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.
19.
Which one of the following, ██ █████ █████ ████ ██████████ ███ █████████ ██ ███ █████████
a
Increases in bird ███████████ █████████ █████ █████████ ███████ ███████ █████████
Saying that bird populations "sometimes" increase after "unusual weather patterns" doesn't tell us anything how or whether a mild winter could cause a population increase. We already know the population increased after a mild winter. We want to understand why it increased. Was it a result of the warmer winter? (A) doesn't help establish that it was.
b
When birds do ███ ███████ ██████ ███ ██████ █████████ ████ ███████ ██████ ████ █████ ████ ███████ ████ ████ ██ ████████
This tells us that non-migrating birds exhibit different mating behaviors than migrating birds. But different how? Does different mating behavior lead to more offspring, fewer offspring, or roughly the same number? We have no idea. Without knowing the direction of the effect, (B) can't strengthen the argument. For all we know, the different mating behaviors could lead to fewer birds, which would weaken rather than strengthen.
Unwarranted assumption
Unwarranted assumption
c
Birds eating at ███████ ███ ████ ██████████ ██ █████████ ████ ███ █████ ████████ ██████████
This provides a causal mechanism that connects the feeder observation to increased population. If birds eating at feeders are more vulnerable to predators, then the fact that fewer birds visited feeders last winter means fewer birds were exposed to that increased predation risk. More birds foraging naturally means more birds surviving, which contributes to the larger-than-usual population.
Notice how this fills the gap we identified. Without (C), the first observation (fewer birds at feeders) just sits there, disconnected from the conclusion about population size. With (C), we now understand why fewer birds at feeders can lead to more birds surviving, which makes the author's hypothesis more plausible.
Causal mechanism
Causal mechanism
Answers that undermine, or help establish, the practical story of how an alleged cause could produce the alleged effect.
d
Birds that remain ██ █████ ██████ █████ ███ ██████ █████ ███████ ████ ███████ ████ ██████ ██████ ███████
This arguably weakens the argument. If birds that stay in their summer range all winter tend to exhaust the food supply before spring, then not migrating could lead to starvation. That would undermine the second causal chain by suggesting that avoiding migration attrition might come at the cost of making it harder to find food.
Directionally wrong
Directionally wrong
Answers that, if they have any effect, do the opposite of what we want (weaken when we're trying to strengthen, or strengthen when we're trying to weaken).
e
Birds sometimes visit ███████ ████ ████ ████ ███ ████ ██ ████ ██████████ ████ ███ ████████ ██ ████████ ██████████
(E) tells us that birds sometimes visit feeders even when they can find enough food by foraging naturally. But the argument's gap isn't about whether birds visited feeders less often during the mild winter. The stimulus already tells us that happened. The gap is about why fewer feeder visits would lead to more birds surviving. (E) doesn't address that question at all. It has nothing to say about the connection between feeder use and bird survival or population size, which is the missing link we want to fill to strengthen the argument.