Year of the Bird Series: May – Cape May Warbler

Editor’s Note: 100+ bird organizations declared 2018 the “Year of the Bird” as it is the centennial of the Migratory Bird Treaty Act’s ratification. Many are pledging to do 1 thing per month to help birds. As Ruthven Park National Historic Site has a bird banding station in collaboration with the Haldimand Bird Observatory, we are featuring one blog post per month on different birds! Below, you will find the fifth installment in this 12-part series.

Article by Mike Furber, Ruthven Park National Historic Site Bird Bander

It was a special fall banding season in 2015 at Oriskany Banding Station in Haldimand County. Previously, we were lucky to catch and band one or two Cape May Warblers a year since 2003. That fall, however, we banded an astonishing 22 Cape Mays (dare we say a “catch twenty-two”) – indeed, more than the previous twelve years put together! How could this happen? Read on.

A striking warbler, especially in the spring in its alternate plumage, the predominantly yellow male has a unique chestnut or rufous cheek patch along with a large white wing strip that sets it apart from other warblers. The drabber female is not as distinct and can be quite challenging to identify – especially in the fall in basic plumage – but there always seems to be a diagnostic yellowish pale marking on each side of the neck. In autumn basic plumages, both birds are not as colourful, yet still attractive.

Many of our wood warblers have strange names that often seem to have little to do with their natural history; Cape May Warbler (Setophaga tigrina) is a good example of this phenomenon. Many warblers are named after where they were first discovered. In this case, Cape May Warbler was first named by Alexander Wilson (1766-1813) based on an anonymously collected specimen from Cape May County, New Jersey in 1811. Ironically, he and other famous ornithologists such as John James Audubon (1785-1859) and Thomas Nuttall (1786-1859) never saw a live one.

To this day, Cape May Warblers have continued to be uncommon or rare as a migrant in southern Ontario – except during occasional years of a spruce budworm (Choristoneura fumiferana) population breakout in the northern coniferous forests. Without these occasional population explosions of this moth caterpillar, you should wonder what might happen to Cape May Warblers. Indeed, they seem to be more dependent on this insect larva than the other so-called spruce budworm warblers, namely Bay-breasted Warbler (Setophaga castanea) and Tennessee Warbler (Oreothylypis peregrina).

Cape May Warbler is the most specialized of the northern forest breeding warblers. Although rather uncommon to rare most years, occasionally, they can be very common in fall migration after a successful nesting year that coincides with a high spruce budworm population. This often results in a year where many birds are caught and banded. Herein lies the answer to why we probably caught so many during that fall of 2015. Earlier that year in the spring, there probably was a spruce budworm population outbreak in much of the northern boreal forests, resulting in a very successful nesting year for Cape Mays. In fact, most large banding stations like Long Point Bird Observatory banded almost twice as many as usual (152) that same year. Furthermore, the Ontario Bird Banding Association totalled 344 banded throughout the province which includes most banders and banding stations in 2015; again, more than twice the normal total.

The preferred habitat is balsam fir mixed with white or black spruce; a more open park-like feel is preferred, with birch and aspen in the mix. As a tree top feeder, Cape May prefers to sing from the top spires of conifers. The song is a very high frequency, thin sounding “seet seet seet seet.” Though rarely observed, the nest is usually placed in the upper branches of a spruce or balsam.

As a favourite of many birders and bird banders, most would agree that Cape May Warbler is a darling of the Canadian Shield and the boreal forest where it nests. The species name “tigrina” is quite appropriate for two reasons: first, the male’s yellow underparts are finely streaked with black; and second, it is a very aggressive warbler toward other species, such that it could be called the “tiger” (or “tigress”) of the wood warbler world.

Although not as long a distance migrant as some warblers such as Blackpoll Warbler (Setophaga striata), Cape May Warbler still travels a respectful distance from its nesting range to its wintering grounds primarily in the West Indies and surrounding islands. A remaining mystery is why Cape May Warblers don’t expand their breeding range into more of British Columbia, the Yukon, and Alaska where there is much of the same spruce-fir habitat. One reason, though not a satisfying one, is that there may be limits on the distance a bird species can travel north during the hurried spring migration. Other possible explanations include competition among other species, geographical barriers such as the Rockies, and physiological limits such as coping with temperature extremes. Ornithologists at the Cornell Lab of Ornithologists continue to research this mystery.

Unfortunately, like many bird species, Cape May Warbler numbers show significant declines since the 1990s per Canadian Migration Monitoring Network data from Long Point Bird Observatory. There was a moderate decrease in the breeding populations recorded during the second Ontario Breeding Bird Atlas (2001-2005) compared to that observed during the first atlas (1981-1985). This difference was thought to be mainly due to lower spruce budworm numbers during the second atlas. It will be interesting to see the results of the upcoming third Ontario atlas data collection during 2021-2025. 

To be sure, much has been learned from bird banding recoveries and encounters. Moreover, along with the use of satellite transmitters and geolocators, and with continued amazing technological advances in bird research, we will continue to learn more about this lovely northern warbler. And to this end, we can better conserve this important avian member of the boreal forest ecosystem. Best of luck to all birdwatchers: here’s hoping you are fortunate to see a Cape May in its resplendent spring plumage this May and in many more years to come.