There's one Gezarah by Bossor Becholov which is that if someone ate bird's meat and milk together it's an Issur Derabonon A prohibited thing from the Rabbis, a Gezairah. There's another Gezairah which is not to eat milk and within half an hour you eat meat, or you're not allowed to eat meat and then within 1,3 or 6 hours depending on your custom. So why don't we say that if you eat bird's meat you should be allowed milk straight after because it's a Gezairah LiGezaira?
Aaron Pacanowski, Melbourne, Australia
Your question is hinted at in the Gemara because Rav Yosef (Chulin 104a) adduced a proof for his opinion that it is forbidden mi'de'Oraisa to eat milk after bird's meat from the fact that one is not allowed to put bird's meat on the same table as milk, because otherwise it would be a Gezera (that by putting the meat on the table, one might come to eat it with the milk) to a Gezera (if bird's meat and milk would be only mi'de'Rabonon). Therefore, since the Halachah does not follow Rav Yosef - since we Pasken that bird's meat and milk is only mi'de'Rabonon - this means that in this case we do make a Gezera to a Gezera. Tosfos 104a DH u'Mina writes that in some places one does make a Gezera to a Gezera, so this could be one of these places.
See TAZ (YD 88:5 at end) that the reason this is not considered a Gezera to a Gezera is because Chazal strengthened their words and made them equivalent to a de'Oraisa (i.e. they gave bird's meat and milk a din equivalent to de'Oraisa) and therefore this is considered only one Gezera. In TAZ YD 88:1 he writes something similar - that sometimes one does make a Gezera to a Gezera. PRI MEGADIM MISHBETZOS ZAHAV, to the latter TAZ, explains that the reason for this is that, because milk and meat are both permitted on their own, people are not so careful about them and are liable as a result, to come to eat them together. Because people consider it as a lighter prohibition, Chazal decided, on the contrary, to make the prohibition more severe than usual and therefore, in this case, they did make a Gezera to a Gezera.
(See also TAZ YD 90:12 at end and see TESHUVAS CHASAM SOFER YD #73 DH ve'Hineh)
Kol Tuv,
D. Bloom