Cape May Warbler

Cape May Warblers breed from mid-to-late June, producing one brood per season — occasionally two when food is exceptionally abundant. Nest placement is extraordinary: the female builds almost exclusively at the very top of a spruce or balsam fir, typically 12–15 m (40–50 ft) above the ground, placing the nest close to the trunk just below the crown. This makes Cape May Warbler nests among the highest of any North American warbler, and among the hardest to find.

The nest itself is a bulky, oval-shaped cup constructed by the female from spruce twigs, grass, pine needles, cedar bark, and plant down, lined thickly with animal hair, rootlets, and feathers. The exterior is often clad in sphagnum moss. Nests average approximately 10.4 cm in diameter and 5.3 cm tall, with an interior cup around 5 cm across by 3.8 cm deep. The female's approach to the nest is deliberately secretive: she flies into the tree low and then creeps up the trunk, and departs by moving down the trunk rather than flying directly away — a behaviour that makes active nests almost impossible to locate from the ground.

Clutch size ranges from 4 to 9 eggs, with an average of 6 — the largest average clutch of any New World warbler. Clutches are consistently larger in years when spruce budworm is abundant, a direct evolutionary response to the predictable food bonanza. Eggs are creamy white with reddish-brown blotches, measuring approximately 1.5–1.84 cm long by 1.15–1.4 cm wide. Incubation is performed solely by the female and is estimated at 11–13 days based on closely related Setophaga species. Both parents feed the nestlings; fledging occurs approximately 9–12 days after hatching. Adults and young remain in the nesting area for several weeks before beginning their southward migration, sometimes joining mixed-species flocks.

Males are monogamous and guard females closely during nest construction, though they do not assist with building. Territory defence is vigorous, directed at rival Cape Mays and other warbler species alike.

Cape May Warblers typically live two to five years in the wild, with most individuals not surviving beyond their third or fourth year. Annual survival rates for small migratory songbirds are generally low — the hazards of two long-distance migrations per year, combined with predation, building collisions, and the challenges of finding food on both breeding and wintering grounds, keep mortality high.

The longevity record for the species was dramatically revised in 2022, when a female banded at the USGS Bird Banding Laboratory's fall migration station in 2013 was found dead in Friendsville, Pennsylvania. She was at least 9 years old — more than double the previous record of 4 years and 3 months, which had stood since 1978. Her cause of death was a building collision, one of the leading killers of nocturnal migrants in North America.

Compared with other Setophaga warblers of similar size, the Cape May Warbler's lifespan is broadly typical. The Prothonotary Warbler, for example, has a recorded maximum of just under 9 years. The principal mortality factors for Cape May Warblers are building and glass collisions during migration, predation by cats and raptors, and — on the breeding grounds — the indirect effects of insecticide spraying aimed at controlling spruce budworm, which can eliminate the food supply and cause breeding failure across large areas simultaneously.

The Cape May Warbler is listed as Least Concern on the IUCN Red List (2021), with a global population estimated at approximately 7 million breeding adults by Partners in Flight. Despite this reassuring headline figure, the species is classified as a Common Bird in Steep Decline: North American Breeding Bird Survey data show an estimated 2.5% annual decline from 1966 to 2015, representing a cumulative loss of roughly 70% of the population over that period. Partners in Flight rates it 12 out of 20 on its Continental Concern Score.

The population trend is inseparable from the boom-and-bust cycle of the spruce budworm. Numbers surged during the 1967–1993 budworm outbreak — which affected an estimated 136 million acres of boreal forest — then crashed sharply through the 1990s as the outbreak subsided. A new outbreak began in Québec in 2006, and warbler numbers have been recovering since. This tight coupling means that the species' long-term fate is partly determined by forest management decisions made across the Canadian boreal zone.

The principal threats are: use of insecticides to control spruce budworm, which eliminates the primary food source and causes steep local population declines; logging of mature boreal forest, which removes the old-growth spruce stands needed for breeding; building and glass collisions, which kill an estimated up to 1 billion birds annually in the United States — nocturnal migrants like the Cape May Warbler are particularly vulnerable; and climate change, which is projected to shift and reduce boreal forest habitat over the coming decades. Feral and domestic cats also take some birds during migration, though the Cape May Warbler's preference for high canopy foraging reduces its exposure compared with ground-feeding species.

