All bird watchers should read and abide by the Birdwatching Code of Ethics.
1 The interests of the
birds come first
Birds respond to people in many ways,
depending on the species, location and
time of year.
If birds are disturbed they may keep away from
their nests, leaving chicks hungry or enabling
predators to take their eggs or young. During cold
weather, or when migrants have just made a long
flight, repeatedly disturbing birds can mean they
use up vital energy that they need for feeding.
Intentionally or recklessly disturbing some birds
at or near their nest is illegal in Britain. ''
FULL TEXT HERE:
http://www.bto.org/notices/birdwatchers_code/bwc.pdf
Knocking on a tree a few times isn't going to cause any serious disturbance to Woodpeckers outside the breeding season and of course they are not migrant birds. However, they are territorial and males will also use the 'knocking' to attract mates as well as using it to 'warn' other males away from the area as early as February, so while not a serious disturbance, it's a potential disturbance none the less.
PISHING is another way birders will try to get hidden birds to reveal themselves (often forgetting why the birds are hiding themselves in the first place! It's often because they are on nests, exhausted after a long migration, trying to conserve much needed winter energy by huddling down between feeds, or because there is a predator around (you?!) - trying to draw birds out will always potentially have some knock on effect. See here:
The Fine Art of Pishing
How far people will go in trying to attract/draw birds to them often seems to boil down to individual and subjective standards of moral behaviour, however, the Code itself is very clear on the obvious impact of repeated tape luring/pishing/throwing stones into bushes etc
Personally, I'm of the opinion that good field craft ie. stealth and patience is not only far more rewarding but has much less impact. In the case of Woodpeckers, it's usually pretty easy to track them down by listening to calls, knocks and even the gentle tapping of feeding - sharp observation skills and good binocular technique will reveal flights between trees, evidence of bark destruction and peripheral movements up tree trunks. In the UK you only have three species to choose from so it's not rocket science which Woodpecker you've seen briefly/heard etc - it's not necessary imo to knock on trees!
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