Avian influenza, commonly known as bird flu, is a highly contagious viral disease that can infect birds and in rare cases, spread to humans or other mammals. There are several strains of avian influenza viruses, with H5N1 being the most common strain known to cause severe disease in birds and humans.
Hummingbirds are a unique and fascinating group of birds found naturally only in the Americas. There are over 300 different species of hummingbirds, all belonging to the family Trochilidae. They are best known for their diminutive size, speedy flight, and ability to hover in midair as they feed on flower nectar. Their extremely high metabolism and rapid breathing and heart rate while hovering necessitates that they feed almost constantly throughout the day in order to survive.
So can hummingbirds become infected with avian influenza viruses like H5N1 that spread in other bird populations? Below we will examine what is currently known about avian flu infections in hummingbirds and the potential risks.
How avian influenza spreads in birds
Avian influenza viruses are spread among birds through direct contact with secretions or feces of infected birds. The viruses are shed in high concentrations in respiratory secretions and feces, allowing rapid transmission to other birds through close contact in large flocks or at feeding/watering areas. Infected birds may exhibit symptoms including sudden death, lack of energy, decreased appetite, coughing, sneezing, lack of coordination, diarrhea, or soft-shelled eggs. However, some infected birds, especially waterfowl like ducks, may show no signs of illness at all while shedding virus.
Waterfowl like ducks are considered a natural reservoir for avian influenza viruses, meaning the viruses have adapted in these hosts over time without causing significant illness. However, some strains like H5N1 when transmitted to other bird species can result in severe disease and mortality events. Domestic poultry flocks are especially vulnerable.
Migratory waterfowl are thought to play a key role in spreading avian influenza geographically during seasonal migrations. The viruses can then spill over into local poultry flocks or wild birds when they congregate in large groups at migratory stopover sites.
Evidence of avian flu in hummingbirds
So far there have been no confirmed cases of H5N1 avian influenza virus infection in hummingbirds, despite the virus becoming endemic in some wild bird populations globally. However, evidence suggests hummingbirds may be susceptible to infection with low pathogenic avian influenza viruses:
- Serological studies have detected antibodies to avian influenza viruses in multiple hummingbird species sampled across North and South America, indicating past exposures. One study found up to 44% of individual rufous hummingbirds tested positive for avian flu antibodies.
- In laboratory experiments, one low pathogenic avian influenza virus (H4N8) was shown to successfully infect and replicate in captive rufous hummingbirds, causing lethargy, loss of appetite, and other signs of illness.
- Avian influenza viral RNA has been detected at very low levels in cloacal swabs from wild hummingbirds during surveillance studies in North America, although presence of viable virus was not confirmed.
While concerning, these findings suggest that encounters between wild hummingbirds and low pathogenic avian flu viruses occur somewhat regularly across the Americas without causing widespread mortality events. The greater threat is if highly virulent viruses like H5N1 are introduced into hummingbird populations that lack prior immunity.
Unique risks for hummingbirds
Hummingbirds may face higher risks from avian influenza than other wild birds for several key reasons:
High metabolism and potential immune tradeoffs
Hummingbirds have the highest metabolic rate of any vertebrate animal, with heart rates up to 1,200 beats per minute and breathing rates of 250 breaths per minute during hover-feeding. This extremely high activity level necessitates that they feed frequently throughout the day on energy-rich foods like nectar and small insects.
However, some research suggests that hummingbirds and other small birds may have weaker immune responses compared to larger birds due to evolutionary tradeoffs between immune function and high metabolic demands. Having a high-performance immune system can be energetically costly, potentially interfering with the metabolic priorities of hummingbirds. This means they may be more vulnerable immunologically to high viral loads of novel viruses like H5N1.
Small body size and rapid virus progression
Due to their tiny body size, hummingbirds have a higher surface area relative to their volume compared to larger birds. This means any virus absorbed through mucous membranes could circulate and multiply faster within a hummingbird’s entire blood volume.
One study found house finches experimentally infected with avian flu showed more rapid virus progression and worse symptoms when they were in a leaner nutritional state. Hummingbirds with extremely high metabolic needs may thus be more strongly affected by a given viral dose.
Reliance on scattered nectar resources
Hummingbirds rely heavily on widely dispersed, ephemeral food sources like nectar from blossoming trees and flowers. This causes hummingbirds to cover large distances seeking out nourishing flowers each day, and regularly encountering other hummingbirds competing for the same food.
At some flowers, many different hummingbird species will gather together to feed, creating opportunities for direct virus transmission. Contaminated water sources are also a potential transmission risk. Compared to flocking birds that may acquire protection from early virus exposure, solitary hummingbirds may be constantly re-exposed while foraging.
Lower genetic diversity
Research shows that some hummingbird species like Anna’s and rufous hummingbirds have lower genetic diversity compared to other birds. This is attributed to relatively small effective population sizes and the solitary nature of hummingbirds. Having lower genetic diversity may reduce the adaptive potential of hummingbirds when faced with new pathogens or strong selective pressures.
