I have pidgeons roosting in an enclosed space behind some decoration
in the old building I live in in Spain. I am looking for a substance
to pour in there which will not kill them (and leave them to smell)
and will render the place uninhabitable for them.

http://pest-bird-animal-control.com/BX-birdproof.html

seems more designed for an open, outdoor space and cannot be applied
in this case becase I cannot get access to the surface floor area
behind  the ornamented grating .

I am concerned that rat poison might leave them to die and smell .

If I could pour something in there that would convince them to leave,
then I could block up the grating and prevent further visits by birds.

declan: For what it's worth, all birds HATE sticky substances. If the
area in which you have a problem is readily accessible, you could have
a professional apply a pigeon repellent. If it were me, I would spray
3M Spray Mount Artist's Adhesive (available at artist supply stores in
aerosol cans) liberally on the area involved. It would provide a
sticky surface which would repel the pigeons.
monroe22

A scarecrow of some sort?

http://answers.google.com/answers/threadview?id=343071

"Safeco Strategy is for the Birds" (Aug. 13, 2001)
http://seattlepi.nwsource.com/baseball/34957_aguy13.shtml

Best regards,

Omnivorous-GA

There is  no D in pigeon in the British or Austalian version of
English at least. Perhaps they spell it with a d in the USA but I
doubt it. Its like many people spell burglar as burgular as in burg-
u-lar, also incorrect. And while I am there all you people who write
out oCCaSSion please note that there is only a total of three Cs and
Ss in that word not four!!

This may sound a little ridiculous, but placing a boiled chicken egg
by the pigeon's nest usually does the trick.  (It fools to bird into
thinking that there is a large bird of pray such as an eagle moving
into their territory and scares them away)

Nuisance birds can be tough to eliminate and nuisance pigeons are
historically harder to control than other birds.  The reasons for this
are numerous but main reasons are their sheer numbers and their strong
nesting instincts.  These nesting instincts make it imperative that
all nests, nesting materials, etc. are removed at the start of your
pigeon control program.  This also means that a combination of devices
is usually needed to keep pigeons out of unwanted areas for good.
Examples of good combinations are sonic repellers that us birds own
distress calls to create a danger zone. Another option is Irri-Tape
its a iridescent foil that combines holography, wind and light to
irritate pest birds' senses and create an "off limits" zone against
them. There are fake prowler owls you can put out. and you can
purchase3Bird spikes that can be Installed.Good Luck
www.syracusePest.com