Listener 3962 Disappearance by Phi

Gregson enjoyed Phi’s Right & Left puzzle and learned a lot about the endangered birds of New Zealand. The unclued entries at 18 were both native birds. The kakapo and the kokako are nearing extinction. The first letters of extra words in the left side of the grid spelled HUIA TODAY, and in the right side GONE TOMORROW. With the unclued TODAY in the left side and TOMORROW in the right the solver had to deduce what should be highlighted and what erased. The word HUIA could be seen diagonally on both sides of the grid and had to be highlighted on the TODAY side and erased on the TOMORROW side.
The “melancholy anniversary” was of December 28 1907, the last recorded sighting of the huia in New Zealand.

Redundant words in clues are shown in square brackets

ACROSS
1 Its nest's abandoned? Stick around - one should get evidence from this (12, two words)
WITNESS STAND anag. IT’S NEST in WAND A clue that hints at the theme.
6 [Gasps] are encountered in terrace - runner having breath to turn (6)
TEARER ARE in TER.
INWIND  2 definitions IN WIND
7 Deficiency in US people [habitually] injecting heroin - Ecstasy repeatedly requires ordered injection of new fragment for the former release (6)
SHORTS H in SORTS (people)
ENFREE N (new) + FR (fragment) in Es (ecstasy)
8 Deceived when circling tree - drew gunfire - airmen in shelter cut down (6)
HAULED ULE (tree) in HAD
RAFALE RAF + ALE(E)
10 Sweet stuff in elaborate trance music's regularly gripping one with native zest (6)
NECTAR anag. TRANCE
GIUSTO I in GUSTO
12 Girl in Scotland [exhibiting] prim restricting manner recalled good chat, apparently over drink (6)
MIRIAM AIR (manner) (rev.) in MIM
GRAPPA G + RAP (chat) + AP (rev.)
16 Feature such as Notre-Dame's became apparent, involving account set out in article for Mitterrand by [Ted] Heath (6)
ROSACE (rose window) AC in ROSE (became apparent)
UNMOOR UN (indefinite article in French) + MOOR (Heath)
17 Memory about the same [University] recalled as newsman penned article in home of the senate all about [our] American charm (6)
EDITOR Id. (the same) in ROTE (rev.)
ENAMOR AN in ROME (all reversed) US spelling of ENAMOUR
18 See preamble (6) KAKAPO KOKAKO
19 In accordance with choice, recalled regulation pregnancies - first hint of [infant] children meeting with approvals (6)
BYELAW BY + WALE (rev.)
CYESES C(hildren) + YESES
DOWN
1 Desires rebukes about [allowing] swarm to set up in stalk - a fungal stalk? (8)
WISHINGS SHIN (swarm) in WIGS
STERIGMA RIG in STEM + A
2 A rise in [men] garbling verses - fashionable woman penning a [tricky] line - it'll make you take a breath (7)
SENARIA anag. A RISE IN
INHALER IN (fashionable) + A L (line) in HER
3 Without getting into sheep, New Zealand's gambling American sweet-talking's elevated [ovine] flesh of Canterbury, ultimately (5)
TWO-UP WO (without) in TUP
TAFFY FAT (rev.) + F + Y (last letters oF canterburY)
4 Anglo-Irish exclamations are mostly [rapturous] - cheers Northern Ireland's opposing player [originally] pinched from Scotland (6)
ARRAHS AR(E) + RAHS (cheers)
NIRLIT N + IRL + IT (the opposing player in children’s games)
5 [Ragamuffins] left in poverty no longer point to board partially laden, [demanding] teriyaki (5)
NEELD L in NEED
ENTER Hidden
9 Hedge-clippings mostly burning, perhaps, going up in pyres for heretics (8)
BRASEROS BRAS(H) + SORE (rev.)
TOMORROW Unclued
11 Elision [angered] company enthralled by a poet, one with attitude, surrounded by cheers [outraging] subarea (7)
APOCOPE Co in A POPE (ref. Alexander Pope)
TALOOKA A + LOOK in TA
13 [Yarrow] equally choking special clover? Again prepare to do some gardening, gathering me up (6)
ALSIKE S (special) in ALIKE
REMAKE ME in RAKE
14 Get upset to an appreciable extent, having to stomach refusal (5)
 ANNOY NO in ANY
TODAY unclued
15 [Welsh] travellers left headscarf when chasing scoundrel in trees (5)
ROMAL ROMA + L
RATAS RAT + AS