Bald Eagles

Bald Eagle Feeding
Copyright © 2011 Bryant Olsen

Bald Eagle in Flight
Copyright © 2011 Bryant Olsen

Juvenile Bald Eagle in Flight
Copyright © 2011 Terry Greene

Flying Immature Bald Eagle
Copyright © 2011 Terry Greene

Flying Immature Bald Eagle

Copyright © 2011 Terry Greene, Photographer

When winter arrives in Utah, a number of our bird species hit the road – some flying thousands of miles to Mexico and Central America in search of a warm winter home.

But there is one notable bird that actually migrates to Utah in the winter – the bald eagle. In general, birds migrate because of seasonal food shortages. Think of the hummingbirds that rely on flower nectar and insects, which Utah cannot provide in winter, but which are abundant other times of the year. The same is true for bald eagles, whose main food source is fish. Winter comes on strong in Alaska and Canada, freezing lakes, ponds, and all but the strongest flowing rivers. So the birds travel to seek out the relatively mild winters found farther south.

One of the largest birds of prey you’ll see in our Utah skies, a mature bald eagle can have a wingspan of 6 to 8 feet and stand almost three feet tall. Only the golden eagle rivals it in size. Pairs are thought to mate for life, and they are also responsible for the largest nests of any bird in North America. One record-setting abode measured 9 feet wide, 20 feet deep, and weighed more than 2 tons.

And to think, we almost lost this incredible species. Since DDT and hunting heavily affected bald eagle numbers in the early half of the 20th century, the birds have made a truly remarkable comeback. From a low point around 4,000 individuals in the lower 48 states, they are now thought to number in the tens of thousands, and have been removed from the Threatened and Endangered Species lists.

Perhaps as their comeback continues, Utah will once again see these majestic animals make their massive nests here, fishing in our many rivers and lakes year round. For now, though, aside from a few rare exceptions, bald eagle enthusiasts will have to make the most of their short winter stay. To observe bald eagles, consider a visit to the Great Salt Lake Nature Center at Farmington Bay. Every year, the Bay plays host to hundreds of eagles, from November to March, and while this milder-than-usual winter has brought in fewer numbers of eagles, you may still be able to catch a glimpse. They will likely be heading north later this month, however, so don’t delay. And be sure to keep your ears tuned into the bald eagle’s haunting song:

[Bald Eagle call from Songbirds of Yellowstone, Kevin Colver]

Thank you to the Rocky Mountain Power Foundation for supporting the research and development of this Wild About Utah topic. For the Stokes Nature Center and Wild About Utah, this is Andrea Liberatore.

For the Stokes Nature Center and Wild About Utah, this is Andrea Liberatore.

Credits:

Audio: Courtesy & Copyright Kevin Colver
Photos: Courtesy & Copyright Bryant Olsen and
            Courtesy & Copyright Terry Greene
Text:    Andrea Liberatore, Stokes Nature Center, logannature.org

Additional Reading:

Parrish, J. and Walters, B., Editors. (2009) Bald Eagle (Haliaeetus leucocephalus). Utah Division of Wildlife Resources: Wildlife Notebook Series No. 3. https://wildlife.utah.gov/publications/pdf/2010_bald_eagle.pdf

National Geographic Society (2012) Animals: Bald Eagle (Haliaeetus leucocephalus)
https://animals.nationalgeographic.com/animals/birds/bald-eagle/

Birds and West Nile Virus

Birds and West Nile Virus: Bald Eagle, Haliaeetus leucocephalus
Bald Eagle
Haliaeetus leucocephalus
Courtesy US FWS
It’s that time of year again when we encounter a barrage of public health messages alerting us to take precautions during cold and flu season. However, a different disease outbreak has been making headlines this winter in Utah: West Nile virus.

Though West Nile virus is not a new name to most of us, our familiarity with it typically comes from summer time outbreaks amongst human populations. Yet this particular flare up has gained attention for causing the deaths of over 50 Bald Eagles and it happened during winter.

West Nile virus is maintained in nature by a transmission cycle between mosquitoes and birds. In this cycle, birds simply act as pathogen reservoirs, while the mosquitoes act as pathogen vectors passing the virus on to their eggs and infecting humans and animals through bites. Historically, in the locations of its origin – Africa, Europe, Asia, and the Middle East – this pathogen is rarely fatal to its avian hosts. In fact, antibodies to West Nile virus have been found in the blood of birds native to this region.

It wasn’t until 1997, when a stronger strain of West Nile virus emerged and caused fatalities in a wide range of avian species, that the infection started to be considered pathogenic to birds. When the disease first reached the United States in 1999 it proved to be highly virulent in North American bird populations. The American crow was particularly susceptible. Within 4 months of detection in New York, nearly 5,500 crows died from the infection. Since this first outbreak, West Nile virus spread across the United States and has been isolated in over 250 species of birds, including Bald Eagles.

While wintertime infection amongst human populations is rare, infection during this season is not so uncommon for birds. This is because birds can contract the disease by a variety of routes other than mosquito bites and direct contact. This is especially the case amongst opportunistic scavengers like raptors. If a raptor consumes the carcass of a bird killed by West Nile virus it can contract the virus orally, as the Bald Eagles did after consuming the remains of infected Eared Grebes. Luckily, while bird-to-bird transmission does occur amongst birds that exhibit roosting and group behaviors, the likelihood of a Bald Eagle, which is typically a solitary bird, directly passing the disease to another Bald Eagle is quite low. Meaning that once the last of the infected food source is gone, hopefully no further infections will occur.

