As staff staged a 24-hour strike over pay with a protest at the broadcaster's annual shareholder meeting, ITV reported net advertising revenue ahead of guidance during a "strong" first quarter but warned of a slower second quarter.Although ITV's share of viewing continued to be down, total external revenue in the three months to end March was up 14% as its ITV Studios production business returned to organic growth.Growth of 12% in net advertising revenues (NAR) was ahead of the 11% guidance issued by the company along with its final result in March, which with 31% growth from online, pay and interactive, helped lift broadcast and online revenue up 10% to £530m.NAR in the second quarter has slowed, however, with ITV Family NAR forecast to be up around 5% for the whole of the first half. A weaker May is estimated to be down 5%, with June down expected to be down 5-7% against a period last year that was boosted by the football World Cup.Non-NAR revenue were strong, however, up 13% to £319m, representing an acceleration from the 10% rise last year.The ITV Family of channels suffered a 3% fall in share of viewing in the first four months of the year, but this was an improvement on the 5% fall in 2014 and management said improving SOV remained a key focus for the year."We've had a strong start to the year with further growth across all parts of the business as we continue to deliver against our strategy," said chief executive Adam Crozier, pledging that the full year was expected to see "further strong growth".As he prepared to face protests from staff during a planned 24-hour strike in rejection of a 2% pay rise, the Scot hailed ITV Studios' return to organic growth, with an 8% increase that contributed around half of Studios 17% revenue growth, which will be boosted by the completion of The Voice and Utopia maker Talpa Media in March.ITV Studios remains on track to increase revenue by around £100m on a constant currency basis with the benefit of Talpa to come.Crozier said he was encouraged by a more robust recent performance, including Britain's Got Talent, Ninja Warrior, Vera and well received new drama including Safe House, Home Fires and Code of a Killer."Our digital channels are growing audience share, up 3% overall year on year, and we are firmly focused on the main channel where we expect to see improvement in the second half of the year when we will have the benefit of the exclusive rights to the Rugby World Cup."However shares were down 1.6% by 09:00 as investors focused on the slowing in NAR this year, slow viewing figures and the strike."2Q is down versus tough comps - some had thought it would be better, so this could be a slight negative," said Investec.Credit Suisse said the stock "is not expensive on our estimates" on a price/earnings ratio of 16.3 times.The Swiss bank expressed appreciation for the above-consensus advertising momentum "with adspend trends likely to improve with post-election confidence".Last year the UK advertising market expanded at its fastest pace since 2010, with total advertising spending up 5.8% to £18.6bn in 2014, according to the latest data from the Advertising Association. The AA forecast that total ad spending would grow 5.8% in 2015 and 5.4% in 2016 to more than £20bn.Credit Suisse also highlighted prospects for a cash return from ITV, the "strategic value" of the Studios content business, and the "likelihood of takeover speculation re-emerging" with US giant Liberty Global a 6.4% shareholder.US media tycoon John Malone's Liberty bought its stake in July last year and then made moves behind the scene to form "alliances" with ITV's major institutional shareholders. Mike van Dulken at Accendo noted that in morning trading ITV shares were still holding above their trendline of rising support from to March 2009 financial crisis lows and remain within touching distance of 274p all-time highs."The uptrend requires a test of the 100-day moving average at 240p to jeopardize it. Are investors more loyal than viewers?"