May 112017
 

One of my greatest pleasures in May is the welcome sight and sound of songbirds returning right on cue from their southern wintering grounds. I was therefore delighted to hear a northern waterthrush belting out its emphatic double-note song at the start of the de Pencier Trail at the Trent Nature Area this week. Having seen these brownish warblers in winter in the mangroves of Costa Rica, the wonder of their annual two-way journey never ceases to amaze me.

The Northern Waterthrush often bobs its tail up and down as it walks. It’s one of many songbirds in the Kawarthas that migrates to and from the tropics each year. (Photo: Robin Williams Blake)

This month sees the biggest push of spring migration with the arrival of nearly all long-distance (neo-tropical) migrants – birds that spent the winter in the Caribbean, Mexico and Central and South America. It is possible to see more species at the height of migration in May than at any other time of year.

An elegant synchronicity of events occurs this month. As the green canopy of leaves develops overhead, countless caterpillars emerge to feast on the verdant bounty laid out before them. And, right on cue, millions of birds pour into central Ontario to regale themselves of this insect banquet. While some species will remain to raise a family here, others pass through quickly as they push forward to northern nesting grounds.

Bird GPS

For thousands of years, bird migration stumped the greatest minds. People used to believe that hummingbirds migrate on the backs of geese and that swallows emerge in spring from the bottom of ponds. Today, scientists have a much better understanding of this fascinating phenomenon, although much still remains unexplained.

Most bird species migrate at night when there is less danger from predators and the air is more stable. The daylight hours are used for feeding and resting. If conditions are favorable, such as with the passage of a northward-advancing warm front, birds will start migrating about one hour after sunset. Songbirds such as the waterthrush usually fly at less than 200 metres and average speeds of about 25 kilometres per hour.

Changing weather conditions during the night can cause “groundings” of these nocturnal voyagers. When a northward moving warm front collides with a cold front, the warm air – and the birds in it – rises over the cold; the air cools, rain develops and the birds are forced to land. This means that rainy May mornings can produce superb birding.

Birds use a variety of navigational cues to find their way, and different species rely on some cues more than others. Indigo buntings, for example, appear to orient themselves in relation to the pattern of stars around the North Star, Polaris. To navigate by stars, birds require a clear view of the sky. However, many birds migrate below cloud level, which begs the question of what “GPS” they are using. Researchers now have conclusive evidence that at least some migratory songbirds are able to get their bearings from the Earth’s magnetic field. The Earth is like a gigantic magnet, with magnetic field lines extending between the magnetic north and south poles. In 1984, it was discovered that the nasal tissues of birds such as bobolinks contain magnetite. This magnetic mineral acts almost like a miniature compass needle. It is thought that birds may actually be able to see the magnetic field as a visual pattern or specific colour. The northern waterthrush may in fact see north and south as a shade of blue, for example, but perceive no colour at all when facing east or west.

When songbirds cannot rely on stars or the magnetic field for direction, they may turn to information from the position of the setting sun on the western horizon. They may also align themselves to the band of polarized light, which extends perpendicular to the setting point of the sun. Invisible to humans, polarized light is created when sunlight scatters as it passes through the atmosphere. Just as the sun’s location changes with latitude and time of year, so does the position of the band of polarized light. These cues can therefore be used by birds to determine direction. Polarized light is visible to birds even when the sky is completely overcast.

Other directional signals may exist as well. They include infrasounds – sounds whose frequency is below the normal limit of human hearing, such as the roar of the ocean surf or the sounds of winds across the mountains. Wind-carried odors like the smell of certain types of vegetation may also provide useful information. Therefore, it may be that the waterthrush I heard this week “remembers” the specific smell of the de Pencier Trail wetland at Trent.

Boardwalk at beginning of de Pencier Trail (Photo: Drew Monkman)

The incredible accuracy of these navigational cues allows birds to return to the same summer and winter territory each year – maybe even the very spot where they hatched as chicks. This is especially true for songbirds like warblers. So, I make a point of saying “Hi! You’re back right on schedule!” to the waterthrushes at Trent each spring (when no one is listening, of course!). They were probably the same birds as last year.

Monitoring

Technology now plays a major role in monitoring nocturnal bird migration. Weather radar, for example, reveals the tell-tale signatures of migrating birds. The radio waves sent out by Doppler radar bounce off birds and return a signal to the receiver. The numbers can be staggering. It was estimated that somewhere in the order of three to four million birds crossed a line between Cornwall and Granby, Quebec on the night of April 15, 1994. A new visualization tool for radar data even reveals birds’ nocturnal journeys as blue streaks that sweep across a map like raindrops on glass.

