There are two kinds of twitcher. The first kind is the birdwatcher who has tipped over the edge into obsession, and will go anywhere, offend anybody simply to have a new species on his personal list; this is the kind discussed this week by Maureen Lipman in a new series of 'social investigations', The Lipman Test (Radio 4, Thursday). The other kind is the person who has been irritated into minor spasms by 30 minutes of listening to Lipman's laboured quips.

Still, if nothing else, The Lipman Test illustrates superbly the huge gulf between acting comedy and writing comedy. Lipman doing other people's scripts is fine - Agony was great, I could sit and watch the ads for hours - but Lipman doing her own scripts is appalling. Here's how she describes the BBC's approach to her to do this series: 'Maureen, they said, you're no Joan Bakewell, but you're fairly tart. You're no Kate Adie, but you're a humorous lady . . . well, they like their little joke of a weekday.' What makes this so peculiarly awful is not just that the jokes are so unfunny they're fascinating (how did they get into the script? why did nobody stop them?), but that she tries to pass them off as somebody else's responsibility. This is wicked.

To be fair, Lipman doesn't deserve all the blame: whoever thought of the series should take some of it (after all, the horror of Lipman let loose was fairly clear from radio serialisations of her books and from her appearances on Hoax, the humourless lying game that's been the resting place for Tim Brooke-Taylor's career for the last couple of years). And it wouldn't sound so painfully chucklesome if the jokes weren't heavily underlined by what sounds like the incidental music from a Laurel and Hardy film.

It's a shame, because underneath the arch surface there was some brilliant material about the sadness of the twitching life; one twitcher explaining how he had had to rush out of his wedding, in Luton, to get to Dorset to see a new kind of shrike. His wife - not the same wife; that marriage hadn't lasted - explained how she had become inured to disappointment, because he would abandon any plans, renege on any promise for the slim chance of a new bird.

There was, too, the story of a nocturnal chase after an American night-hawk (very rare, apparently) which turned out to be a cowpat. This seemed somehow emblematic for the whole enterprise.

Against this, the deliberately half-baked comedy of The Masterson Inheritance (Radio 4, Saturday) begins to look more appealing. This is the one where the cast improvises a family saga with audience suggestions to help them along. Having followed a more-or- less normal narrative progression in the first series, now the saga has turned more episodic, looking at the Mastersons through the ages.

This week we had a scamper through ancient Rome, with all the usual cliches (orgies, circuses, bath-houses, etc). It's all a bit chaotic, and the pleasure is less in the jokes themselves than in knowing they're made on the spur of the moment. Still, the hit-rate was well up on the last series, and there were some very good bits - Paul Merton (by some way the funniest thing here), momentarily playing a gladiator, tells a comrade that although they are about to die, they must be happy: 'We're not sadiators.' Well, perhaps you had to be there. This is possibly true of the whole thing, in fact - it sounds frustratingly as though the studio audience is having more fun than you at home. But you're both having more fun than anybody listening to The Lipman Test.