Spruce Budworm in Michigan’s Eastern Upper Peninsula

Male Cape May Warbler in a spruce with two Spruce Budworms

Periodic outbreaks of Eastern Spruce Budworm (every 30-40 years) are part of the natural cycle in spruce and fir forests of Eastern Canada and Michigan’s Upper Peninsula. Although budworms are always present in these forests they periodically get out of control. Then we have an “outbreak”. Their life cycle goes from eggs to larva to pupae to moths. In the moth stage eggs are placed in microscopic holes in the needles of spruce and fir trees (and much less often – pines). The larvae hatch out and feed on needles or expanding buds. As larvae grow, needles are severed at the base. After many years of defoliation, the treetop (or the entire tree) may die. The larvae that escape the mouths of birds mature to become moths that drill holes in more needles, plant eggs, and continue the cycle for the next year.

Two warbler species in Canada and the Eastern Upper Peninsula (EUP) – Cape May and Tennessee, are known as spruce budworm specialists, and they dramatically increase their numbers near infested areas as this cycle progresses in years. They feast on the larval worms and eventually help bring the cycle to an end – that may take 8 years or more. Then most of them will likely move on to another area of North America where spruce budworm is more abundant.

I began noticing dramatically increased numbers of nesting Cape May and Tennessee Warblers in the EUP in the spring of 2012. I searched for and found the center of the outbreak well inland, away from the northern shore of Lake Huron. There was about a one acre patch of spruces completely defoliated, but less than 1/2 mile away there was very little defoliation. The outbreak has since spread to the entire region, and I have documented many other species taking advantage of the bounty of budworms, which makes finding food for themselves and feeding nestlings much easier.

Male Cape May Warbler in a spruce with a large budworm

Living full-time in the EUP during the duration of the current outbreak I have witnessed and documented the feeding habits of many Cape May Warblers less than 5 miles from my home. When the birds arrive in late May, after a winter in the Tropics, they find a mate and a suitable nest tree – a tall spruce (35-60 feet high) with sufficient crown density near the top, appropriate for a well-concealed nest. And they make sure the nest tree is near healthy spruces that are infested with worms. They search for worms which are usually buried within the spruce needles before the worms make cocoons. After cocoons are made they probe the cocoons to get at the worms.

Cape May Warblers are the only warbler with a semi-tubular tongue, which allows them to have a diet of mainly nectar and juices during the winter, after they use their sharp bills to pierce flowers and fruit. When migrating north through the Eastern United States they can stop to feed on cut oranges, a bowl of jelly, and/or a hummingbird feeder. They are known to feed on sap from Yellow-bellied Sapsucker wells when insects are scarce. And they can chase a sapsucker from it’s own drill holes when no other food is available.

Male Cape May Warbler battles a Spruce Budworm

When a Cape May Warbler captures a large budworm the worm battles by fiercely wriggling and contorting. High speed photography reveals that this warbler often uses its tongue tip as pictured above – I have many images that document this behavior. It seems this warbler may be using its unique (among warblers) tongue to suction the worm during the battle, to help subdue it – possibly to hang on to it or injure it. High speed (very high frame rate) video, viewed in slow motion, would be very revealing.

Male Cape May Warbler in a spruce with a medium-sized budworm

Cape May Warblers seem to time their nesting cycle to Spruce Budworm development. If the spring is colder than normal they will wait until worms are hatched before they nest. On three of the past four years the EUP Lake Huron north shore spruce forests were colder than normal and nesting was significantly delayed, as compared to similar forests inland.

