If you only have 1 camera and its the 5DMK2, and for a 2nd camera, 7D is by far, and absolutely one of the best body out there, not just within Canon, but among all other bodies.
7D is almost 90% a 1D, except that its a crop body with APSC sensor.
But I am literally drooling as I read the spec here
http://www.dpreview.com/news/0909/09...canoneos7d.asp
These 2 functions alone is worth the money
19-point cross-type AF system including Spot AF
The AF system has been completely redesigned using the same architecture as the EOS-1 series and includes a separate processor to handle AF calculations. This, along with AI SERVO II AF, enables the EOS 7D to offer accurate, reliable and continuous shooting at 8fps. Uniquely at this level, all 19 points in the EOS 7D AF sensor are cross-type points with f/5.6 or faster lenses, which allows sophisticated tracking, accuracy and performance throughout the frame.
EOS 7D also includes Spot AF, a new mode that uses a smaller area of the sensor to determine focus. This is useful for small subjects where there is background detail that can distract the AF sensor. It also includes Zone AF which limits automatic selection to one of five zones. This allows photographers to ensure their subject is automatically selected.
iFCL metering system with 63-zone Dual-layer Sensor
The iFCL system uses Focus, Colour and Luminance information to determine consistently exposed shots. All focus points provide distance information to the metering system to determine proximity to the subject and allow the algorithm to weight the exposure accordingly. EOS 7D has a completely new metering sensor with 63 zones compatible with 19 AF points. Typically, metering sensors are more sensitive to red subjects which can lead to overexposure. EOS 7D combats this with the dual layer sensor, which has one layer sensitive to red and green light and one that is sensitive to blue and green light. The metering algorithm then compares the level of the two layers and adjusts the meter reading accordingly.
Did I mention that if you have a zoom lens, say 24-70, or 17-55 f2.8, it is said that at ALL 19 point focus point, you will have the same photo quality at all points, whereas the current 50D and even the 5D cannot do.
So... 7D.. a bit pricy at $1800 for a crop body, but hell, if I didnt have a 50D already, I'd grab 7D.
For your bird photography, I'd assume auto focus speed, accuracy, and ability to focus moving objects are KEY.. heck, I thnk you have no choice, but to go for 7D, unless budget is an issue.