The story of the lazy birds

This is one of the question from my Maths Tutorial 1:

Ornithologists have determined that some species of birds tend to avoid flights over large bodies of water during daylight hours.

It is believed that more energy is required to fly over water than land because air generally rises over land and falls over water during the day.

A bird with these tendencies is released from an island that is 5 km from the nearest point B on a straight shoreline, flies to a point C on the shoreline, and then flies along the shoreline to its nesting area D.

Assume that the bird instinctively chooses a path that will minimize its energy expenditure.

Points B and D are 13 km apart.

If it takes 1.4 times as much energy to fly over water as land, find the distance between B and C.

Arrggg!!! Stupid lazy birds! Why can't they just fly from point A to B without stopping? That would make life simpler right? No no... they just have to stop midway to take a break and even plot to minimize their energy expenditure. Ha.

My answer to this question? I don't know. Because I was busy trying to shoot the damn birds. Ha.