[R] Subsetting dataframe with missing values

[Luciano La Sala]

Dear R-community, 

I am using R (V 2.14.1) on Windows 7. I have a dataset which consists of 19
variables for 91 individuals or rows. Two of my variables are Age
(adult/chick, with no NA values) and Sex (0 for females/1 for females, with
quite a few NA values). The sex of many adult birds is unknown (entered as
NA in dataframe). At some point of my analyses, I happen to need to need to
work with only male adults, so I tried subsetting the dataframe as follows
(see code below) but I get a new dataframe containing all the males but also
a lot of unneeded information such as data in rows 1-7 (NAs), 13, 14, 19 and
21-30. I suspect this is caused by NAs in the variable Sex because
everything goes fine (I get a dataframe containing adults) if I run the same
code but without the "& Data$Sex == 1" part. 

How can I fix this problem? I there a straightforward way of subsetting
efficiently when NAs are present in the original dataset? 
Thank you so much!

Luciano        

adult.males <- Data[Data$Category == "Adult" & Data$Sex == 1,] 
adult.males

         ID Category Sex  Beak   Head 
NA     <NA>     <NA>  NA    NA     NA 
NA.1   <NA>     <NA>  NA    NA     NA 
NA.2   <NA>     <NA>  NA    NA     NA 
NA.3   <NA>     <NA>  NA    NA     NA          
NA.4   <NA>     <NA>  NA    NA     NA          
NA.5   <NA>     <NA>  NA    NA     NA          
NA.6   <NA>     <NA>  NA    NA     NA          
9     LAA10    Adult   1 57.40 121.95       
10    LAA11    Adult   1 56.40 113.00       
11    LAA12    Adult   1 52.00 111.85       
13    LAA14    Adult   1 56.55 124.85       
15    LAA16    Adult   1 57.15 120.10       
NA.7   <NA>     <NA>  NA    NA     NA          
NA.8   <NA>     <NA>  NA    NA     NA          
21    LAA22    Adult   1 56.85 117.35       
22    LAA23    Adult   1 54.80 117.45       
27    LAA28    Adult   1 59.00 116.75       
28    LAA29    Adult   1 55.95 124.25       
NA.9   <NA>     <NA>  NA    NA     NA          
30    LAA31    Adult   1 57.70 112.80       
NA.10  <NA>     <NA>  NA    NA     NA          
NA.11  <NA>     <NA>  NA    NA     NA          
NA.12  <NA>     <NA>  NA    NA     NA          
NA.13  <NA>     <NA>  NA    NA     NA          
NA.14  <NA>     <NA>  NA    NA     NA          
NA.15  <NA>     <NA>  NA    NA     NA          
NA.16  <NA>     <NA>  NA    NA     NA          
NA.17  <NA>     <NA>  NA    NA     NA          
NA.18  <NA>     <NA>  NA    NA     NA          
NA.19  <NA>     <NA>  NA    NA     NA