To predict Y based on only one sample of X and Y

I want to predict wing length using regression commands. (lm and  
predict)

The data details are as followed
Data:

Bird physiological data

1. body mass
2. body length
3. wing length

Data type:

Order A: consisting of 20 species, body mass and length of all 20  
species are measured

Order B: consisting of 2 species, body mass and length of only 1  
species is
measured

Order C: consisting of 5 species, body mass and length of only 1  
species is
measured

Method:

1.      for order A, using predict and lm

datam.lm <-lm(Wing.Length~Body.mass+Body.Length)

However, for order B and C, please kindly advise any method to get  
wing
length based on body mass and length.

(phylogenic relationship is tentatively neglected)


Elaine

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