After four consecutive victories, the leaders were undone as their batsmen failed to deliver for the first time this term.
They looked to be very much on course for another win having reduced their visitors to 61/6 before Dan Marston rescued Helperby’s innings with a rapid knock of 64 from just 54 balls.
In the end, the away side were all out for 137, Jon Millward capturing 4-45.
Opener Tom Croston managed 26 when Birstwith replied, though none of his team-mates were able to surpass that score as the table-toppers were sent packing 10 runs short.
Tom Messenger claimed 3-21 and Al Darnell also impressed, finishing up with 3-34.
Second-placed Killinghall bounced back from last week’s maiden loss of the campaign, routing Kirk Deighton on home soil.
A 117-run first-wicket stand between Ed Paxton (74) and Stephen Lennox (44) paved the way for the hosts, then Dan Atkinson added an unbeaten 43 as they put 244/7 on the board.
Michael Malthouse was in superb form for Deighton, helping himself to a six-wicket haul.
The away batsmen would however struggle when they took their turn at the crease, Atkinson (4-11) and Hamish McIntyre (3-27) leading the way as Louis Toseland’s men were blown away for 88.
Nathan Smith struck a half-century to pave the way for Arthington’s 52-run triumph over Harrogate 3rds.
His knock of 73 was the highlight of the home side’s 196 all out, Dominic Bradburne bowling superbly to claim 6-41.
Rob Stanworth 41 and Mark Roberts contributed at the top of the order to keep the Roosters in contention before their chase lost momentum and they were eventually dismissed on 144.
Naveed Andrabi shone with the ball for Arthington, finishing the contest with 5-39.
Harry Lamb’s excellent innings of 72 set West Tanfield on their way to a 27-run success over Pateley Bridge.
The home opener was the mainstay of his team’s 179/9, Jos Overend scooping 3-22.
Tom Fryer struck 50 not out for the Badgers, though he was left stranded as 3-21 from Jake Stubbs helped ensure that their response did not progress beyond 152/9.
Defending champions Darley recorded back-to-back league wins for the first time this summer when they inflicted more woe on struggling Goldsborough.
Having posted 167/6 from their 45 overs, the Crayke Lane outfit dismissed their hosts for just 108.
Masham required only 35 overs to complete a comfortable seven-wicket victory over bottom-of-the-table Blubberhouses, who made their way to 183/7 having been put into bat.