September Nature Notes

September is a month of migrations in Minnesota, including journeys by animals both large and small.

Monarchs 

Even a creature as small and delicate as a butterfly can travel thousands of miles in migration. Every fall, monarch butterflies from Minnesota fly all the way to conifer forests in the mountains of central Mexico. These butterflies have never been to Mexico; they are the descendants of the monarchs that last overwintered there. The butterflies that started the journey north from Mexico in the spring are the great- or great-great-grandparents of the monarchs that summer in Minnesota. Between March and August, three or even four generations of monarch butterflies are produced as the insects gradually make their way north, laying their eggs on the milkweed plants that sustain their caterpillars.  

Monarch numbers in Minnesota peak in late August and early September as resident butterflies are joined by those from even further north that are beginning their fall migration. South-bound monarchs are larger than their predecessors, built for the rigors of a migration that they will complete in one generation without pausing to lay eggs like the north-bound butterflies months earlier.  
 
There are many opportunities to see these butterflies and learn about their migration at the Minneapolis Monarch Festival in early September.

Nighthawks 

Nighthawks are much larger than monarch butterflies, but may be overlooked during their migration high overhead in August and September. These remarkable birds are not true hawks: they belong to a group of birds called “goatsuckers” due to a bizarre belief long ago that they would fly into a barn at night and nurse from the teats of goats. Perhaps the large gape created when they open their stubby bills inspired this odd notion; nighthawks instead use their prodigious mouths to capture insects that they actively hunt in the afternoon and evening while flying like enormous stiff-winged swallows. Often, they are heard before seen; listen for their raspy “peent” calls then look overhead. In the spring and early summer, male nighthawks also make a buzzing (or “booming”) sound caused by the rush of air past their wing feathers when they swoop during courtship display flights; listen to their sounds on this video about nighthawks in Oregon. Thousands of nighthawks fly south through Minnesota to their wintering areas in South America. Many of these birds have been city dwellers, nesting on flat roofs in metropolitan areas where their peenting calls are a part of long languid summer evenings for those who listen. Those calls are becoming less frequent: nighthawk numbers are down and it’s thought that part of the reason may be due to rubberized roofs replacing the gravel roofs they prefer for nesting.  

Raptors 

Minnesota’s most impressive bird migration in September takes place at Hawk Ridge in Duluth. It’s the place to go to see birds of prey in the fall, but can be disappointing if winds are not from the west or northwest. Westerly winds push birds toward Lake Superior; birds reluctant to cross that large body of water then follow the shoreline south to Duluth, an inevitable concentration point for all migrating raptors from the north. Hawk flights begin in August and continue into November—the numbers and types of birds vary throughout the season. The greatest numbers of birds occur in mid-September, when it is possible to see tens of thousands of broad-winged hawks in an afternoon when conditions are right. The Nature Conservancy helped establish Hawk Ridge, which overlooks the city and the lake; observers there can watch broad-wings circling on thermal updrafts, forming massive “kettles” of birds that climb high in the sky then glide overhead, hundreds of hawks at a time. Other raptors, such as sharp-shinned hawks and kestrels often fly past at eye-level, providing exciting encounters. Come later in the fall, and watch for goshawks and rough-legged hawks that migrate in October and November (as well as greater numbers of bald eagles and red-tailed hawks).