The professor is discussing a type of animal behavior called kleptoparasitism.
This refers to one species stealing food from another species, and in the case of birds, the stealing of nesting materials, too.
The first example he gives is about seagulls. Seagulls steal clams from ducks because they are not physically able to dive to the bottom of the sea to collect their own clams.
So, they just wait for ducks to collect the clams, and then the seagulls steal them.
His second example is about American Redstarts. These kleptoparasites steal parts of nests from other species to reduce the amount of time it takes to build their own nests.
This means there's less chance that they'll be spotted by predators, and it lets them spend more time looking after their own nest.