High mortality rates if infected
While the metabolic, immune, and behavioral reasons above may make hummingbirds more prone to contracting avian flu, they also lead to predictions that the illness severity and mortality rate would be extremely high once infection occurs.
Hummingbirds are constantly on a fine energetic edge. Even a moderate virus infection that reduces feeding activity for days could be life-threatening. One lab study found over 65% of rufous hummingbirds died within 7 days of low pathogenic avian flu infection. If a virulent strain like H5N1 became established, impacts on wild hummingbird populations could be devastating.
Risk of spillover from poultry operations
Thus far, surveillance studies have not detected highly pathogenic H5N1 virus in North American wild birds. However, the legal poultry trade and illegal smuggling still pose risks of introducing these viruses from overseas into domestic American flocks. Backyard poultry operations can provide a bridge for transmission between wild waterfowl and hummingbird populations.
Several factors that increase the risk of hummingbirds being exposed to such poultry-origin viruses include:
- Use of outdoor ornamental water features, bird baths, and plantings that attract both hummingbirds and free-roaming poultry in backyards.
- Locating poultry enclosures near flowering trees, gardens, or other habitat features that draw in wild hummingbirds to the same area.
- Lack of sufficient biosecurity measures in small poultry flocks allows contact with wild birds or their secretions.
- Improper poultry carcass disposal can lead to contamination of wild bird feeding stations.
Strict biosecurity protocols, separation of poultry and hummingbird areas, and proper hygiene are critical to reduce risks posed by outbreaks from domestic flocks. These measures protect both poultry health as well as limit opportunities for adaptation and spread through wild bird populations.
Surveillance needs
While hummingbirds appear susceptible based on experimental infections and antibody evidence, there are still substantial knowledge gaps about how avian influenza viruses may be currently circulating and evolving in wild hummingbird populations across the Americas in real-world conditions.
Enhanced wildlife surveillance efforts focused on hummingbirds would provide valuable data, including:
- Tracking changes in viral antibody prevalence regionally and seasonally in common hummingbird species.
- Molecular surveys using PCR to detect viral RNA in oral/cloacal swabs from wild hummingbirds, combined with attempts to isolate viable virus.
- Phylogenetic analyses to trace virus lineages detected in hummingbirds, which could identify adaptation trends and key interactions with waterfowl reservoirs.
These types of systematic, large-scale surveys over time are needed to elucidate patterns of avian influenza transmission dynamics in hummingbirds and how they may contribute to viral diversification and spillover potential.
Targeted sampling of hummingbirds utilizing backyard feeders, migratory corridors, and poultry operations can also help assess relative exposure risks posed by anthropogenic factors like supplemental feeding and livestock contact. This surveillance data would directly inform management policies aimed at preventing future outbreaks.
Prevention and control strategies
While many knowledge gaps remain, preliminary evidence indicates that avian flu introduction into hummingbird communities could potentially have severe effects. Here are some measures that can be taken by wildlife agencies, researchers, and backyard bird enthusiasts to reduce risks:
Monitoring and reporting of unusual die-offs
– Educate the public to recognize signs of avian influenza in hummingbirds, including lethargy, tremors, lack of flight control, and sudden mortality events. Report observations to wildlife authorities for rapid investigation and sample collection.
Targeted surveillance
– Conduct opportunistic cloacal sampling of rehabilitated or banded hummingbirds to expand testing pools. Prioritize locations with backyard poultry.
Suspension of bird feeders
– Temporarily removing hummingbird feeders in areas with confirmed H5N1 outbreaks in wild or domestic birds reduces congregation and virus spread.
Enhanced backyard biosecurity
– Advise homeowners to separate backyard poultry from areas frequented by hummingbirds through fencing, eliminating ornamental bird baths, etc.
Vaccine development
– Explore potential of developing an avian flu vaccine that could be administered to high-risk hummingbird populations via nectar feeders if a severe H5N1 outbreak emerges.
Bird-friendly gardening
– Promote widespread planting of native nectar flowers in backyards, parks, and open spaces to provide dispersed food resources. This can reduce potentially contagious aggregations.
Poultry flock regulations
– Strengthen biosecurity requirements, mandatory outbreak reporting, traceability measures, and other poultry regulations to limit trade-associated flu risks.
Conclusion
While avian flu has not yet emerged as a widespread threat to the over 300 hummingbird species found in the Americas, their unique metabolic physiology, feeding behaviors, small size, and genetic factors increase potential susceptibility to novel viruses like H5N1. Enhanced surveillance efforts focused on hummingbirds are needed to quantify current exposure levels and viral evolutionary patterns in wild populations over time and space.
Outreach campaigns educating backyard poultry owners on practices that prevent human-facilitated transmission between their flocks and visiting hummingbirds can help reduce spillover risks. Promoting dispersed and abundant natural nectar plantings provides supplemental food resources without congregating birds. Targeted closures of feeders may be warranted in some outbreak scenarios.
Continued research and proactive prevention measures are advised to protect hummingbird communities against the threat of highly pathogenic avian influenza emergence and establishment in the Americas. With their specialized adaptations and crucial pollination services, conserving hummingbird diversity remains an important wildlife issue into the future.
