That is an interesting phrase "sort of vague reverence". Were I to write that passage, I would have not added that phrase. A careful writer picks his words carefully & a careless editor, thinking they were superfluous, would be tempted to delete them. Yet he did, which suggests his words here are intentional, & worth looking more closely at.
"Reverence" is related to the verb "revere", an act one performs to things considered sacred. We can understand Smith considering a wild creature sacred, so that part is understandable. However, when one reveres someone or something, one is attentive to it, one gives it close attention. Which seems to contradict the adverb "vague", which implies that Smith's action was done in an off-handed or inattentive fashion.
In other words, Orwell could be understood to say here that Smith watched the thrush carefully, yet did not really see it.
If you haven't read the Q&A Spagirl linked to yet, this is the point where you may want to now. Some interesting ideas are presented there.