Many migrants make soft chirps, tweets and buzzes as they fly overhead under cover of darkness. The sounds, which are unique to each species, may serve as a way to maintain in-flight associations and stay on course. On a good night, it is possible to hear hundreds of these faint vocalizations and, with practice, put a species name to some of them. Now, the sounds are being captured by specialized microphones and other acoustic monitoring equipment that can record, analyze and identify the call makers.

Across North America, such monitoring is allowing both research scientists and citizen scientists to discover everything from what species are flying over the backyard on a given night to how migrating birds interact with the landscapes around them. This is important information, since at least 70 percent of birds migrating to and from Canada fly over urban landscapes and many are attracted to blinking communications towers and illuminated skyscrapers. The mortality that results from collisions with these structures can be staggering. Data from acoustic monitoring can therefore be used to identify high-threat and low-threat zones in urban areas, and measures can be taken to help birds migrate successfully.

Why migrate?

Why would a neo-tropical migrant such as a waterthrush have evolved to undertake a dangerous 6000 km journey from Costa Rica all the way to the Kawarthas and back? The short answer is that it allows them to raise more young. Protein-rich insects are abundant during the Canadian spring and summer, there is a much more habitat available and the long days allow birds to feed their young for up to six hours longer than if they had stayed in the tropics. By flying north in the spring, they also free themselves from competition for food from tropical resident birds.

Using data from weather surveillance radar and eBird checklists, it has now been determined that climate change is causing the seasonality of bird migration to shift. Many birds are arriving at their northern breeding grounds earlier in the spring. This seems to be especially true for temperate migrants like robins and tree swallows, which overwinter in the southern U.S.

Get outside

You don’t have to go far afield to see neo-tropical migrants such as warblers, vireos, flycatchers, orioles, tanagers and grosbeaks. As long as there is sufficient cover, even city backyards can have their own coterie of migrants. Habitat edges such as wooded roadsides are especially worth checking. Get out early, however, preferably before 8 a.m. Keep an eye on the weather forecast, too. Winds from the south usually bring in the most migrants. The largest concentrations generally occur when these south winds are met by some change in the weather such as light rain or drizzle. Even a passing line of local thunderstorms can result in a surprising array of species.

By visiting different habitat types, an experienced birder can usually record 80 or more species on a single morning in mid-May, including 10 or more kinds of warblers. Don’t just rely on your eyes. Be sure to track down any songsters you hear uttering a strange vocalization. Be sure to pish, as well, since warblers are quite responsive to these sounds. If you’re not quite sure which migrants are arriving when in the Kawarthas, visit my website at drewmonkman.com and click on the “Seasonal Bird Abundance” tab. Most of the better known species are listed here.

City parks such as Beavermead and Ecology Park can be great spots for finding warblers and other songbirds. I would also recommend the first kilometre or two of the Rotary Greenway Trail, starting at East Bank Drive at Trent University. This is usually a great spot to hear my waterthrush friends, as well! Learn the song at allaboutbirds.org

 

May 052016
 

The time that birders have awaited since the lonely, frigid days of winter is now upon us. With May comes the peak of spring migration as long‑distance migrants pour into the Kawarthas from the neo‑tropics ‑ Mexico, Central America, the West Indies and South America. In Peterborough County alone, a day of birding in mid-May can produce over 100 different species. Not only do their flamboyant colours symbolize the tropical habitats whence they’ve come, but their vigorous singing heralds that high spring is finally here.

An elegant synchronicity of events is happening before our eyes. As the green canopy of leaves develops overhead, countless caterpillars emerge to feast on the verdant bounty laid out before them. And, right on cue, hundreds of millions of birds arrive to regale themselves of this insect banquet. While some species remain to nest in the Kawarthas, others are only passing through on their northward journey and won’t be seen again until their flight south in the fall.

Rose-breasted Grosbeaks - Drew Monkman

Rose-breasted Grosbeaks – Drew Monkman

You may wonder why an eight inch, two-ounce bird like a rose-breasted grosbeak would risk a perilous 4000 km journey from Costa Rica all the way to Kawarthas just to nest? Obviously, there must be compelling reasons. The short answer is that they are able to raise more young than had they remained in the tropics. Protein‑rich insects are abundant during the Canadian spring and summer; there is a much larger geographical area over which to spread; and the long days allow birds to feed their young more than four hours longer than had they remained in the south.

