Myiarchus

Question: what Family of birds is the most speciose?

Answer: Tyrannidae (tyrant flycatchers), although you wouldn't necessarily know it from the way the Family is represented in Ontario.

According to the Handbook of the Birds of the World, there are 432 species of tyrant flycatchers, all of which are endemic to the nearctic and neotropical ecozones (a.k.a. the western hemisphere). Only 24 species have been recorded in Ontario, and of that number only 11 occur regularly. These are primarily birds of the South American continent.

Great Crested Flycatcher
(Myiarchus crinitus)

Great Crested Flycatcher
(Myiarchus crinitus)

Here is another noteworthy thing about the tyrant flycatchers: some of them are difficult to tell apart. Species in the genus Empidonax are perhaps the most notorious in this respect, but there are others. Take the genus Myiarchus for example. Since Great Crested Flycatcher is the only regularly occuring Myiarchus flycatcher in Ontario, this may not present itself as a major problem. But what is it about the bird in the above two photos that readily distinguishes it from Ash-throated Flycatcher? With the extensive, bright yellow underparts hidden from view, and the tip of the tail somewhat obscured by shadow, the best field mark is probably the broad white edges to the tertial feathers (the wing feathers closest to the body).