By the beginning of April, early nesters like woodcocks, killdeers, and bald eagles may have eggs ready to hatch; really early nesters like great horned owls will already be feeding nestlings. The shorebirds of March are joined by flocks, sometimes huge, of American golden-plovers, as well as lesser yellowlegs, solitary sandpipers, spotted sandpipers, and the first least sandpipers and dunlins. The numbers of passerby waterfowl tail off steeply except for late migrants like teals, but coots peak in April, along with their cousins the moorhens, and April is the best month for loons of all species. The larger marsh birds—bitterns, night-herons, herons, egrets, soras, rails—arrive in increasing numbers, the clamor of their calls dominating the night in fruitful marshes by monthís end.
Wintering gulls depart, replaced by migrant Bonaparteís along Lake Erie, and Caspian, then Forsterís, then common terns appear inland, gathering in larger numbers in the Lake marshes. Tree swallows are early in the month joined by rough-wings, barns, purple martins, and cliff swallows; bank swallows wait till mid-month to appear, but come in large flocks in favorable places near the marshes. Many resident hawks and owls are feeding young by mid-April, when ospreys and flocks of broad-winged hawks appear in the sky for the first time in the year.
Spring migrant woodpeckers—flickers, sapsuckers, and red-headeds—peak in April, as do winter wrens, brown creepers, fox sparrows, and hermit thrushes. The numbers of winterers—half-hardy birds like brown thrashers, and sparrows like white-throated, white-crowned, savannah, vesper, field, and swamp sparrows—swell before those of other migrants sharing their habitats. Hummingbirds arrive later in the month. Some of the vireos—blue-headed, white-eyed, yellow-throated—have arrived by mid-month.
The first warblers in decent numbers as early as mid-April include black-throated green, prairie, palm, cerulean, black-and-white, prothonotary, worm-eating, ovenbird, Louisiana waterthrush, and hooded. All of these except palm warbler breed commonly in Ohio, and the best places to see them this early are in the southernmost counties, where breeding commences earliest; those headed further north may not appear for another week or so. For neotropical migrants breeding north of Ohio, however, crossing the state may take only 1-3 days, and arrival dates donít differ much throughout the state. Another factor that may influence arrival dates is the rare but regular phenomenon of overflight, in which over-eager spring migrants, riding quirky winds, can arrive even a week or two ahead of their normal schedules; most often these overflights tend to be noticed along the Lake Erie shore, where their headlong progress may be halted by weather conditions or by the Lake itself.
Noticeable waves of northbound migrants appear toward the end of the month: swifts, kingbirds, nightjars, house wrens, gnatcatchers, waxwings, catbirds, still more sparrows, more flycatchers like great-crested and least, along with tanagers, orioles, grosbeaks, cuckoos, and indigo buntings. Many of these birds are fairly scarce in April, but May will bring many more.
The last ten days of April and the first twenty days of May represent the most concentrated and noticeable migration of the year, and it is during this period that observers can find more birds, in more easily recognizable plumages, as well as at their most vocal state, than at any other time.