The Deans have been immensely active on the incoming front this summer, bringing in a host of new faces to Five Heads Park.
Meanwhile, goalkeeper Ryan Rochester,17, has joined from Gosport Borough while Petersfield Town pair Charlie Merritt and Marvin Orepo, along with dual-signed Hawks academy prospect Max Connolly, former US Portsmouth skipper Tom Jeffes and ex-Fareham youngster Louis Udeh, have all followed suit.
With virtually the whole of last season's squad who helped Horndean achieve a fourth-placed Wessex Premier finish re-signing, it means Birmingham is presently working with a near 30-strong squad.
The Deans boss admitted he's faced questions from some of his own players on the club’s summer activity, but he's says it's up to them to show him why they should be in his plans for next season.
‘All I know is we've got a six-week period from the start of pre-season, which we started on Tuesday last week, they've got six weeks to impress me and that's it,’ said Birmingham.
'They've got six weeks to stamp their authority on the shirt. One thing I will say is, the year we finished second to Sholing (2018-19 Wessex Premier season) I made a big, big error – I didn't make enough new signings.
'I was very loyal to the squad who got me to second place and it backfired. The players knew they didn't have players coming in who wanted their shirt.
‘My phone is already ringing from some of my players going, ‘hold on a minute, you're signing some of these players in my position?’ but they're in the same position as them, it's wiping the slate clean and there's six weeks to impress.’
By the time Horndean's Wessex Premier season-opener at home to Moneyfields comes around on Tuesday, August 2, Birmingham insisted he'll be operating with a first-team squad of around 20-22 players.
But he explained why he has brought in so many summer additions ahead of their pre-season friendly opener at Wessex League new boys Fleetlands on July 2.
Birmingham added: ‘At the end of the day, if I didn't get these numbers in ... I've got a squad of about 28 (players) registered at the moment - I would like it down to about 20 or 22 (when the season starts).
‘They've got six weeks to prove to me, we've got some tough friendlies, and players are coming back from big, big, long injuries.
‘Why am I going to sign a squad of 16 when I've got five (players) out already? In pre-season people have got holidays as well, rightly so, people should go on holiday with their kids and their families.
‘So they go on holiday and I can't sign anyone else? That's part of football. I know which players are going on holidays, what dates they're going on holidays, so I have to make sure I've got 22 players available for my first three friendlies.’