fields_for for nested attribute returns nothing

I'm trying to create a nested model form in Rails 3.0.3. Here are my models:

class Bird < ActiveRecord::Base
  has_one :taxon, :as => :organism
  accepts_nested_attributes_for :taxon
end

class Taxon < ActiveRecord::Base
  belongs_to :organism, :polymorphic => true
end

Here's the controller method:

def new
  @bird = Bird.new
  @bird.build_taxon
end

And here's the form:

New Bird
<% form_for @bird do |f| %>
<p>
    <%= f.label :wingspan %>
    <%= f.text_field :wingspan %>
</p>
<p>
    <%= f.label :body_length %>
    <%= f.text_field :body_length %>
</p>
<% f.fields_for :taxon do |builder| %>
    <%= builder.label :common_name %>
    <%= builder.text_field :common_name %>
    <%= builder.label :genus_name %>
    <%= builder.text_field :genus_name %>
    <%= builder.label :species_name %>
    <%= builder.text_field :species_name %>
<% end %>
<%= f.submit %>
<% end %>

When I run the new method, The fields for taxon don't show up. There's no hint of them in t开发者_运维知识库he html source. I've heard that this can happen if the nested model is nil (i.e. if I had forgotten to build it in the controller method), but it's there. I added some conditional code in the view just to make sure.

So, who will make me smack my forehead here? What am I missing?

Thanks!

Are you using Rails 3? If so it should be:

<%= form_for @bird do |f| %>

and

<%= f.fields_for :taxon do |builder| %>

Note the equals.