The Ontario Breeding Bird Atlas

Common Grackle (fledgling)

Common Grackle (fledgling)

This year marks the start of a five-year breeding bird survey for the third Ontario Breeding Bird Atlas, which is a collaboration between Birds Canada, the Canadian Wildlife Service, the Ministry of Natural Resources and Forestry, the Ontario Field Ornithologists, and Ontario Nature.  Approximately 300 bird species breed in our province, and the goal of the atlas is to map the distribution and relative abundance of these species by looking for evidence of breeding for as many species as possible. By conducting surveys every 20 years researchers are able to determine which species are expanding their range, which ones are shrinking, which species are increasing in abundance, and which ones are declining.  Although data collection began on January 1, 2021, breeding bird surveys don’t really kick into high gear until mid-May once almost all of our breeding birds are back from their wintering grounds in central and South America to Ontario.  As I was not a birder when data was being collected for the second atlas (2001-2006), this was my first chance to participate as a volunteer atlasser, and I jumped at the opportunity. Over the last few years, and especially during the lockdowns of the COVID-19 pandemic, I’ve gotten to know the birds within my own area quite well, and after looking at a list of the species found in my area during the second atlas, I knew I could contribute some new data on species that were missing. For instance, Red-shouldered Hawk wasn’t found in the last atlas in my area, although I found a pair occupying a nest in Stony Swamp back in 2016. Barn Swallow was recorded only as being in suitable habitat in the last atlas, while they used to nest under the bridge at the Eagleson ponds before the city put wire mesh underneath it. And Killdeer was last reported as showing agitated behaviour, while I’ve seen a fuzzy newly-fledged bird at the Eagleson ponds once.

I quickly discovered that atlassing is not quite the same as birding. The goal of birding, generally, is to find birds and identify as many as possible, with secondary goals of photographing them, adding to one’s life list or year list, finding rarities, or adding new species to the list of birds for an area such as a yard or a hotspot. The goal of atlassing is to look for breeding evidence, especially evidence that confirms breeding, rather than evidence that indicates it is merely possible or probable. Atlassing is work; it means spending more time watching birds and seeing what they’re doing rather than just identifying them and moving on to the next bird.

There are four levels of evidence, with different types of observations falling under each. The lowest evidence is merely observing a species during its breeding season, but it is not in suitable nesting habitat – for example, seeing a Double-crested Cormorant flying over the suburbs. The next three levels of evidence are “Possible”, “Probable” and finally the “Confirmed” level of breeding evidence. Atlassers try to obtain Probable or Confirmed breeding evidence for as many species as possible in their area, and to upgrade any low categories of evidence to a higher category of evidence during the survey period.

American Robin on nest

Watching birds carrying nesting materials may lead you to their nests. I saw this robin building the nest; a later check revealed the robin to be incubating eggs.

Possible breeding evidence includes observing a species in suitable nesting habitat during its breeding season, or hearing a bird singing or producing other sounds associated with breeding (such as a Woodpecker drumming) in suitable nesting habitat.

Probable breeding evidence is usually limited to territorial or courtship behaviours in suitable nesting habitat, including:

  • Seven or more singing/drumming individuals heard during one visit to a single area and in suitable nesting habitat
  • Observing a pair in suitable nesting habitat
  • Observing an adult bird singing or drumming in the same area on at least two visits, one week or more apart
  • Courtship or displays involving a male and female (e.g., courtship feeding, copulation) or antagonistic behavior between two or more individuals (e.g., territorial disputes or chases)
  • Observing a bird visiting a probable nest site
  • Agitated behavior or alarm calls of an adult
Agitated Song Sparrow

Agitated Song Sparrow

Agitated birds are easy to spot. Birds that aren’t nesting usually fly off when a human gets too close; however, one that has a nest nearby will usually remain in the area, bouncing around a tree or shrub fairly close to the intruder while chipping or vocalizing with increased agitation. Song Sparrows and blackbirds are both notable for voicing their displeasure, and blackbirds may even hover above people or dive-bomb them in an attempt to scare them away.

