Harris’s Sparrow is one of only four bird species with its breeding range entirely in Canada. Its wintering range is also unusual, and it can be expected only in the central United States from central Texas to Nebraska and from Colorado east to Arkansas. This central Canada to central Great Plains migration is unique among North American birds. The periods of passage show up well on the animation, with migration beginning in mid-September and peaking in October. Spring withdrawal is mostly in April, and the northward flow continues into mid-May. Please note, however, that the occurrence metrics on this map are relatively low, peaking at 6% and fading from view at about 0.6%.
Every STEM occurrence map raises questions. Notice how the last areas to “blink out” in spring are the areas at the core of the winter range in Kansas, Nebraska and Iowa. Instantly one must wonder: is this real? Where are late May Harris’s Sparrows more likely to be found on the ground: in North Dakota or Minnesota or in more southerly areas like Nebraska?
Although most of us probably would have drawn a map showing bird migration proceeding like a northward flowing river to the breeding grounds, bird migration from the perspective of an observer on the ground is not always an orderly northward procession of birds. It is likely only a minority of the wintering population that stops in the United States on northward migration; some may move to the breeding grounds in two nights of migration, stopping over in southern Canada for the daylight hours in between. If this happens, the bulk of the population might seem to disappear from Nebraska without ever showing up on the ground in the northern United States.
It may in fact be true that you are more likely to encounter outlying late migrants in areas where the species winters abundantly, rather than on passage in North Dakota or Minnesota. Fortunately one can investigate questions like this quite easily in eBird. The frequency graph for Harris’s Sparrow in Nebraska drops sharply in mid-May, plummeting from 22% on 8 May to 1.5% on 15 May. In Minnesota and North Dakota, the peak occurrence is 6% on 8 May but drops to 2% by 15 May. By the final week of May, eBird frequency of Harris’s Sparrow is indeed higher in Nebraska (0.45%) than in North Dakota and Minnesota (0.28%); in June there are records from Nebraska, but not for Minnesota or North Dakota. In this course analysis, it does in fact seem that late lingering Harris’s are more likely to stay on the wintering grounds than to turn up somewhere en route to the breeding grounds. This is certainly true for many ducks and shorebirds too, which may spend all summer on the wintering grounds at times.