Afton State Park Phenology, April 15-21

This is when the woods and waters start to get busy.




5 minute read

Phenology: The scientific study of periodic biological phenomena, such as flowering, breeding, and migration, in relation to climatic conditions.

American Heritage® Dictionary of the English Language, Fifth Edition. (2011).


Saturday April 16th is the Full Pink Moon. It gets its name from the pinkish tinge  to swelling leaf-buds on trees and shrubs. 


White-throated Sparrows are passing through our area heading north. Some people  say their whistling call sounds like “O Sweet Canada Canada”, others think it sounds like “Old  Sam Peabody Peabody”. Song Sparrows have returned and some will stay here the whole  summer. American Robins are starting to build nests. This time of year, before the trees leaf  out, is a good time to look for last year’s nests. 

And don’t forget to look UP for birds. Flocks of Pelicans overhead are easy to identify – they are very large white birds with black on their wings. Turkey Vultures have returned to our part of Minnesota. Look for them circling. Their wing feathers are darker toward the leading edge of the wing and lighter at the back. They hold their wings in a “V” shape as they glide and they wobble a lot (in contrast to Eagles and Hawks, which hold their wings straight out and glide smoothly. Sandhill Cranes pass by high overhead. They hold their necks out ahead of them as they fly, with their legs trailing out behind them. This distinguishes them from Herons, which fly with their necks curled back toward their bodies. And Canada Geese and Ducks fly near the river.

Amphibians and Reptiles

By mid-April there are FOUR species of frogs calling in the evening  hours: Spring Peepers, with their high-pitched peeping, Chorus Frogs, with their trilling call  that sounds like running your finger along a comb, the chuckling call of the Wood Frogs, and  now the low snoring call of the Northern Leopard Frogs, ending in a croak or cluck.


The Monarchs are coming! The butterflies that overwintered in Mexico have left the oyamel pine forests of Mexico and begun their journey north. These individuals won’t be the ones to return to Minnesota – the butterflies that spent the winter in Mexico mated and laid eggs in Texas or Oklahoma, and first reports are coming in that those eggs have hatched and the Monarch larva are busy eating milkweed plants. This new generation of Monarchs will make another stop to mate and lay eggs, and those eggs will lead to the generation of butterflies that will return to Minnesota in May or June. Here in Minnesota the seed pods of last year’s milkweed plants have opened, and the seeds are blowing in the wind, dispersing over the landscape. Some of them will land in hospitable places and grow into milkweed plants, just in time for the return of the Monarch butterflies.

While we’re waiting for the Monarchs to return, keep a lookout for Spring Azures. Spring Azures overwinter as pupae. Last fall the larvae finished eating and made their cocoons, where they rested until the spring sunshine and longer days signaled that it was time to complete metamorphosis and emerge as butterflies. As the name suggests, Spring Azures are a vibrant blue on the top side of their wings. But they are rather pale on the underside of their wings, and when they land they almost always hold their wings closed. Sometimes people are reluctant to believe that the pale colored butterflies they see perched are the blue butterflies they just saw on the wing.


Spring ephemerals season continues! Hepatica, Bloodroot, Dutchman’s Breeches, Yellow Trout Lily, Large-flowered Bellwort, Wild Ginger, and Wild Violets are still in bloom, and joining them about mid-month are Jack-in-the-Pulpits and Spring Beauties. They’re called “ephemerals” for a reason – they won’t be around long! So take the opportunity to hike through the woods and enjoy these early flowers soon.

Weather observations

Here are some weather observations from past years:

Friday, April 152002: record high of 91°; 2018: record snowfall of 3.5”
Saturday, April 162017: 3/8” rain; 2019: temperature in 60s
Sunday, April 172016: high in 70s; 2019: rain with thunder and lightning
Monday, April 182013: record snowfall of 6.4”; 2020: sunny and mild, in the 50s
Tuesday, April 192013: 6” heavy wet snow overnight; 2020: partly sunny with a cold wind, temperature in the 50s.
Wednesday, April 202013: record low of 21°; 2019: sunny and in the low 70s.
Thursday, April 212002: Record snowfall of 6.6”; 2016: ¼” rain overnight, in the 50s.

Photo/Image credits

All photos copyright Nina Manzi, except:

  • Dean Lokken: American Robin, Canada Geese, Northern Leopard Frog, Turkey Vulture
  • Gary Sater: Song Sparrow, Sandhill Cranes
  • Allen Blake Sheldon, MN Conservation Volunteer: Boreal Chorus Frog, Spring Peeper