I don't believe that
orcs are outright killed by daylight like
trolls are, but I'm getting the impression, from what I'm reading in Lord of the Rings, that they somehow damaged by it, but that
Saruman's
Uruk-hai didn't have that problem, in part because they were part Man.
On the flip side, if orcs were stronger at night, having the Fellowship
travel during the night instead of the day seems to have put them more open to encountering orcs. I think there were orcs out in broad daylight, but that was in the point where
Sauron had messed with the sky or something in Return of the King and there was "no dawn".
Furthermore, the Uruk-hai mocked
Aragorn at
Helm's Deep for thinking that he'd fare better than them in sunlight than in the dark, as they didn't have the limitation that Sauron's orcs did.
Why would you believe Orcs are outright killed by daylight? There are several times in LotR and The
Hobbit that Orcs are out in the daylight and don't die from it.
Sauron's Uruk-hai don't seem to have had much problem with the sunlight either. In the chapter The Uruk-hai, it is specifically the northern orcs that are called out for it:
"‘But what are we going to do at sunrise?’ said some of the Northerners.
‘Go on running,’ said
Uglúk. ‘What do you think? Sit on the grass and wait for the Whiteskins to join the picnic?’
‘But we can’t run in the sunlight.’
‘You’ll run with me behind you,’ said Uglúk. ‘Run! Or you’ll never see your beloved holes again. By
the White Hand! What’s the use of sending out mountain-maggots on a trip, only half trained. Run, curse you! Run while night lasts!’"
and:
"In the afternoon Uglúk’s troop overtook the Northerners. They were flagging in the rays of the bright sun, winter sun shining in a pale cool sky though it was; their heads were down and their tongues lolling out."
LotR, The Uruk-hai
As for the Uruk-hai being part Man, they are not half-orcs.
The word 'uruk-hai' means 'orc-folk', and '
uruks' is just the anglicized form.
Tolkien distinguishes between half-orcs and the uruk-hai/uruks:
"He emptied
Isengard. I saw the enemy go: endless lines of marching Orcs; and troops of them mounted on great
wolves. And there were battalions of
Men, too. Many of them carried torches, and in the flare I could see their faces. Most of them were ordinary men, rather tall and dark-haired, and grim but not particularly evil-looking. But there were some others that were horrible: man-high, but with goblin-faces, sallow, leering, squint-eyed. Do you know, they reminded me at once of that Southerner at
Bree; only he was not so obviously orc-like as most of these were.’
‘I thought of him too,’ said Aragorn. ‘We had many of these half-orcs to deal with at
Helm’s Deep. It seems plain now that that Southerner was a spy of Saruman’s..."
LotR, Flotsam and Jetsam
And:
"This was barely done when disaster came. Saruman’s eastern force came down with unexpected speed; it was much smaller than the western force, but more dangerous. In its van were some Dunlending horsemen and a great pack of the dreadful
Orcish wolfriders, feared by
horses. Behind them came two battalions of the fierce Uruks, heavily armed but trained to move at great speed for many miles. The horsemen and wolfriders fell on the horse-herds and picketed horses and slew or dispersed them."
UT,
Battles of the Fords of Isen
...and later:
"As soon as the enemy had gained possession of the eastern end of the Fords there appeared a company of men or orcmen (evidently dispatched for the purpose), ferocious, mail-clad, and armed with
axes."
op.cit.
Saruman seems to have bred two types of half-orc:
"Finally, there is a cogent point, though horrible to relate. It became clear in time that undoubted Men could under the domination of
Morgoth or his agents in a few generations be reduced almost to the Orc-level of mind and habits; and then they would or could be made to mate with Orcs, producing new breeds, often larger and more cunning. There is no doubt that long afterwards, in
the Third Age, Saruman rediscovered this, or learned of it in lore, and in his lust for mastery committed this, his wickedest deed: the interbreeding of Orcs and Men, producing both Men-orcs large and cunning, and Orc-men treacherous and vile."
Morgoth's Ring, Myths Transformed, Orcs
