I thought that to build a nest you had to attract 5 different birds to build your nest.
My nest has been partly built by a mallard and a chafflinch, however a wren and another one i forget which didn`t add to the build.  WHY!!!!

you have had 22 wrens visit and 17 yellow wagtails visit when you have had 25 wrens and 20 wagtails they will help build the nest

you need 5 of the same bird for it to help the nest

http://www.freedomfairies.com/bird-nesting-guide.html#Nes    copy from that website

You will need to attract 5 DIFFERENT TYPES OF BIRDS to your Garden overall to build your Nest. For example, you could start with the Pair of Lovebirds, once a multiple of 5 Pair of Lovebirds have been spotted in your garden they will give you a 20% part of your Nest. Then you need to move to a different Bird, for example Macaws, once you have a multiple of 5 Macaws spotted in your garden another 20% part will be added to your Nest, (these are of course just examples and you can attract any type of Bird from the list below to help build your Nest). You need to repeat this for 5 DIFFERENT BIRDS. You will also receive a 20% part of your nest if it is the first time that a particular bird has been spotted in your garden.

All Adult Birds (except the Cuckoo) will add a a 20% part to your Nest, you can use our list below to see which Birds you could attract to your Garden to build your Nest. Remember if you spot Birds in someone else' Gardens they DO NOT help build your own Nest, you need to have the  Birds spotted in your own Garden. However, the Birds you spot in other Gardens will help to build that Garden Owners Nest. You can build as many Bird Nests as you want to and you can send them to storage at any time for safe keeping when you want to do something else.
** IMPORTANT **
Please note when building your Nest that birds will only nest and lay eggs in a Nest which they have helped to build.
So for example, a Wren will only nest and lay an egg in a Nest which another Wren helped to build.
You may want to bare this in mind when building your nest.

you can also see it on
http://fairylandfaq.wordpress.com/index/page-15/

The chaffinch helped build because it was your first one.

I just spotted my 60th Singing Bush Lark, why did it not complete my nest?

I also just spotted my 155th Yellow Wagtail, it also didn't repair my nest, can someone help please.  I must be doing something wrong.  I haven't done birds and nests for years.