Heermann's Gull Larus heermanni
Version: 2.0 — Published April 9, 2020
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Movements and Migration
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Dispersal and Site Fidelity
Natal Philopatry and Dispersal
No information on natal dispersal, though young disperse widely along the Pacific Coast of North America from breeding sites.
Adult Fidelity to Breeding Site and Dispersal
Little information. On Isla Rasa, Gulf of California, Mexico, site tenacity, group adherence, and mate fidelity of 60 banded pairs whose reproductive success was known from the previous year were studied over 2 breeding periods by Velarde and Urrutia (102). Site tenacity, mate fidelity, and group adherence were all higher for pairs that had successfully nested the previous year compared to unsuccessful breeders, although these differences were not statistically significant.
No information on dispersal, though adults disperse widely along the Pacific Coast of North America from breeding sites.
Fidelity to Overwintering Home Range
Partial, short-distance migrant. The majority of the population disperses out of the Gulf of California into the Pacific and along the Pacific coast, north to British Columbia and south to Guatemala, during spring, summer and autumn. Northern individuals then retreat south to winter along the coast only from California south. The small population at Seaside, California (Monterrey Bay) seems to be resident.
Timing and Routes of Migration
Dispersing adults and immatures from breeding colonies in Gulf of California, Mexico make their first appearance off coast of southern California during last week in May. Peak numbers reach California’s north coast by mid-June, late July in Oregon, and during July and August in Vancouver Island, British Columbia, Canada. Some birds linger off coast of Washington into November but southward migration begins in late July and August off British Columbia, Canada, during September off Oregon coast, and from October through mid-December off California’s central coast. Some birds remain along southern California coast year-round but majority depart for breeding grounds by mid-March (82, 103, 83, 31, 104, 105, 106, 86, 28, 84). Most individuals migrate along the Pacific coast but a few appear at inland sites like the lower Colorado River (98). However, the migratory pattern determined by observers within the United States and Canada may be biased because few observers are located along the Pacific coast of Mexico, where Heermann´s Gulls are more likely to go undetected.
In Isla Rasa, the main breeding site of the species, arrival of early nesters that are exploring and establishing territories but not laying eggs, occurs by mid-February with the peak occurring by late March. By the first week of April, laying starts with peak occurring during the second week to mid-April (EV, personal observation). By the same argument, early arrivals of adults in California during June may be failed nesters that are returning to their nonbreeding areas. Alternatively, during the times of earlier research, breeding dates may have been different (earlier) than during the last decades of the 1990s and the current century.
No information on flock size or if Heerman's Gull migrates during the day or at night.
Control and Physiology of Migration