How to attract more birds to my balcony garden?

Hello fellow gardeners,
I have a north west facing balcony and it attracts some birds. Seeing them makes my day a good day. And I can imagine it pleases you to see your visitors too.
My balcony garden is made up few plants, flowers and a small holly tree. It has a couple of bird feeds, a mini greenhouse, table and chairs.
There are a variety of birds in the area I live because I've seen and heard them. I know they prefer the proper gardens than my balcony one.
I would like to attract more birds but not sure what I need to buy more of.
I have room for another small tree, a large ceramic pot, two hanging baskets and maybe room for one more medium pot. Anymore items then it'll make the area look cluttered and cramped.
I would like another small tree. One that doesn't grow really tall and attracts more birds!! :)
Please could you suggest which tree I should go for? Or shall I just opt for buying more plants?
I don't know what to do :/
Many thanks,
Sultana Wahab, London, uk
P.s please don't mention bird food & feeders because that's sorted now!!! ;)
--
Sultana Wahab

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On Sat, 15 Mar 2014 12:03:31 +0100, Sultana Wahab

Perhaps a bird bath. Also a lot of birds prefer fresh fruit, apples will attract birds, also suet cakes. How high is your balcony? And if there are more balconies near yours with people on them that will keep birds away. Even if you are inside but visible the birds will be put off. I feed birds on my rear deck but when I get too close to the glass sliders they all fly off.
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