Killdeer and the spring tide rhythm

The mud flats along Burrard Inlet stretch wide this morning, exposed by a tide that pulled back further than usual. The water retreats in measured stages, leaving behind a glistening expanse where amphipods burrow and marine worms thread through the sediment. Somewhere in the distance, a Killdeer (Charadrius vociferus) calls its sharp, repetitive cry, the sound carrying across the flat terrain like a signal.

This small plover has arrived with the late spring migration, but its presence here is no accident of timing. The Killdeer synchronizes its breeding cycle to the twice-monthly spring tides, when the moon's pull combines with the sun's to draw water away from shore with unusual force. These extreme low tides expose feeding grounds that remain submerged during the smaller daily cycles. The bird's speckled brown and white plumage blends against the mottled substrate as it moves in quick, measured steps, pausing to probe the mud with its short, straight bill.

What the Killdeer finds in these exposed zones sustains both its own energy needs and those of its developing chicks. Small amphipods, no larger than rice grains, tunnel through the upper layers of sediment, processing organic matter that settles from the water column above. Marine polychaete worms, segmented and bristled, emerge from deeper burrows when the pressure of overlying water disappears. The spring tides create a predictable abundance, a reliable calendar written in the behavior of water and moon that the Killdeer has learned to read. During the two weeks between spring tide cycles, when daily tides expose less area, the bird shifts to hunting terrestrial insects in the salt marsh edges and muddy margins where fresh water meets salt.

The Killdeer's ground nest, a simple scrape lined with pebbles and shell fragments, sits on slightly higher ground above the tide line. The timing of egg-laying aligns with these foraging cycles. When the chicks hatch, they are precocial, able to walk and feed themselves within hours, but they still depend on their parents to lead them to the richest feeding areas. The adult birds guide their young to the newly exposed flats during each spring tide, where the small shorebirds learn to recognize the subtle movements in the mud that signal prey below. The rhythm of the tides becomes the rhythm of the family's daily routine, a pulse that governs when to forage intensively and when to rest and preen in the higher marsh grasses. Close your eyes and listen for that sharp call again, cutting through the morning air above the gleaming flats.