Courtship in the Cape May Warbler is rarely observed, partly because it takes place high in the boreal canopy and partly because the breeding season is short and concentrated. The most detailed early account comes from ornithologist James Bond, who in 1937 described watching a male fly above a female with rigid, outstretched wings — a display posture quite unlike normal flight — while she was engaged in nest construction. This stiff-winged display flight has since been noted by other observers and appears to be the primary visual courtship signal.

Males arrive on the breeding grounds slightly ahead of females in mid-May and immediately begin singing from prominent perches near the tops of spruce trees, establishing territories of approximately one acre. Once a female is present, the male guards her closely throughout nest construction — following her movements and singing persistently — though he takes no part in building the nest itself. This mate-guarding behaviour is consistent with the monogamous mating system typical of most Setophaga warblers.

Territory defence against rival males involves both song and direct chasing. Males will confront not only other Cape May Warblers but also other warbler species that attempt to establish territories in the same spruce stand. The intensity of territorial behaviour scales with food availability: when spruce budworm is abundant and territories are small, interactions between neighbouring males are frequent and vigorous.

The Cape May Warbler is a mega-rarity in Europe, classified as such by BirdGuides in the UK. As of late 2023, there have been at least three confirmed UK records. The first two were in Scotland, both documented prior to October 2013. The third — and most celebrated — was a female found on Bryher in the Isles of Scilly in November 2023, making it the first record for England. In the same autumn, a male was recorded in County Mayo in the Republic of Ireland.

Several of these 2023 arrivals were associated with Storm Babet, a powerful Atlantic storm that struck the British Isles in October 2023. Fast-moving Atlantic depressions can carry exhausted North American passerines across the ocean in as little as three to four days, depositing them on western coasts of Britain and Ireland. The Cape May Warbler's autumn migration route along the northeastern Atlantic seaboard of North America places it in exactly the right position to be caught up in such systems.

Norway recorded its first Cape May Warbler in September 2020, suggesting that transatlantic vagrancy — while still extremely rare — is not confined to the British Isles. For UK and Irish observers, any small warbler showing a yellowish rump, streaked underparts, and a slightly decurved bill in autumn should be examined carefully. The Isles of Scilly, the Outer Hebrides, and the west coast of Ireland are the most likely landfall points, particularly in October and November following westerly storm systems. The species' thin tsip call and buzzy seet flight call are worth learning for anyone seawatching or patch-watching in these locations during autumn.

No other aspect of Cape May Warbler biology is as consequential as its relationship with the spruce budworm (Choristoneura fumiferana), a native moth whose caterpillars periodically defoliate vast areas of boreal spruce-fir forest across Canada. Budworm populations follow a roughly 30–40-year boom-and-bust cycle: numbers build slowly over decades, then explode into outbreaks that can affect tens of millions of acres simultaneously, before collapsing as the forest is stripped of foliage and the caterpillar population crashes.

The last major outbreak ran from approximately 1967 to 1993, affecting an estimated 136 million acres of boreal forest — an area larger than France. During this period, Cape May Warbler populations surged as birds exploited the superabundance of caterpillars. When the outbreak ended, warbler numbers crashed sharply through the 1990s. A new outbreak began in Québec in 2006, and Cape May Warbler populations have been recovering since — making the species a living barometer of boreal forest health.

The warbler's biology is shaped by this cycle at every level. The extraordinarily large clutch size — up to 9 eggs, averaging 6 — is thought to be a direct evolutionary response to the predictability of budworm outbreaks: when caterpillars are abundant, pairs can raise far more young than in normal years, rapidly rebuilding the population. The species' preference for mature spruce stands, its canopy-top foraging style, and even its migration route through the boreal zone all reflect this deep ecological dependency. Ironically, the same budworm outbreaks that benefit the warbler prompt forestry managers to apply insecticides — which eliminate the food supply and cause the very population declines the birds would otherwise avoid.