Enjoying the show

May’s bounty of birds can be enjoyed right here in Peterborough, especially if you have tree cover on your property and an offering of sunflower seeds and sugar water. Early May sees the arrival of ruby-throated hummingbirds, Baltimore orioles and rose-breasted grosbeak, all of which will come to feeders. House wrens, too, are arriving from the tropics and are easy to attract to nest boxes. In the downtown, watch and listen for swallow-like chimney swifts coursing above George Street like chattering “flying cigars”.

Male Indigo Bunting at feeder - Greg Piasetzki

Male Indigo Bunting at feeder – Greg Piasetzki

By mid-month, an indigo bunting may also make a guest appearance at your feeder – an unforgettable sight in its radiant blue plumage. Watch and listen too for up to 16 species of warblers, many of which put in a brief appearance in city backyards. Decked out mostly in yellows, oranges, whites and blacks, warblers are the true gems of spring migration. Bringing up the rear, late May ushers in species such as the red-eyed vireo, which have flown all the way from the Amazon Basin. If you live in a part of the city with mature trees, listen for the vireo’s repetitive, robin-like song as it forages high in the treetops.

Red-eyed Vireo - Karl Egressy

Red-eyed Vireo – Karl Egressy

If you want to take in the entire migration spectacle, however, you will need to be out looking and listening almost every day, especially when the weather is damp and mild. Bird activity is usually most intense in the morning between about 6 and 9 a.m. Song is the key to the birds’ presence, so it’s important to pay attention to the different voices. Warblers, for example, tend to have high-pitched, buzzy songs, while birds like scarlet tanagers, orioles and grosbeaks sing in rich, musical notes. The good news is that many May migrants show up in loose, mixed-species flocks. If you find one variety of warbler, for example, other species are probably nearby as well. Pishing will often bring them in closer for great views.

 

Where to go

Although migrants can turn up anywhere, some habitats and specific locations are consistently better than others. Habitat edges are most productive, including wooded roadsides, the trees along rail-trails, hedgerows, and the shrubby borders of wetlands. Among my favorite places for spring birding are Herkimer Point Road (east of County Road 31 in Hiawatha), Beavermead Park and Ecology Park on Little Lake, Jackson Park,  the Rotary-Greenway Trail (especially the Promise Rock section north of the science complex at Trent University), Lynch’s Rock Road and Sawer Creek Wetland (northeast of Lakefield), Hubble Road (east of County Road 44, north of Havelock), Petroglyphs Provincial Park and Sandy Lake Road (east of County Road 46, just south of Lasswade).

Favorable winds

Flying at an elevation of about 1000 metres, most songbirds migrate at night, which allows them to see the stars for navigation purposes and to avoid predators such as hawks. It is quite common to hear their contact calls as they pass overhead in the inky darkness. Songbirds almost always wait for a tail wind – a wind blowing in the same direction they are headed – before migrating in large numbers. A tail wind allows the birds to expend less energy in flight. In the spring, tail winds are associated with warm fronts advancing from the south or southeast. However, a sustained south winds may cause birds to fly right over your favorite birding destination without stopping. The key to great May birding is to watch the local forecast for some change in the weather such as a forecast of rain and fog. When a northward-moving warm front collides with a cold front, the warm air ‑ and the birds flying in it ‑ rises over the cold. The air cools, rain develops and the birds are forced to land in what is called a “fallout”- sometimes right in your own backyard! This means that rainy mornings in May can produce superb birding, especially when the precipitation is light and starts after midnight. During spring fallouts, I’ve seen trees hopping with dozens of warblers of ten or more species.

 

Song

To the practiced ear, a chorus of bird song is like a symphony in which you recognize each of the individual instruments. As a beginner, though, you should learn to focus on one song at a time and not the entire symphony, which can be quite overwhelming. Focus your attention first on the closest, loudest and most obvious songs. You can then move on to the softer voices. Cupping your ears can be very helpful. American Redstart in full song  - Karl Egressy

There is no doubt that some species sound similar to others. However, when you take into consideration the context of the song ‑ habitat, time of year and the bird’s behaviour ‑ the choice usually comes down to only a handful of species. It is also crucial to learn the memory-aids or “mnemonics” for the songs. To me, a rose-breasted grosbeak sounds like a robin that has taken voice lessons, while a scarlet tanager is reminiscent of a robin with a soar throat! Go to fernbank.edu/Birding/mnemonics.htm for a great list. My favorite bird song app is the Sibley eGuide to Birds. Allaboutbirds.org is another superb resource.

Being able to recognize bird song is one of the most satisfying ways to enjoy nature. To step out the back door or walk down a forest trail on a May morning and hear the expected birds singing in the expected locations provides reassurance that the bird community is healthy, and the seasonal rhythms of the natural world are occurring, as they should.