calypte anna
• Calypte costa
• Selasphorus rufus
• Selasphorus sasin