Spring is the sweet spot. April and May bring huge concentrations of migrating waders and songbirds passing through on their way north from Africa, and resident species — flamingoes, spoonbills, purple herons, bee-eaters — are at peak visibility before the summer dry-down contracts the marshes. Temperatures are comfortable, wildflowers are out in the scrubland, and the deer and boar are active. Autumn, from late September through November, offers a second migration pulse with different species: raptors moving south, ducks and geese arriving to winter in the marismas.
Summer is honest about its limitations: the marismas largely dry out, birds disperse to wherever they can find water, and temperatures in the dunes can be genuinely punishing. The beach is popular with Spanish families, which is fine, but it is not the time for wildlife. Winter is underrated — cold enough for woolly layers in the early morning but mild by northern European standards, and the marshes fill again with tens of thousands of ducks and waders. Visit in January or February for sheer numbers if you can handle the mist.
El Rocío's Romería — one of Spain's biggest pilgrimages — takes place on the Sunday after Whit Sunday, typically in late May or early June, drawing over a million visitors to the park boundary. The area becomes overwhelmed and accommodation disappears weeks in advance. Either plan around it deliberately and embrace the spectacle, or give the park a wide berth for that weekend entirely.