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Phenology Report: How do birds decide when to migrate?

A songbird sits on a bare branch. It is mostly yellow with patches of beige and tan. The wings are black with white wing bars. The image is labeled "Molting American Goldfinch."
A mid-molt male American Goldfinch sits on a branch.
Do you have observations to share? We’d love to hear from you! Get in touch with me (, John Latimer (, or text "phenology" to 218-326-1234.

Persistent snow

Maple sap boils in a metal pan. Clouds of steam rise from the surface. The pan is raised on walls of rocks, and a large pile of wood is nearby. In the background is a forest with buckets hanging from some of the trees.
Steam rises from maple sap boiling in a metal pan.

Snow, snow and more snow — I don’t know about you, but I’m pretty sick of it. John has measured 20 inches of snow on the ground, with more in the forecast. Luckily, warm weather is on its way! It will bring with it melting snowbanks, increased sap flow in the sugarbush, and a whole lot of happy Minnesotans.

Maple sap runs best when temperatures get into the 40s in the day and drop below freezing at night. Syrupers in the Twin Cities have already begun gathering sap, but trees “up north” haven’t begun to produce yet. (Are you tapping trees? We’d love to hear how your season goes — keep in touch!)

Phenology is local

A screen capture of google maps is shown, showing the distance between Akeley and Grand Rapids in context of the entire state. An arrow points from the words "5-day gap in phenological events" to the relatively short distance between the cities. The image is captioned "Phenology is local!"
Modified from the original
Google Maps
A short distance can make a big difference when it comes to phenology.

Dallas, a phenologist in Akeley, saw his first geese Monday, 11 days later than the average first sighting in his 20-year records. He’s also seen coyotes and otters. Many of the critters and behaviors he tracks are late, including grouse drumming and Sandhill Cranes returning. Here is a list of the birds Dallas has not yet recorded this year, as well as their average date of first sighting.

  • Ducks (March 10) 
  • Horned Larks (March 13) 
  • Mallards (March 14) 
  • Robins (March 19) 
  • Red-tailed Hawks (March 20) 

John’s records in Grand Rapids show these average dates:

  • Ducks (March 15) 
  • Geese (March 15) 
  • American Robins (March 24) 
  • Red-winged Blackbirds (March 29) 
  • Red-tailed Hawks (March 28) 

What causes birds to migrate?

A Great Blue Heron heron stands on ice. Wind ruffles its feathers. Its neck is pulled close to the body., and it appears chilly. The image is captioned "Great Blue Heron".
A chilly, windy day for a Great Blue Heron.

The cues prompting these birds to migrate vary by species. Some, such as the Great Blue Heron, begin to migrate based on day length. Therefore, their return is quite regular year –to year. (The earliest return date John’s recorded is March 23 and the latest is April 5, a range of just 10 days.) With that in mind, it’s the right time to look for the first Great Blue Heron of the season: the 10-day window begins Thursday, March 23.

Other migrators, like robins, are cued by temperature. Unseasonably warm or cold temperatures will alter their timing quite a bit: John’s recorded their arrival as early as March 8 and as late as April 18 — a 41-day gap!

Eagle nesting

Two Bald Eagles sit in a massive nest. The nest is comprised of large sticks and branches and is roughly 10 times as big as the eagles themselves. The eagles are silhouetted against a blue sky. The image is titled "Nesting Eagles".
Two Bald Eagles sit in their nest.

In March, John spends a lot of time with his spotting scope trained on a nearby eagle’s nest. Monday, he spent an hour looking for them and didn’t see them near the nest. Sunday night, however, he saw two eagles fly to the nest around dusk. One of them dropped into the nest, causing another eagle to fly out of the nest and give chase. The third eagle turned tail and flew off to the southeast.

The eagles didn’t spend the night in the nest, so they have not yet laid eggs. Once eggs are laid, one of the parents must be on the nest to keep them warm: otherwise, the young will die in the egg. Average nesting date in Grand Rapids is March 9, so keep an eye out if you have a nesting pair in your area! Judd Brink, a naturalist at the Sax-Zim Bog, monitors two eagle nests. Both mated pairs are brooding eggs.

“You know, this is going to be a late spring. How late, I don’t know. I don’t believe it will be as late as 2013, which was the latest spring I have on record, but it could rival some nearby winters where spring just doesn’t want to break out and show us anything."
John Latimer

In 2022, John’s eagles laid on March 8 and the nest failed, likely due to the brutally cold March and April we had last year. John’s not sure if the female eagle has any control over the timing of egg development, but if she does, she likely will want to hold off for better weather. (I checked, and it's unclear how much control individual birds exert over the timing of egg-laying. However, this article suggests that Blue Tits [a species of European songbird] can adjust their reproductive schedule to coincide with optimal food availability.)

Catkins and furry buds

In the foreground is a close-up shot of the end of some aspen branches. The stems are smooth and light brown, and have four small, fuzz-covered buds on the tips. The fuzz is white and looks almost furry. At the base of the buds are reddish scales. The background is blurred and indistinct. The image is captioned "Aspen Buds".
Aspen buds emerge from twigs.