Male Cape May Warbler in a spruce with six Spruce Budworms

Witnessing males gathering budworms to feed young gave me an appreciation for how easy the feeding process can be during an outbreak. This bird (above) gathered worms in one spruce less than 30 yards from the spruce where it nested, probing cocoons that were everywhere, and pulling out worms with almost every attempt. I knew where its nest was because when it had a beak full of worms it always flew straight to that tree, to a point well below the crown. It is known that, from there, it walked up while remaining close to the tree trunk and concealed, to arrive at its nest in the crown. This way potential predators cannot pinpoint its nest. Its nest was presumably placed near the trunk, a few feet from the top, as is typical for this species. This climbing behavior is necessary because if the bird habitually flew to the outside of the tree at nest level it would repeatedly return to a spot only two or 3 feet from its nest, because spruces are very narrow near the top. A predator could catch on very easily, and destroy the nest and/or capture the nestlings. Every time this bird returned from the nest tree its beak was empty.

Merlin falcon with a captured nestling

The most dangerous predator to a Cape May Warbler in the area is a Merlin falcon, but there are others such as Sharp-shinned Hawks and Red Squirrels. Merlin falcons specialize in preying upon songbirds. All warblers are on the menu and this falcon nests in the area, and is especially prevalent close to the northern Lake Huron shoreline. Many migratory warblers feed and rest in the nearshore forests in the spring and fall, and a high density of warblers nest there because of the abundant food supply of midges and spiders. This falcon is adept at negotiating spruces. The Merlin pictured above is in a spruce. It stopped briefly before bringing its prey item (a nestling bird) to its nest in a spruce about 20 yards away. Merlin falcons in the spruce forests of our area often hunt patiently from a concealed position within a spruce, waiting for the right moment to make a stealthy attack on an unwary songbird. This all but proves that the Cape May’s climbing behavior is absolutely critical in our area.

Tennessee Warbler in a spruce with a Spruce Budworm

I found Tennessee Warblers beginning to nest in the EUP in significant numbers in the spring of 2012 as well. Before that there were very few in the region. Nesting Cape May Warblers could reliably be found before that (with much effort along creeks with tall spruces), but Tennessee Warblers were rare. Tennessee Warblers were formerly classified in the genus Vermivora (before 2010), which contains Blue-winged Warbler, Golden-winged Warbler, and their hybrids. All of these birds (and the Tennessee Warbler) feed on larva that infest the growth tips of deciduous trees – tag alders, maple saplings, and other related trees and bushes. But Tennessee Warblers always nest near areas of a Spruce Budworm outbreak. I have found many nesting Tennessee Warblers utilizing a habitat which includes these deciduous trees, and feeding on them as well. The bird above is in a spruce but the blurry leaves on the right side of the image above are tag alder leaves.

Tennessee Warbler in Tag Alder with a larval worm

Above a Tennessee Warbler captured a little green larval worm by probing the center of the emerging tag alder leave cluster right under its head. There were infested spruces nearby.

Tennessee Warbler in tamarack with a Spruce Budworm

The Tennessee Warbler above captured a budworm in a tamarack in a colorful stage of cone development. This was the only time I have witnessed the presence of Spruce Budworm in a tamarack. It is known to occur. Young Spruce Budworm larvae are known to hang from threads and disperse by wind. This area had many large infested Spruces so I presume the captured budworm arrived in the tamarack this way. It would be able to feed on the tamarack needles, but once in the moth stage it would have to choose a tree which does not lose its needles, when the time came to drill holes in needles and plant eggs. Because tamaracks lose their needles in fall.

Male Blackburnian Warbler in a spruce with Spruce Budworm

The Blackburnian Warbler was the first non-Spruce Budworm specialist to increase its numbers in response to the outbreak. They occupy the same habitat as Cape May Warblers and nest and forage well up in spruces, but not near the top like the Cape May Warbler. They also nest in a variety of other evergreens. They forage for budworms exactly as a Cape May does.

Male Blackburnian Warbler in a spruce with Spruce Budworm

The image above was taken in 2019 on July 15 , and the one before that in 2010 on June 2, both near the north shore of Lake Huron. The budworm in the second image is only slightly larger than the one in the previous image, but the worms were captured more than 6 weeks apart. In 2010 we had a warm spring, but in 2019 we had a very cold spring, which greatly delayed the development of Spruce Budworm.

