The average is pitifully low, the majority don't make it through the first months of their first year, but should they get lucky and avoid the cats, sausage-fingered ringers, hawks and cars, the odds get somewhat better. I'd say a reasonable lifespan for a Robin-sized bird is about three to four years, but some become right little veterans. As well as the Blackbird example given above, I had one (ringed) Coal Tit return to winter in my garden til he was the ripe old age of at least seven ...not bad given the number of days down to minus 30 he had to endure!