Science is shedding new light on how, where, when and why birds migrate 

Peterborough Examiner  – May 3, 2024 – by Drew Monkman  

Spring and fall migration are the best times to be a birder. For me, however, spring takes top billing. May sees the biggest push of spring migration with the arrival – right on cue and decked out in spectacular breeding plumage – of nearly all the long-distance migrants. These are birds like hummingbirds, warblers, orioles, vireos and thrushes. Having spent the winter in the Caribbean, Mexico and Central and South America, they travel thousands of kilometres to their North American nesting grounds.

An elegant synchronicity of events occurs in May. As the green canopy of leaves develops overhead, countless caterpillars emerge to feast on the verdant bounty. And, at just the right moment, millions of migrants arrive to regale themselves of this insect banquet. While some species will remain to raise a family here in the Kawarthas, others are only passing through on their way further north.

It’s possible to find more bird species at the height of migration in mid-May than at any other time of year. An experienced birder can often record 80 or more species on a single morning, including 10 or more kinds of warblers.  

Why do birds migrate?

            Why have some birds evolved to take a dangerous long-distance journey from Latin America all the way to the Kawarthas and back? Simply put, it allows them to raise more young. Protein-rich insects are abundant during the Canadian spring and summer; there is much more habitat available for nesting; and the long days allow birds to feed their young for up to six hours longer than if they had stayed in the tropics. As winter approaches and the availability of insects and other food drops, the birds move south again. Despite the arduous journeys involved, over 400 of Canada’s birds are migratory.

It appears that the urge to migrate triggered by a combination of factors such as changes in day length, food availability and genetic predisposition. Spring and fall restlessness has long been observed in migratory caged birds. This includes repeatedly fluttering toward one side of their cage. 

These twice-yearly long distance flights are fraught with risk. Among these are finding adequate food along the way, bad weather, and increased exposure to predators like free-roaming cats. One of the greatest causes of bird mortality during migration, however, is fatal collisions with residential windows, communication towers and tall buildings. Many species are attracted to the lights of tall buildings and millions are killed each year in collisions with these structures. On the night of Oct. 4, 2023, nearly 1,000 songbirds died in Chicago after colliding with just one of the many buildings along the waterfront. Simply turning out or redirecting the direction of the lights in these buildings can make a huge difference.

May sees the arrival of spectacular long-distance migrants from the tropics. Some, like orioles, buntings and grosbeaks will even visit feeders. From top left clockwise: Baltimore oriole, indigo bunting, rose-breasted grosbeak and black-and-white warbler. Indigo bunting photo by Greg Piasetzki. Others by Drew Monkman

How do birds navigate?

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. Depending on the species, songbirds like warblers fly at altitudes ranging from 200 metres to several kilometres and can travel in excess of 500 km in a single night.  

Changing weather conditions during the night can cause “groundings” of these nocturnal voyagers. When a northward moving warm front collides with a cold front, rain develops, and the birds are forced to land. This means that rainy May mornings can make for excellent birding.

Although many questions remain, we now know that birds use different senses to  navigate. Some get information from the position of the setting sun and from landmarks seen during the day. Indigo buntings, for example, appear to orient themselves in relation to the pattern of stars around the North Star, Polaris. Bobolinks may depend more on the earth’s magnetic field. 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. Other sources of directional information include bands of polarized light and even the wind-carried odors of certain types of vegetation. The incredible accuracy of these navigational cues allows birds to return to the same summer and winter territories each year.

Monitoring migration

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 of birds that show up on radar screens can be in the millions.

Many migrants utter species-specific call notes as they fly overhead under cover of darkness. Now, the sounds are being captured by acoustic monitoring equipment – often located on roof tops – that can record, analyze and identify the call makers. One such station is located in Millbrook. The Merlin Bird ID app is also able to identify many of these chirps, thanks to its Sound ID feature. Last fall, I used Merlin to identify various thrushes as they flew south over my house in the pre-dawn darkness.

Another tool now being used is Motus Wildlife Tracking. Developed by Bird Studies Canada, it is a method of using radio telemetry to follow the movements of birds and other wildlife. Individual birds are equipped with a tiny radio transmitter that can be detected by specialized radio receivers (Motus stations) at over 1,500 locations worldwide. One such station is located north of Buckhorn. This method contributes to our understanding of migration routes, timing, and stopover habitats for different species, and does not require re-capture of tagged individuals.    

               Migration monitoring is also being done by attaching lightweight devices to birds. They are known as light-level geolocators. A geolocator records sunrise and sunset times to estimate location. When the birds are recaptured and the locator recovered, the data can be analyzed to map migration routes and identify important stopover and wintering locations. Data from all of these monitoring techniques is being used to identify high-threat and low-threat zones, especially in urban areas.  


An online resource called BirdCast can help you explore the wonder of migration and even find more birds. Go to to find forecast maps that predict how much, where, and when bird migration will occur; live maps of bird migration in real-time; and local bird migration alerts that tell you when intense bird migration will occur near you. Although BirdCast is mostly limited to the U.S., it does provide migration alerts for Toronto. These alerts are largely applicable to Peterborough, as well.  I recommend starting with the three-day forecast maps, which are the first maps you see on the BirdCast home page.

Impact of climate change

            Climate change is making migration monitoring more important than ever. What’s most disturbing is a potential mismatch between when migrants arrive on their breeding territory and when food resources are at their highest. Long-distance migrants are particularly at risk. Despite earlier springs and earlier peak insect numbers, these birds aren’t migrating any earlier. They are therefore at risk of arriving after insect numbers have peaked and therefore experiencing lower nesting success.

Take in the show

You don’t have to go far afield to see spring and fall migrants. As long as there is sufficient cover, they can even show up in city backyards – sometimes even at feeders. Keep an eye on the weather forecast, too. Winds from the south usually bring in the most migrating birds in spring. Be sure to make full use of the Sound ID feature on the Merlin app, too. City parks such as Beavermead and Meadowvale are often birding hotspots during migration. I would also recommend the various Trent nature areas and our many rail-trails.

Remember that migration is the best time to be a birder. Don’t miss the show!

Drew Monkman

I am a retired teacher, naturalist and writer with a love for all aspects of the natural world, especially as they relate to seasonal change.