Since Pidgey need only 12 Pidgey Candies, it is easier to evolve than other Pokemon. That means you can gain 500XP faster.
3. Good combination with Lucky Egg
Unlike other XP resources, you can evolve Pokemon and gain XP whenever you want based on the number of Pidgey and Pidgey Candy you have. By collecting the enough number of Pidgey and Candy, you can gain 1000XP many times within 30 minutes (The time limit of Lucky Egg).
1. Capturing Pidgey
Pidgey is easy to catch. You can capture a bunch of Pidgeys while looking for PokeStops or doing other things.
13 Pidgeys per unit. Stock 4 Pidgeys and transfer 9 Pidgeys to Professor Willow to spend Candy efficiently. You can earn 5,300XP per unit during the effect of Lucky Egg.
2. Transferring Pidgey to Professor
Transfer Pidgey to Professor Willow to get the enough amount of Pidgey Candy. Pidgey can evolve with 12 Candy. Note that you should stock some Pidgey to be evolved.
3. Using Lucky Egg
Finishing step 1 & 2, you will use Lacky Egg to double gained XP for next 30 minutes. To increase XP earned during the effect of Lucky Egg, you may evolve other Pokemon as well.
4. Transferring Pidgeotto
Now you may have a bunch of Pidgeottos. To get more Candy, transfer them to Professor Willow. You can go back to step 1 and gain more XP.
Why is 'Pidgey Marathon' the Best?
900XP gained by a Pidgey evolution
Pidgey need 12 Candy to evolve into Pidgeotto. To get 12 Candy, 4 Pidgey have to be captured. You can gain 400XP by capturing 4 Pidgey and 500XP by one Pidgey evolution.
500XP × 2 = 1,000XP!!
While having the effect of Lucky Egg, gained XP is doubled. You can earn 1,000 XP per Pokemon evolution.
PokeLevel is very nice tool to optimize the effect of Lucky Egg. Tap the link below to see the details of the tool and how to use it.