婉蕠呀，做為讀了英文五十幾年的我，看你這麽認真我高興.

a gull on sands (in fact, i would prefer 'sand') sounds like a gull that is standing/sitting/sleeping on sand, not flying. sand本身就是在earth上。如果一個沙鷗在sand上， 它就很難在heaven/sky 和earth中間 (除非是數學家們把端點也放入）。
沙鷗無非就是gull or seagull (or possibly sand gull or beach gull). therefore i would propose something like this:
a lonely gull hovering between heaven and earth