Confirmed breeding evidence is the highest evidence and includes:

  • Nest building or the carrying of nesting material by all species except wrens and woodpeckers
  • Distraction display, injury-feigning, or other displays attempting to draw attention away from a nest or young
  • Recently fledged young
  • Adult entering, occupying, or leaving a nest site (visible or not) or whose behavior suggests the presence of an occupied nest
  • Adult carrying a fecal sac
  • Adult carrying food for young
  • Nest containing eggs seen
  • Nest with young observed (either seen or heard)

When birding I would be happy if I came across a nest or two during the breeding season, though I never purposely looked for them. Disturbing nesting birds can have fatal consequences, often by reducing protection and concealment if branches are moved out of the way, or creating a track that leads predators directly to the nest. Collecting evidence of breeding means watching the birds and seeing what they are up to – a singing bird might not reveal very much, but a bird carrying nesting material or food in its beak might lead you to its nest. I’ve found two Eastern Kingbird nests in my area by watching them carrying nesting material to the nest; only one of them seems to be occupied so far.

Eastern Kingbird with nesting material

Eastern Kingbird with nesting material

Red-winged Blackbird

This female Red-winged Blackbird was carrying nesting material, although I was not able to find out where the nest was.

Be careful, though! Not all birds carrying branches or vegetation are using them as nesting material. I saw this Black-crowned Night Heron carrying a large stick and waited hopefully for it to fly off somewhere, indicating that it was taking it to a nest. Not only did the heron NOT fly off with the stick, it started dipping the stick into the water before leaving it there! Instead of building a nest, it looked to me a though the heron was using the branch as a tool to try to catch fish – something I’ve heard of Green Herons doing, though they usually use bread or buoyant objects and place them on the water to lure the fish closer to the surface.

Black-crowned Night Heron

Black-crowned Night Heron using a stick to lure fish

The next code is for actually seeing recently fledged young. These are for altricial songbirds and precocial ducks and shorebirds incapable of sustained flight. Most songbirds can be recognized as fledglings by the downy feathers on their heads, fleshy gapes at the corners of their mouths, and underdeveloped tails or wings. They are also usually quite vocal, as they still depend on their parents to feed and protect them. Grackles and robins are among the earliest species to fledge, and I found this baby grackle calling for its parents – who weren’t very happy with me and flew in with the whole neighbourhood gang and clucked at me from the trees above me until I left.

Common Grackle fledgling

Common Grackle fledgling

This Red-winged Blackbird’s mother was also nearby, and both she and the baby were vocalizing a few feet apart from each other until the mother eventually flew deeper into the reeds, while the baby followed.

Red-winged Blackbird fledgling

Red-winged Blackbird fledgling

The next highest code on the list is observing an occupied nest: either seeing an adult settling on the nest as though incubating the eggs, or entering or leaving a nest site in a way that suggests the nest is occupied. After watching the Eastern Kingbird building the nest, I returned to the same site a little later in the season and found it was indeed occupied by an incubating bird, with the other parent close by.

Eastern Kingbird

Eastern Kingbird on nest

This Blue Jay nest was found the same way, before the trees had fully leafed out. It makes me wonder just how many nests I pass by without seeing due to the foliage every outing!

Blue Jay on nest

Blue Jay on nest

I also stumbled across a Baltimore Oriole nest entirely by chance while watching a few other species in the same area! A male was singing and vocalizing and moving around in the trees above me, and while trying to determine whether a Red-winged Blackbird’s nest was being occupied, I happened to glance up and spot the oriole’s distinctive pouch-like nest right above my head. Orioles use different plant fibers, including milkweed silk, manmade fibers such as yarn, and recycled material from an old nest to weave a new nest each spring. The female first secures the rim of the nest to the tree branch, then hangs upside down to weave the fibres together to make the outer pouch. Next she builds an inner layer inside the pouch before lining the interior with softer material including wool, hair, or fine grasses. The resulting nest contains a small opening at the top, and is sturdy enough to provide protection from predators and parasites such as cowbirds.

