The precondition the panel printed
The menu shows a progress bar with the figure it is waiting for. In the recorded session that bar read 21/40 Speed before the Rebirth and 33/40 Speed later in the same menu, which is the closest thing to a published requirement this game offers: the panel tells you how far along the bar is rather than printing a rule. The same menu printed the reward side by side with it - Renacimiento 0 at x1 Dinero against Renacimiento 1 at x1.5 Dinero - so the trade the panel is offering is visible even though no per-level table is published for the levels after this one.
What the reset clears, in the panel's own words
The menu carries a red warning that the Rebirth resets your speed, stamina and money, and next to the confirm button sits a second option that skips the Rebirth while keeping the speed. Together those two facts answer the question that most guides leave open: the flight attributes and the Cash balance are the things the reset touches, and the game sells a way to keep the speed if you would rather not give it up. What is not stated anywhere is how much of each value returns or how the multiplier interacts with later upgrades, so this page does not print a reset table.
What to prepare before you confirm
The safe preparation is on the income side, because the panel's own warning says the money goes with the reset. Finish any hatch that is already running, place your best Animals so the plot is producing rather than holding an unplaced egg, and write down the Cash-per-second figure your plot prints before you confirm. After the reset, place Animals again and read the same figure. The recorded session showed the HUD afterwards reading Speed 20 and Wing Stamina 19/19, so the attribute side starts again from a low base - which is exactly why the animals you can place immediately after matter more than the balance you are leaving behind.
What the client announced when it happened
The Rebirth itself was announced by four lines: all eggs have been reset, 1000% stronger, an ETERNO notice that a three-headed snake egg had appeared in the cloudy lands, and a banner that Rebirth 1 had been reached. The first two tell you what the game just took and gave back; the third is an event spawn in a specific area, which is why the zones page tracks the live window; and the fourth is the confirmation that the multiplier has moved. Nothing in those lines states the threshold for the next Rebirth, so no target can be published here.
the Animal income values — Measure the income a reset would clear
the payback calculator — Turn an income change into a payback time
the wing and attribute cards — See which flight purchases a reset also clears
the zone labels — Check where the flight progress currently sits