Self-study
Support Last winter was mild enough to allow most bird species to forage naturally, which explains why the proportion of birds visiting feeders was much lower than usual. ███ ████ ██████ ████ ███████ ████ ███████ ██ ████ ██ █████ ██████ █████ ███ ██████ ███████ █████████ ██████ ███████ ████████ ███ █████ █████████ ████████████ ██████████ ██████ ████ ██████ ████ ██████ ██ ███████████ ███ ████ ██████ █████████████████ ████ ███████████
A Missing Link
The author concludes that last year's mild winter is responsible for this year's larger-than-usual bird population. To support this, the author offers two observations about what the mild winter caused:
First, because the mild winter allowed birds to forage naturally, fewer birds visited feeders than usual. Second, because the mild winter allowed birds to stay in their summer range without migrating south, fewer birds were lost to the usual attrition that accompanies migration.
The second observation connects cleanly to the conclusion. Less migration means less attrition (fewer birds dying from the migration), which means more birds survive. That makes sense.
But what about the first observation? How does the fact that fewer birds visited feeders contribute to a larger bird population? The author seems to think it does, but the connection is never spelled out. Why would visiting feeders lead to fewer surviving birds? There's a gap here: we're missing a reason why foraging naturally (instead of eating at feeders) would help birds survive.
Anticipation
To strengthen this argument, we want to fill in the gap between the feeder observation and the population conclusion. We need a reason why birds visiting feeders less often would lead to more birds surviving. In other words, we need a causal mechanism that explains how eating at feeders is bad for birds' survival. If we can establish that, then the first observation becomes a genuine contributor to the larger bird population, and both of the author's causal chains support the conclusion.
Analysis by Kevin_Lin
19.
Which one of the following, ██ █████ █████ ████ ██████████ ███ █████████ ██ ███ █████████
a
Increases in bird ███████████ █████████ █████ █████████ ███████ ███████ █████████
b
When birds do ███ ███████ ██████ ███ ██████ █████████ ████ ███████ ██████ ████ █████ ████ ███████ ████ ████ ██ ████████
c
Birds eating at ███████ ███ ████ ██████████ ██ █████████ ████ ███ █████ ████████ ██████████
d
Birds that remain ██ █████ ██████ █████ ███ ██████ █████ ███████ ████ ███████ ████ ██████ ██████ ███████
e
Birds sometimes visit ███████ ████ ████ ████ ███ ████ ██ ████ ██████████ ████ ███ ████████ ██ ████████ ██████████