Baltimore Oriole nest

Baltimore Oriole nest

The nest didn’t look occupied, but when I moved around to the other side, I saw the female’s tail sticking out, suggesting she is incubating!

Occupied Baltimore Oriole Nest

Occupied Baltimore Oriole Nest

The male didn’t spend much time visiting the nest, but he did stop by once, presumably delivering food.

Baltimore Oriole

Baltimore Oriole visiting nest

These behaviours all show that the birds are attempting to nest, or have successfully raised their young to the fledgling stage, but as even fledglings have some mobility, they don’t confirm successful nesting at a specific place. The next four breeding codes all confirm successful breeding in a particular spot.

The first is observing the adults carrying a fecal sac. Young birds eat a lot, which means they also produce a lot of waste. In order to keep the nest clean and prevent its discovery by predators, the adults carry this waste away and deposit it somewhere out of view. When baby birds defecate, their waste is contained in a thick, strong sac made of mucus that makes it easy for their parents to carry away. Grackles in particular like dropping the fecal sacs into water, and the only time I’ve seen a bird carrying a fecal sac was at the Eagleson ponds. I was talking with friends (pre-pandemic) and watched grackles fly over the water and drop a small white sac into the water three times. If a natural pond or stream isn’t available – for instance, in a suburban backyard – nesting grackles will often drop the fecal sac into a swimming pool instead.

Even better evidence than an adult carrying a fecal sac is an adult carrying food. Sometimes when a bird becomes agitated and starts chipping at me from the shrubs or cattails I notice it is carrying a bug or two in its mouth. Other times I will watch a bird fly off with its beak full of insects, wings and legs sticking out at crazy angles. My best sighting was of an American Kestrel flying high over a marsh with a snake clutched in its talons! These birds are all busy trying to feed their young, looking for enough food to stuff into a hungry mouth before flying off to find more.

Red-winged Blackbird (female)

Red-winged Blackbird (female) with a crane fly

The highest levels of breeding evidence are seeing a nest full of eggs, or observing the young in the nest – either heard or seen, as babies make distinct begging noises that you can’t help but notice everywhere once you become attuned to them. I found my first nest with several nestlings quite by accident – while checking several small spruce trees in a park for robins’ nests, I found one agitated bird, but no nest, and this nest with no adults attending to it. I thought it might have been an old nest from the previous year, but when I looked inside I was shocked to see several young birds with barely any feathers! A robin was hopping through the grass nearby, probably looking for food. I took two photos and then quickly backed away.

Nest with baby robins

Nest with baby robins

I found other nests with young simply by watching the adults flying in with food. The first was a phoebe’s nest tucked under a bridge – the adults kept flying in and out of it, and I wasn’t sure if the adults were still building the nest, or bringing food, as I couldn’t see what they were carrying. By getting underneath the bridge I was able to find a well-developed nest and hear the cries of the young inside, confirming my first phoebe nest with young.

Eastern Phoebe nest

Eastern Phoebe nest with young (heard only)

The second nest was a Tree Swallow nest. They nest in bird houses and natural tree cavities, and I found this one by watching the parents fly in with food. Not long after they left, the baby inside stuck its head out as though looking for more food – based on its size, it looks as though it is almost ready to fledge.

Tree Swallow nestling

Tree Swallow nestling

It feels great to put my birding skills to use and contribute something meaningful to our knowledge of the distribution and abundance of Ontario’s breeding bird species. So far I’ve really enjoyed my time in the field, watching the birds and looking for breeding evidence. Many, many species that I’m logging so far have low evidence of breeding, such as birds heard singing several times in the same spot more than a week apart (including almost all the warblers I’ve heard, Gray Catbirds, Rose-breasted Grosbeaks, Swamp Sparrows, Alder Flycatchers, and more), or a pair seen in suitable habitat, such as the male and female Indigo Bunting I saw interacting together. Hopefully as the season progresses I’ll be able to upgrade those species to a higher level of evidence, and observe them carrying food, visiting a nest site, or feeding their young. This is a five-year project, however, which gives me ample time to find that evidence!


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