Purple herons hunting the mid-summer shallows

The lagoons near Sesimbra hold still water in mid-summer. The reed margins have dried at their tips, the mud at the edges has cracked into pale plates, and the water level has dropped enough to expose a shelf of shallows maybe knee-deep, maybe less. This is where the purple herons are now. Breeding is finished. The birds that spent spring hidden deep in reedbeds, standing motionless in the stems while they incubated eggs, have moved outward into open water. They are visible now in a way they rarely are.

The purple heron is a large bird, though it carries itself narrowly. Standing still, it looks almost thin: the neck folded tight against the chest, the body tilted slightly forward, the legs bent at the joint just above the waterline. The plumage is dark. The back and wings run to slate gray, but the neck is a deep rufous-brown streaked with black, and in certain light it reads almost purple, which is where the name comes from. These are threatened birds in the Iberian context, their wetland habitats reduced by drainage and disturbance, which makes the Sesimbra lagoons genuinely important ground. When a purple heron stands in the shallows here, it is not a casual visitor. It is using one of the few places along this coast where the conditions still work.

The hunting posture is one of the more deliberate things you can watch a bird do. The heron moves in increments: one foot lifted and placed without a ripple, the body held low over the water, the neck beginning to extend. It can hold that extension for a long time, the bill angled down at the surface, before anything happens. When it strikes, the movement is a single fast contraction of the neck, the bill entering the water and closing. What it is after in these lagoons is primarily fish. Common two-banded seabream move through the shallows here, young fish using the warm marginal water as feeding and refuge habitat. They are not large, typically a few centimeters in the juvenile and sub-adult sizes that frequent the shallows, and the heron takes them from just below the surface. The seabream are themselves threatened along this coast, pressured by overfishing in the wider marine environment, so both species are operating with reduced numbers relative to what this coastline once held.

The heron also takes frogs, large invertebrates, and occasionally small mammals when hunting along the marsh edge. In mid-summer the shallows warm quickly, and the fish tend to be most active in the early morning and again in the evening, which is when the herons hunt most intensively. During the heat of the day the birds often stand in shade at the reed margin, waiting. The black-crowned night heron, which also uses these lagoons, shifts its hunting into darkness entirely, which means the two species partition the same water across different hours. The night heron is stockier, shorter-necked, and takes smaller prey on average. The purple heron works the deeper edge of the shallows where its longer legs give it access to water the night heron cannot easily stand in.

Post-breeding dispersal in mid-summer means the birds are less territorial than they were in spring. Young birds from this year's nests, now independent, are moving through the region, testing habitats. Some will push south into Africa before autumn. Others will remain on the Iberian wetlands through the season. The Sesimbra lagoons, sitting between the limestone hills and the coast, collect water from the surrounding catchment and hold it into the dry season, which is why they continue to function as hunting ground when other wetlands have dried entirely.

If you are near the water now, look along the reed margin where the stems thin out and the open shallows begin. The heron, if it is there, will not move much. You may see it as a dark vertical shape before you recognize it as a bird. The cicadas are loud in the scrub above the lagoon. The water at the heron's feet is barely moving.