I would be grateful if anyone can offer help.

I have just put down the evening meal and my cat (6 months old) didn't
eat a single mouthful (quality canned food that he normally devours).
He has gone to sleep beside the fire, looking very drowsy.  He has slept
since 5pm when he came in after three hours of playing outside.  I have
just noticed that his third eye-lid is closing very sluggishly, and
moving reluctantly from being about one-eighth closed.  

He's plainly ill.

I can't imagine why.  His twin brother is in fine spirits.

The only thing I can think of is that three days ago I found him playing
with a dead greenfinch (he had it tightly in his jaws between tossing it
around).

Now, my neighbours have been throwing birdseed on one piece of ground
outside their living-room window for the past year and they recently
told me they were puzzled that they were finding lots of dead
greenfinches round their property.  I let them know that some healthy
wildbirds carry salmonella and that from December to March particularly
it can be transmitted to faeces and rotting food in feeding areas which
are not being regularly cleaned and disinfected with bird-friendly
disinfectant.  (Of course, the ground on which they have been throwing
birdseed for the last year must be crawling with salmonella and it has
now to be either disinfected or the top three inches of soil needs to be
removed.)   Anyway, the point is that my cats are finding all these
greenfinches that have recently died from salmonella.  (The twin found
one today: I wonder if he will come down ill in three or four days'
time.)

Can oral contract with salmonella make a cat ill?

If so, what needs to be done?  (Tomorrow's Sunday: my vet is closed all
day.)

Thanks.

Nell.