The Ruby-throated Hummingbird (Archilochus colubris) is the only species that regularly breeds in Wisconsin. These tiny birds undertake one of the most remarkable migrations in the avian world, traveling from their wintering grounds in Central America and Mexico to the northern breeding range. Their stay in the state is precisely timed to the window of warm weather and abundant floral resources required for reproduction.
Spring Arrival Timing
The first wave of Ruby-throated Hummingbirds typically appears in the southern regions of Wisconsin near the end of April. The main influx of birds generally occurs from the beginning of May through the middle of the month. These early arrivals are almost exclusively adult males, who precede the females by approximately one week to ten days.
The males move quickly to establish and aggressively defend feeding territories rich in nectar sources. This early establishment is a strategy to secure the best resources for attracting a mate once the females arrive. As the spring progresses, the arrival pattern moves northward, following the wave of blooming flowers and warming temperatures.
Summer Residency and Breeding Cycle
Once the females arrive, they quickly mate with a male and take on all parental duties alone. The female constructs a tiny, cup-shaped nest, often using plant down and lichens held together by spider silk.
The female typically lays two pea-sized eggs and begins incubation for a period lasting 11 to 16 days. The young hatch blind and helpless, requiring the female to forage constantly for small insects and spiders to provide the protein necessary for their rapid growth. Nestlings remain in the nest for about three weeks before they are ready to fledge.
The entire cycle from nest construction to the independence of the young takes approximately seven weeks. Because the female may raise two, and occasionally even three, broods during the Wisconsin summer, reproductive efforts extend the residency well into August. The necessity of raising young is the primary factor dictating the length of their stay.
Autumn Migration and Departure Dates
The signal for the southward journey is not a shortage of food but the decreasing length of daylight hours. The departure from Wisconsin is a gradual process that begins in late summer.
Adult males are the first to depart, typically beginning their migration in late August. Adult females and the newly fledged juveniles follow through the month of September. Peak migration activity occurs during the first two weeks of September.
While a few individuals, often juveniles, may linger into early October, most Ruby-throated Hummingbirds have left Wisconsin by the end of September. Maintaining nectar feeders does not delay their instinctive departure but provides a vital energy source for birds already committed to migration.
Variables Affecting Seasonal Duration
The seasonal windows for arrival and departure are subject to annual variation based on environmental conditions. Geographical latitude is a strong determinant, with the spring arrival in southern Wisconsin occurring up to two weeks earlier than in the northernmost counties.
Local weather patterns can also influence the specific dates, as an unusually warm spring may accelerate the blooming of nectar-rich flowers, allowing for earlier arrival. Conversely, an early cold snap in the fall can hasten the final departure of stragglers.
The maintenance of food resources, such as natural floral nectar and artificial feeders, dictates how long individual birds can remain in a specific location within the broader migration window. These factors create the typical range of dates observed each year, rather than a single fixed date for the entire state.