Follow these steps to copy and edit the alpha channel:
Drag the Alpha 1 channel to the New Channel icon at the bottom of the Channels palette to duplicate it. The new channel is named Alpha 1 Copy.
Choose Image ’ Adjustments ’ Invert to reverse the colors of the channel, making the black areas of the channel white and the white areas black (see Figure H4.6).
Figure H4.6: The inverted alpha channel
Double-click the channel thumbnail. Name it Gradient Mask .
The Gradient Mask channel should be targeted . Click and drag the Gradient Mask channel to the Load Selection icon at the bottom of the Channels palette to select the white area of the alpha channel. You can also select the white area by clicking it with the Magic Wand tool.
Press the D and then the X key to make the foreground color black and the background color white (or click the Switch Colors double-arrow icon on the Tool palette swatches). Choose the Gradient tool and click the Radial option in the Options bar. Click and drag from the center out toward the edge of the document to modify the areas surrounding the chickens with a gradient, as in Figure H4.7. Your alpha channel should then look like Figure H4.8. Deselect.