Male Magnolia Warbler in a spruce with a Spruce Budworm

Magnolia Warblers also increased their numbers in response to the outbreak, but it took a few years before it was noticeable to me. They nest close to the ground in smaller spruces, and do not roam very far from their small nesting territories. The first few years of the outbreak affected large spruces almost exclusively, but small spruces later became widely infested as well. This was probably the reason for the delayed increase.

Male Magnolia Warbler in a cedar with a Spruce Budworm and inchworm

The bird above gathered food for its nestlings last year. The small spruces of its territory had plenty of budworms and the cedars had inchworms.

Male Black-throated Green Warbler in a spruce with a Spruce Budworm

Black-throated Green Warblers did not seem to increase their numbers in infested areas inland. But near the north shore of Lake Huron they did increase in numbers in certain mixed forests dominated by cedars. Those are the areas where they typically nest near the north shore. I do not believe their increase was related to Spruce Budworm because there are not enough affected spruces in those forests. But as an opportunist feeder they certainly took advantage of the budworms in their nesting territories.

Male Golden-winged Warbler with a Spruce Budworm and a large larval worm

This bird nested about 40 yards from the nest tree of the Cape May Warbler at the beginning of this article. This Golden-winged Warbler’s habitat, dominated by tag alders and willow bushes, was abutted to a few large spruce trees infested with Spruce Budworm. It took advantage of the bounty, but the Cape May Warbler chased it away from the mid-level and upper level regions of those spruces, so it stayed low to collect budworms. I observed it probing the cocoons at the tips of spruce branches in exactly the same way as a Cape May does. It may have learned by observing that bird.

Male Nashville Warbler in a spruce with a Spruce Budworm

Last year this bird foraged in the smaller infested spruces within the territory of a nesting pair of Cape May Warblers. It also seemed as if it learned from a Cape May how to probe the cocoons. There are two cocoons right near the head of this bird in the photo. It probed them to extract a worm from both of them.

Male Nashville Warbler in a spruce with six Spruce Budworms

This is the same bird. It was exclusively gathering budworms for its nestlings. It nested at a significantly later date than is typical for its species. The reason may have been due to the cold spring, a failed first attempt, or a delay because it learned to wait for Spruce Budworm abundance to nest. I found a few other Nashville Warblers, the last two years, which nested late as well.

Male Northern Parula in a spruce with a Spruce Budworm

This bird and its mate fed on budworms in the territory of a nesting Cape May Warbler. They also probed in the same way.

Here is short video of another male Northern Parula which demonstrates the feeding behavior: https://www.youtube.com/watch?v=bmobSVl9yv0

At the 3 second and 15 second marks of that video a Cape May Warbler is singing, and could be the bird this Northern Parula learned from.

Red-breasted Nuthatch in a spruce gathering Spruce Budworms

Last year in early June I took a client (Mark Halonen) to photograph the warblers and other songbirds within 5 miles of my home, for 2 days. We found a pair of Red-breasted Nuthatches exclusively gathering spruce budworms from a spruce about 20 yards from their nesting cavity. After capturing a few worms each would fly directly to the cavity and then come back with an empty beak. Mark did a good job of firing a burst of images when this bird probed, hoping that it would raise its head up and come to a stop point with a budworm when it had a good head angle and body position. The nuthatch pair also fed on budworms in the same way as a Cape May. And a singing Cape May Warbler was nearby.

Male Cape May Warbler in a spruce with a Spruce Budworm

We also found this beautiful Cape May Warbler feeding on smaller budworms. Its nesting area was directly exposed to the predominant cold wind off the cold waters of northern Lake Huron during the unusually cold spring of 2020, and this delayed budworm development even more at that location.

For more of Mark’s images from those 2 days see: https://paulrossibirds.com/m-halonen-gallery-2020/

My workshops in late May and early June typically provide clients with opportunities to photograph some birds feeding on Spruce Budworm and/or other insects. See: WORKSHOPS | paulrossibirds