Along roadsides, willows are brightening up the scene with vivid yellows, reds and corals. John’s investigated the catkins on the speckled alder and found no change: they’re still hard as bricks! As they develop, the catkins soften and elongate. Eventually, they will puff out pollen if disturbed. This generally occurs around April 1 or 2, but it’s likely to be much later this year.

Some of the trembling aspens John monitors are showing signs of bud development and growth. One tree, however, is not. Last year, it had furry buds in February. This year, there’s no sign of them: John looked carefully.

A glance through John’s records

Next, John relates some of historical records for March 21:

A Northern Harrier swoops over a dry field. It has a brown body, barred tail, and large head, with grey primary and secondary feathers, and an owl-like facial disc. The image is captioned "Northern Harrier".
A Northern Harrier swoops over a dry field.

  • 2022: American Robins returned to Akeley and Red-winged Blackbirds were sighted in Nevis. A black bear roamed around Bemidji. 
  • 2021: John observed the first migratory robin. Aspens had furry buds. Tag alder catkins produced pollen. John heard the first Sandhill Crane. Wood Ducks returned, and chipmunks were active. 
  • 2020: Chipmunks were active. 
  • 2019: Saw-whet owls were heard. The first Comptons Tortoiseshell butterflies emerged. 
  • 2017: Northern Harriers, a species of migratory hawk, returned.  
  • 2012 (one of the earliest springs on record): John heard chorus and wood frogs singing. First wood tick. Elderberries broke bud. Silver maple male flowers fell from the trees (meaning they already flowered).  
  • 2011: First kestrel sighting. 
  • 2009: First kestrel, Northern Harrier, Red-winged Blackbird and bluebird sighting. Sandhill Cranes and geese returned. 
  • 2007: First Red-tailed Hawk sighting. Ruffed Grouse were drumming. 
  • 2005: Red-tailed Hawks returned. 
  • 2004: Ruffed Grouse heard drumming. 
  • 2003: Killdeer returned. 
  • 2000: Dandelions bloomed. 
  • 1998: Dandelions bloomed. 
  • 1994: John observed robins, woodchucks, Great Blue Herons and Red-winged Blackbirds. 

We’re a long way away from most of those events this year! The first robin appeared in Palisade last week. John isn’t sure if it was a winter resident or a migrator, but he knows the robin migration has reached the Twin Cities.

Signs of spring in Minneapolis

A graphic of a swan is superimposed on an image of a water-filled pothole. The image is captioned "Swans in every pothole".
Sarah Mitchell
Canva Design
A graphic illustrating John Latimer's remarks on the springtime prevalence of Trumpeter Swans on Minnesota's waterways.

While in Minneapolis last weekend, he strolled along the Mississippi. He saw cattail seeds exploding from their seedheads, Canada Geese covering every available patch of open water, and swans occupying areas not taken by the geese. He also saw Red-bellied Woodpeckers and White-breasted Nuthatches.

On his drive to Minneapolis, he saw Trumpeter Swans everywhere! “Every little pothole that was open, every little stream, every river — there were Trumpeter Swans in every one of them,” he claims. (Some of the potholes are large enough to hold a swan, so I believe him!)

At his sister’s place in the Twin Cities, John spotted an American Goldfinch sporting its vibrant yellow breeding plumage. The goldfinches visiting his feeders in Grand Rapids are beginning to molt, with only a few spots of yellow here and there. If you have goldfinches nearby, be sure to take a good look: it’s fun to watch the molting process!

John concludes, “You know, this is going to be a late spring. How late, I don’t know. I don’t believe it will be as late as 2013, which was the latest spring I have on record, but it could rival some nearby winters where spring just doesn’t want to break out and show us anything. Stay with it and keep looking for those signs of spring! When you find them, send them along. It’s encouraging for us to know that it’s coming this way.”

For more phenology, subscribe to our Season Watch Newsletter or visit the Season Watch Facebook page.

Funding for this project was provided by the Minnesota Environment and Natural Resources Trust Fund as recommended by the Legislative-Citizen Commission on Minnesota Resources (LCCMR).

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As a mail carrier in rural Grand Rapids, Minn., for 35 years, John Latimer put his own stamp on a career that delivered more than letters. Indeed, while driving the hundred-mile round-trip daily route, he passed the time by observing and recording seasonal changes in nature, learning everything he could about the area’s weather, plants and animals, and becoming the go-to guy who could answer customers’ questions about what they were seeing in the environment.
Charlie Mitchell (she/they) joined the KAXE team in February of 2022. Charlie creates the Season Watch Newsletter, writes segment summaries for the website, and coordinates our Engaging Minnesotans with Phenology project. With a background in wildlife biology, she enjoys learning a little bit about everything, whether it's plants, mushrooms, aquatic invertebrates, or the short-tailed shrew (did you know they can echolocate?).