【正确答案】The lecture is about bird migration, and there are two types of changes birds exhibit before actually migrating.
One change has to do with body weight: birds eat a lot and gain a lot of weight just before migrating because they have little time to look for food during the long trip. Since much energy is needed in order to fly long distance, birds will eat a lot to store fat before migration. The example the professor gives is of the red-winged blackbird, which gains about 20% more weight.
The second one is the change of social habit. Those birds are usually not sociable, but they start to flock together when migrating. Birds do this for safety reasons.

【答案解析】[听力原文]
W: Today, I'd like to introduce the fascinating topic of bird migration. Birds do not just get up and decide to fly south or north in the winter or spring. It's like any trip: preparation and planning are necessary. Let me mention a couple of examples.
OK, here on the Great Plains we all know the red-winged blackbird, right? Well, one thing the blackbirds-and all species-do before migration is eat a lot. I mean, the birds will be flying for a couple of months most of the day. So they need a lot of energy, and they'll have very little time to look for food. On average, blackbirds gain about 20% more weight before they set out on migration. Another change is social. Blackbirds live in the same marshy areas, but you may have noticed they more or less ignore each other for most of the year. But, in the period of migration, the birds start flocking together. Why? It's safer to travel in groups. There may be some predators or other dangers while migrating. So they flock together before setting out.

Alternative Wording(替换表达)
Synonyms (a)gain weight—put on weight (b)flock—gather
(c)safety—security
Paraphrases 1. Energy expenditure is a consideration during migration; therefore, birds eat heavily prior to setting out.
2. Solitary species become gregarious as migration time draws near.