Supreet,
Pick up a 1.4x TC. It would give you better reach. Is there any possibility of going down lower, as I guess you were clicking from the tank bund? Keep on trying. I love this species and would like to see some more images of these beautiful birds.

As far as spot metering is concerned, when I was shooting with the Canon EOS 1V (it was the film era), the spot metering used to measure from 2.4% of the area from centre. In the digital era, the spot metering reading from the 7D is about 3.2% from centre. I am not sure why the reading for the spot metering area can't be narrowed down further.

Also, one should not think that just selecting a metering mode like spot metering will give instantaneously good results. You need to know the metering theory. One can check the tutorial on metering here: Indiawilds: Land of the Tiger. Conservation, Wildlife Photography, Communities

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Cheers,
Sabyasachi