The first, a Canadian study published in Feb. 2010, found that bats which had been drinking didn’t appear to have significant problems flying and echolocating.
“We fed wild caught Artibeus jamaicensis, A. lituratus, A. phaeotis, Carollia sowelli, Glossophaga soricina, and Sturnira lilium (Chiroptera, Phyllostomidae) sugar water (44 g of table sugar in 500 ml of water) or sugar water with ethanol before challenging them to fly through an obstacle course while we simultaneously recorded their echolocation calls. We used bat saliva, a non-invasive proxy, to measure blood ethanol concentrations ranging from 0 to >0.3% immediately before flight trials. Flight performance and echolocation behaviour were not significantly affected by consumption of ethanol, but species differed in their blood alcohol concentrations after consuming it.”
See: Drinking and Flying: Does Alcohol Consumption Affect the Flight and Echolocation Performance of Phyllostomid Bats? PLOS ONE, February 1, 2010
But a second study, published later that same year, found that inebriated bats’ echolocation skills were considerably affected by alcohol.
“[…] we hypothesized that, if ingested, food containing >1% ethanol is toxic to these [fruit] bats, probably causing inebriation that will affect flight and echolocation skills. We tested this hypothesis by flying Egyptian fruit bats in an indoor corridor and found that after ingesting ethanol-rich food bats flew significantly slower than when fed ethanol-free food. Also, the ingestion of ethanol significantly affected several variables of the bats’ echolocation calls and behavior.”
See: Ethanol ingestion affects flight performance and echolocation in Egyptian fruit bats , Behavioural Processes, Volume 84, Issue 2, June 2010, Pages 555-558
Note: As far as can be ascertained, no subsequent studies have resolved answers to the question.
BONUS : What is it like to be a bat? by philosopher Thomas Nagel, The Philosophical Review, LXXXIII, 4 (October 1974): 435-50.
BONUS: Learn to count bats