Thursday, January 16, 2014

Does Bad Weather Affect Bald Eagle Egg Laying?

April 20th, 2011: Spring Snowstorm
Facebook fan Dianne Wisse asked the following question about the Decorah Eagles: "With the extreme winter weather predicted to continue and even worsen--more 'polar vortex' cold, snow and dangerous high winds--is there any chance it will affect Mom/Dad Decorah's chances of laying viable eggs, or might nature step in to prevent the loss of eggs/eaglets to the unusual weather?"

We didn't begin tracking the eagles egg-laying schedule until 2009. It is as follows:
  • 2013: Egg laying unknown. Hatch believed to have started on 3/25/2013
  • 2012: Egg laying began on 2/17/12. Hatch began on 3/27/2012
  • 2011: Egg laying began on 2/23/11. Hatch began on 4/01/2011
  • 2010: Egg laying began on 2/25/10. Hatch began on 4/03/2010
  • 2009: Egg-laying began on 3/02/09. Hatch began on 4/09/2009
2013 was, like 2014, cold and snowy. 2012 was unusually warm, although the first half of 2011 was 'on the cool side', according to the Iowa Department of Agriculture. If weather impacted the timing of egg-laying, we would expect egg laying to happen a little later in cold,snowy years. But that's not what happens. Instead, Mom lays eggs a little earlier every year, regardless of the weather. 

I did a little reading about the regulation of bald eagle reproductive rates. In a study of bald eagles in southeast Alaska, researcher Andrew J. Hansen found that breeding activity is related to food abundance and habitat quality. He wrote: "Laying date was influenced by proximity to spring food supplies. Pairs nesting within 3 km of food patches laid eggs earlier than those nesting farther from food clumps." Hansen also found that laying date was advanced - that is, it happened sooner - where food was abundant. He believed that earlier laying was probably advantageous, since juveniles that fledge earlier have more time to acquire the skills required for surviving harsh weather and unpredictable food supplies. 

So cold and harsh weather don't influence egg-laying dates directly. But both can influence food availability, which does. During extremely cold and snowy winters, rodents might remain concealed under thick snow pack, fish and fish carcasses might be sealed under ice, and roadkill might be plowed under snow drifts, unavailable until the snow melts. In years where food is scarce, egg-laying happens later. If food can not be obtained, the nesting territory might be abandoned altogether. 

Fortunately, the Decorah eagles have an abundant and highly available food supply. It includes dead and live trout from the nearby hatchery, suckers and other fish from the creek, animals attracted to the 'food oasis', pigeons and other non-migrating birds that live in the area, and road kill. The eagles also have a lot of experience living here - they know their territory and its seasonal rhythms very well. The abundance and reliability of their food supply allows Mom to lay eggs a little earlier than she might otherwise. Experience might be the reason her laying date has crept up since we began keeping track in 2009.  

So in short, I don't expect her to lay egg #1 later significantly later than she has the past few years, although I'm not willing to predict a date. We've seen Mom gently footing and pecking Dad, and both eagles have increased their visits to the nest. Watch for copulation - that will help us estimate a time frame!


Regulation of Bald Eagle Reproductive Rates in Southeast Alaska
Andrew J. Hansen
Ecology , Vol. 68, No. 5 (Oct., 1987) , pp. 1387-1392
Published by: Ecological Society of America
Article Stable URL:

Did you know?

Latitude also influences nesting chronology. From the US Fish and Wildlife Service:
  • In the Southeastern US, egg-laying can begin in mid-to-late October
  • In the Chesapeake Bay region, egg laying can begin in mid-January
  • In the Northern US (including Iowa) and the Pacific region, egg-laying can begin in late January
  • In the Southwestern US, egg-laying can begin in late December
  • In Alaska, egg-laying doesn't start until the beginning of April.
Follow this link to learn more. The nesting chronology is on page 6.

For more on eagles and cold weather, follow this link: