Conclusion: This decline, however, poses no serious threat to the migrating shorebird populations:
Premise: ...by remaining longer in Delaware, the birds will be able to eat enough crab eggs to allow them to complete their migration.

Reasoning: This is a causal relationship between "eating enough crab eggs" and "migrating shorebird populations". The cause is "eating enough crab eggs," and the effect is"migrating shorebirds populations." Additionally, there is another, embedded relationship: "by remaining longer" and "be able to eat enough crab eggs" and "successfully breed." A->B->C

Thus, the conclusion can be weakened if (1) an alternative cause is proposed; (2) cause occurs and the effect does not occur; (3) cause does not occur and the effect does; (4) the relationship is reversed; (5) the validity of the data is weakened.

A. No other food available to the birds on Delaware's beaches is as nutritious and accessible as are crab eggs. Neutral. No change to the cause or effect whatsoever.

B. The decline in the number of crabs on Delaware's beaches is due to the harvesting of crabs by humans. Shell game. Whether or not the crabs are declining because of humans, the length of the stay of shorebirds does not affect the shorebirds populations.

C. There are more crabs on Delaware's beaches than in any other area that the migrating birds pass through. Neutral. Whether or not there are more crabs on the beach than migrating birds, the length of the stay of shorebirds does not affect the shorebirds populations

D. The crabs do not conceal their eggs. Out of scope. No affect on conclusion.

E. The earlier in the season the shorebirds complete their migration, the more likely they are to breed successfully. OK - The length of time does matter in how successful their breeding is. If the birds stay longer than normal, then the migrating shorebird populations could be affected. Thus, the cause of "remaining longer in Delaware" occurs, but the effect "shorebird populations" does not occur.

IMO E