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. 2021 Apr 13:9:e11131.
doi: 10.7717/peerj.11131. eCollection 2021.

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

Pranav S Pandit #  1 , Ruta R Bandivadekar #  2 , Christine K Johnson  1 , Nicole Mikoni  2 , Michelle Mah  3 , Guthrum Purdin  4 , Elaine Ibarra  5   6 , Duane Tom  7 , Allison Daugherty  4 , Max W Lipman  4 , Krystal Woo  4 , Lisa A Tell  2
Affiliations

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

Pranav S Pandit et al. PeerJ. .

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; California; Calypte anna; Calypte costa; Selasphorus rufus; Selasphorus sasin; Wildlife rehabilitation; Wildlife rescue.

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Conflict of interest statement

The authors declare that they have no competing interests. Elaine Ibarra and Duane Tom were employes of the Santa Barbara Wildlife Care Network and LAWild, respectively during the data collection period.

Figures

Figure 1
Figure 1. Wildlife rehabilitation centers contributing hummingbird rehabilitation data.
Geographical map of the state of California highlighting the locations of the rehabilitation centers from where the hummingbird rehabilitation data records were obtained for this study (1991–2016).
Figure 2
Figure 2. Hummingbird rehabilitation from 1991 to 2016 in Californian rehabilitation centers.
(A) Temporal distribution of the number of birds admitted to California wildlife rehabilitation centers from 1991 to 2016. (B) Overall disposition distribution for rescued hummingbirds over a period of 26 years. (C) Distribution of nestling disposition by month rescued over 26 years (1991–2016).
Figure 3
Figure 3. Reasons for hummingbird admission.
Distribution of the rescued 6908 hummingbirds from the three Californian-based wildlife rehabilitation centers based on the reason for admission category.
Figure 4
Figure 4. Monthly distribution of hummingbirds admitted to California wildlife rehabilitation centers and their disposition for each admission category (n = 7).
(A) Found on the ground, (B) unknown, (C) nest-related, (D) caught by domestic animal, (E) window collision, (F) found inside and (G) suspect torpor-like state. Boxplots show the median, first, and third quartiles of the data. Whiskers extend to the data range and outliers are presented as separate diamonds. Green horizontal line represents 10 monthly cases for visual comparisons across subplots.
Figure 5
Figure 5. Risk factors affecting release of hummingbirds from rehabilitation centers.
Odds ratios for all risk factors and their confidence intervals for rescued hummingbirds (n = 6908) with available treatment information. Categories are color-coded according to the group of independent variables.
Figure 6
Figure 6. Risk factors and treatment options affecting the release of hummingbirds from rehabilitation centers.
Odds ratios for all risk factors and their confidence intervals for the subset of hummingbirds with treatment information available in the database (rescued hummingbirds’ n = 3779). Categories are color-coded according to the group of independent variables (Heat = supportive care and/or shock treatment; Oral Fluids = hydration and/or energy supplementation; NSAID = Non-steroidal anti-inflammatory drug; Steroids = Anti-inflammatory and antipyretics).
Figure 7
Figure 7. Predicted probabilitiy of release for hummingbirds.
Distributions of predicted probability using the model vs risk factors for the subset of birds with available treatment information. Boxplots show the median, quartiles, and outliers (diamonds). Each gray dot represents the probability of survival of individual hummingbirds ((A) Season; (B) Sex; (C) Age; (D) Species; (E) Reasons for admissions; (F) Oral Fluids = hydration and/or energy supplementation; (G) Steroids = Anti-inflammatory and antipyretics; (H) NSAID = Non-steroidal anti-inflammatory drug; (I) Antibiotics; (J) Heat = supportive care and/or shock treatment).
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