The annual southward journey of hummingbirds from Iowa marks the end of summer. The sudden disappearance of these tiny, energetic birds prompts the question of when they will leave. This departure follows a summer spent breeding and is fueled by instinct, preparing them for an epic flight to their wintering grounds in Central America. Understanding the timing helps observers appreciate the impressive feat of these avian travelers.
Identifying Iowa’s Primary Migrant
The vast majority of hummingbirds seen in Iowa are Ruby-throated Hummingbirds (Archilochus colubris), the only species that commonly breeds in the eastern United States. These birds are diminutive, measuring less than four inches long and weighing about as much as a nickel. Males are easily identified by their iridescent emerald green back and a brilliant, shimmering ruby-red throat patch, or gorget, which can appear black in poor lighting.
Females and juveniles share the metallic green back but lack the red gorget, instead having a plain white or lightly streaked throat. These birds arrive in Iowa in mid-to-late April, with males typically preceding the females to establish territory. They spend the summer nesting and feeding before their instinctive drive compels them to begin migration south in late summer.
Peak Departure Timeline
The first wave of Ruby-throated Hummingbirds to leave Iowa consists of adult males, who often begin their departure in mid-to-late August. Once breeding duties are complete, they quickly head south to begin the demanding journey. Females and the young of the year follow a couple of weeks later, moving out primarily from late August through mid-September.
By the end of September, the bulk of Iowa’s hummingbird population has passed through or moved on. While the peak window closes in mid-September, it is not uncommon to see stragglers or late-migrating birds from northern regions throughout October. These late sightings, particularly in southern Iowa, are usually the last birds passing through before colder weather arrives.
The Biological Drive to Migrate
The timing of this departure is governed by an internal biological clock, not by a lack of food or a cold snap. The primary trigger for migration is photoperiod—the decreasing number of daylight hours as summer turns to fall. This change in light exposure triggers hormonal shifts that signal the birds to begin their southward journey.
Before leaving, hummingbirds enter a period of intense feeding known as hyperphagia. They feed voraciously, often consuming up to half their body weight in sugar daily, to double their overall body weight with fat reserves. This stored fat is the concentrated fuel required for the non-stop flight, including the 500-mile crossing over the Gulf of Mexico. This instinctual drive means they will migrate even if feeders are full and flowers are still in bloom.
Late Season Feeding Strategy
Keeping feeders accessible during the fall migration period is a beneficial strategy for supporting these travelers. Contrary to a common misconception, leaving feeders out will not prevent hummingbirds from migrating. The instinct to leave is too strong, and the food source only aids their necessary fat accumulation.
It is helpful to keep feeders clean and filled throughout September and into early October to support local birds or those passing through from Canada and Minnesota. To help these late-season visitors build up energy reserves quickly, you can increase the sugar concentration in your nectar solution. While the standard ratio is four parts water to one part sugar, a 3:1 ratio provides a higher caloric density that supports rapid fat storage for their demanding flight.