For Wild About Utah, I’m Anna Bengtson.

Credits:
Image: Courtesy US FWS
Text: Anna Bengston

Sources & Additional Reading:

World Health Organization. West Nile Virus https://www.who.int/mediacentre/factsheets/fs354/en/
Utah DWR. Wildlife News: DWR Learns What Killed Bald Eagles https://wildlife.utah.gov/wildlife-news/1330-utah-dwr-learns-what-killed-bald-eagles.html

Koenig, W.D., Hochachka, W. M., Zuckerberg, B., and Dickinson, J.L. 2010. Ecological Determinants of American Crow Mortality Due to West Nile Virus During its North American Sweep. Oecologia, 163: 903-909. https://www.nbb.cornell.edu/wkoenig/K156TA_10.pdf

Powell, H. 2010. Counting Crows. BirdScope: Autumn 2010. https://www.allaboutbirds.org/Page.aspx?pid=1956

Rappole, J.H., Derrickson, S.R., and Hubálek, Z. 2000. Perspectives: Migratory Birds and Spread of West Nile Virus in the Western Hemisphere. Emerging Infectious Diseases, Vol. 6, No. 4, July-August 2000, pp. 319-328. https://wwwnc.cdc.gov/eid/article/6/4/pdfs/00-0401.pdf

Reisen, William K. 2013. Ecology of West Nile Virus in North America. Viruses, Vol. 5, Issue 9, pp. 2079-2105. https://www.mdpi.com/1999-4915/5/9/2079

Bald Eagles

bald eagle feeding, Courtesy and Copyright 2011 Bryan Olsen, Photographer

Bald Eagle Feeding
Copyright © 2011 Bryant Olsen

 bald eagle in flight, Courtesy and Copyright 2011 Bryan Olsen, Photographer Bald Eagle in Flight
Copyright © 2011 Bryant Olsen

juvenile bald eagle in flight, Courtesy and Copyright 2011 Terry Greene, Photographer Juvenile Bald Eagle in Flight
Copyright © 2011 Terry Greene

flying immature Bald Eagle, Courtesy & Copyright 2011 Terry Greene, Photographer Flying Immature Bald Eagle
Copyright © 2011 Terry Greene

Flying Immature Bald Eagle

Copyright © 2011 Terry Greene, Photographer

When winter arrives in Utah, a number of our bird species hit the road – some flying thousands of miles to Mexico and Central America in search of a warm winter home.

But there is one notable bird that actually migrates to Utah in the winter – the bald eagle. In general, birds migrate because of seasonal food shortages. Think of the hummingbirds that rely on flower nectar and insects, which Utah cannot provide in winter, but which are abundant other times of the year. The same is true for bald eagles, whose main food source is fish. Winter comes on strong in Alaska and Canada, freezing lakes, ponds, and all but the strongest flowing rivers. So the birds travel to seek out the relatively mild winters found farther south.

One of the largest birds of prey you’ll see in our Utah skies, a mature bald eagle can have a wingspan of 6 to 8 feet and stand almost three feet tall. Only the golden eagle rivals it in size. Pairs are thought to mate for life, and they are also responsible for the largest nests of any bird in North America. One record-setting abode measured 9 feet wide, 20 feet deep, and weighed more than 2 tons.

And to think, we almost lost this incredible species. Since DDT and hunting heavily affected bald eagle numbers in the early half of the 20th century, the birds have made a truly remarkable comeback. From a low point around 4,000 individuals in the lower 48 states, they are now thought to number in the tens of thousands, and have been removed from the Threatened and Endangered Species lists.

Perhaps as their comeback continues, Utah will once again see these majestic animals make their massive nests here, fishing in our many rivers and lakes year round. For now, though, aside from a few rare exceptions, bald eagle enthusiasts will have to make the most of their short winter stay. To observe bald eagles, consider a visit to the Great Salt Lake Nature Center at Farmington Bay. Every year, the Bay plays host to hundreds of eagles, from November to March, and while this milder-than-usual winter has brought in fewer numbers of eagles, you may still be able to catch a glimpse. They will likely be heading north later this month, however, so don’t delay. And be sure to keep your ears tuned into the bald eagle’s haunting song:

[Bald Eagle call from https://wildstore.wildsanctuary.com/collections/special-collections Kevin Colver https://wildstore.wildsanctuary.com/collections/special-collections]

Thank you to the Rocky Mountain Power Foundation for supporting the research and development of this Wild About Utah topic. For the Stokes Nature Center and Wild About Utah, this is Andrea Liberatore.

For the Stokes Nature Center and Wild About Utah, this is Andrea Liberatore.

Credits:

Audio: Courtesy & Copyright Kevin Colver
Photos: Courtesy & Copyright Bryant Olsen and
            Courtesy & Copyright Terry Greene
Text:    Andrea Liberatore, Stokes Nature Center, logannature.org

Additional Reading:

Parrish, J. and Walters, B., Editors. (2009) Bald Eagle (Haliaeetus leucocephalus). Utah Division of Wildlife Resources: Wildlife Notebook Series No. 3. https://wildlife.utah.gov/publications/pdf/2010_bald_eagle.pdf

National Geographic Society (2012) Animals: Bald Eagle (Haliaeetus leucocephalus)
https://animals.nationalgeographic.com/animals/birds/bald-eagle/