Why is this bird rare?
In answering this question, where you look is critical.
In a good day of birding, one encounters a myriad of species. Let's say it's spring in the eastern United States. We'd see warblers, sparrows, blackbirds, orioles, kinglets, nuthatches, woodpeckers, hummingbirds, sandpipers…the queue of groups goes on.
Inevitably, after even a little time in the field, the birder's mind unconsciously begins an analytic process, one which demands more and more data from more and more field experience. The birder's mind starts categorizing species by how likely they are to see it. Based on this unconscious categorization, the birder gauges their excitement about a given sighting. Yellow-rumped Warbler in early spring? 'Ah, more warblers are on the way.' Kirtland's Warbler? 'I BETTER GET THE WORD OUT.'

Indeed, it is expected for the observer to separate the common from the rare. Our pattern-seeking minds are naturally attuned to the variance in birds' frequency, and it is often this tuning that guides our decision-making in where to go for a morning's birding.

It's a lucky thing that many of the most common birds are still quite beautiful. This is a puffed up, bathing Yellow-rumped Warbler photographed by Chris Thomas.
Too often, we take this process for granted, as if some birds are more common by default than other birds. But why? In the eastern U.S., why are Yellow-rumped Warblers more common than Kirtland's Warblers? Why are birders so much more excited by Slaty-backed Gulls than by Herring Gulls? What's so much more thrilling about a Common Rosefinch than a Purple Finch?
Birds depend on every ecosystem they visit, be it to breed, to refuel, or to overwinter.
Again, we must remind ourselves that there are natural processes, evolutionary and ecological in nature, behind these questions. Birds, like the rest of life, have evolved to fill various roles in various ecosystems. Each species fancies a particular habitat, a certain mix of vegetation, with unique food preferences. So in asking why a certain bird species is rarer or commoner than another species, we are indirectly asking about the ecosystem in which it participates.

What makes these ponderings even more fun is that birds move a lot. Many birds are long-distance migratory champions, covering hundreds of miles in a day (or night) during their seasonal forays. These journeys literally connect the ecosystems of different continents. As such, the abundance of a given bird species depends on more than the one ecosystem with which we associate it conceptually. It depends on every ecosystem it visits, be it to breed, to refuel, or to overwinter.
So, when we ask, "why is this bird rare," we're really asking a more intricate question. We're querying the series of ecosystems it participates in.
The breeding range of the Gray-cheeked Thrush from the encyclopedia Birds of North America Online
I invite you, now, to consider the Gray-cheeked Thrush, a far northern Taiga-breeding songbird of the Canadian Shield. Unlike its abundant relative the American Robin, this species is uncommon, a fact compounded by its furtive migratory behavior. It travels so far north to breed that its breeding habits are rarely observed, and with infrequent observation comes even more infrequent study. As a result, little known about its life history.

Though its breeding habits are poorly known, at least its breeding range is fairly well established. We could make no such statement about its nonbreeding range. Travelling southward from Canadian taiga, along the Atlantic seaboard, and through the Caribbean, these gunmetal gray migrants settle down east of the Andes for the winter, known to be nonbreeders in Colombia, Venezuela, Trinidad, Guyana, and northwest Brazil — and that's about as specific as it gets.

A Gray-cheeked Thrush singing…but focus more on the habitat. This is an example of the ecosystem they breed in, photographed along Alaska's Denali Highway by clickfun.
Overall, we know that this species has a broad breeding range, and appears to have a broad nonbreeding range. Furthermore, while its behavior is reclusive, it's not that much more reclusive than other members of the genus — like Veery, Hermit Thrush, or Swainson's Thrush. During migration, it is often seen by birders, only not in the same abundances as its thrush kin. As such, we can deduce that its lower frequency can be attributed to a generally lower commonness instead of under-observation.
So, why is Gray-cheeked Thrush as uncommon as it is?
As of right now, we do not have the complete answer to this question. Too little is known about its breeding habits, about its interactions with other species, about its habitat preferences, etc. But one piece of the puzzle was recently uncovered.

Scattered across the Amazon basin are white-sand terra firme forests — rainforests that are not seasonally flooded, and whose substrate is almost entirely white sand. Sandy soil means low nutrient content, which has resulted in uniquely woody, hard-leaved vegetation. This is just the habitat that overwintering Gray-cheeked Thrushes prefer.

Conducting a survey of understory avifauna, Judit Ungvari-Martin captured and banded 57 Gray-cheeked Thrushes that were frequenting this white sand terra firme forest, doing so in 19 sites for three consecutive years. Remarkably, 3 of these 57 returned to the same overwintering location between years. While this may seem like a small number, this approximately 5% site fidelity was observed in spite of the immense mortality rate of neotropical migrants (some estimate near 50% die), and the ability of migrants to simply disperse into other suitable overwintering spots. What this indicates, then, is that there may not be many suitable spots to begin with.
Limited overwintering habitat significantly constrains the number of birds that can survive a full year.
White sand terra firme forest covers a measly 3% of the Amazon Basin. If Gray-cheeked Thrushes only overwinter in this habitat, then we may have at least part of the explanation of their rarity. Limited overwintering habitat significantly constrains the number of birds that can survive a full year. If all the birds that breed across the Canadian Shield must fit into this 3% of Amazonia, then we can expect that there probably isn't a very large breeding population to begin with. The breeding population must be either clustered in a few Canadian locales, or be thinly distributed across the Shield…predictions to be tested. Either way, we would definitely not expect them to be abundant across their established breeding range, and would therefore not expect them to be notably common during migration.

In this case, a birder's reality — the uncommonness of migrating Gray-cheeked Thrushes — fits these expectations.
A Gray-cheeked Thrush singing during migration, photographed by Leslie Starr.
We learn two lessons from these expectations. First, a large distribution may not necessarily mean vast expanses of suitable habitat for that species. Though birds can theoretically show up anywhere, their habitat preferences make it unlikely that they will. Second, sometimes looking by default at the breeding range is the wrong place to look. If limits on population size don't appear to exist in a bird's breeding range, they may yet exist in their nonbreeding range. One must holistically study a bird's full annual cycle — including all the ecosystems it visits — to understand just why a bird is abundant or rare.

So, we see that observing which birds are common and which are rare is one thing. But asking why they are common or rare connects us not only more deeply to a bird's annual cycle, but also to the ecosystems that support it.

Birds are global citizens. In asking questions about them, we have to think more globally ourselves.

Where around the globe might your bird questions bring you?
Citations:
Ungvari‐Martin, Judit, Christopher M. Heckscher, and Keith A. Hobson. "Inter‐annual site fidelity and breeding origins of Gray‐cheeked Thrushes in white sand forests of the Peruvian Amazon."Journal of Field Ornithology (2016).

Lowther, Peter E., Christopher C. Rimmer, Brina Kessel, Steven L. Johnson and Walter G. Ellison. 2001. Gray-cheeked Thrush (Catharus minimus), The Birds of North America Online (A. Poole, Ed.). Ithaca: Cornell Lab of Ornithology; Retrieved from the Birds of North America Online:http://bna.birds.cornell.edu/bna/species/591
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