Why are they catcalling for their lovely new member?

Question

"Brocklehurst, Mandy" went to Ravenclaw too, but "Brown, Lavender" became the first new Gryffindor, and the table on the far left exploded with cheers; Harry could see Ron's twin brothers catcalling.
(Harry Potter and the Sorcerer's Stone)

American and British version all use ‘catcalling’ for the cheering sound which exploded when they met their first new student. But dictionaries have negative meaning for catcalling, and so I’m confused why they are doing so for their lovely new member. Is the word used when they welcome someone?

Oxford: a noise or shout expressing anger at or disapproval of somebody who is speaking or performing in public
Longman: a loud whistle or shout expressing disapproval of a speech or performance

Answer 1

From OED's "draft additions, 2006" ...

catcall orig. U.S. A whistle, cry, or suggestive comment intended to express sexual attraction or admiration (but usually regarded as an annoyance), typically made by a man to a female passer-by. Cf. wolf-whistle n.

In OP's context, the catcalling for Brown is obviously favourable, because we're also told the boys exploded with cheers. It probably means they were boisterously and noisily enthusiastic about the whole process of "team selection", rather than that they were specifically keen on having Brown in Gryffindor.

Answer 2

I have never read the book and only watched two parts of the movie by force against my own will but I do not think that cat call has a positive usage:

What is a cat call ?

The history of theater is not always pretty, as evidenced by the arrival in the 16th century of the derisive whistle known as a cat call. Audience members who wanted to express their displeasure at a specific actor would often launch into a loud and jeering whistle said to resemble the plaintive wail of a cat. This noise could be heard onstage, much to the chagrin of the targeted performer or playwright. Instead of a receptive curtain call, a number of actors received a cat call at the end of an unpopular performance.

The cat call remained part of an unreceptive audience's arsenal for centuries. Modern audiences have largely abandoned the practice, but an occasional jeer or Bronx cheer may still be heard whenever a performer fails to win over the crowd or deliberately insults his or her audience. Hecklers at a comedy show, for example, may still issue a piercing call whenever a comedian's material fails.

A cat call is often paired in people's minds with the wolf whistle, a two-toned whistling noise usually directed at attractive members of the opposite sex. A cat call may be a series of loud cries used as an attention getter, while the salacious wolf whistle essentially seals the deal. The noise isn't always meant to be derisive, but it is meant to be noticed. Whoops, hollers, Bronx cheers and other rude noises could all fall under this term.

Answer 3

I disagree with @FumbleFingers' answer: specifically the part of it that says It probably means they were boisterously and noisily enthusiastic about the whole process of "team selection", rather than that they were specifically keen on having Brown in Gryffindor.

The definitions you gave in your question are not wrong, but there is a specific usage of catcall to indicate (usually male) approval of an attractive member of the (usually) opposite sex. I put the 'usuallys' in because it could in theory be used for women catcalling at men, gay or bi men catcalling to other men, etc. - however it is almost exclusively used for straight men calling at women.

From oxforddictionaries.com:

1 a shrill whistle or shout of disapproval made at a public meeting or performance: he walked out to jeers and catcalls
1.1 a loud whistle or a comment of a sexual nature made by a man to a passing woman: women were the objects of catcalls when they walked by the men’s barracks

I think catcalling is used here to indicate the boys' approval of the Lavender joining the team. In other words, the whole table "explodes with cheers" and the Weasley brothers specifically make some kind of call that indicates either that they are happy for girls (any girls) to be joining Griffindor, or that they particularly approved of this girl (because she was good-looking).

As @Persian Cat pointed out, catcall is very similar to wolf whistle: it's used in the same way, the only difference is that it's some kind of shout rather than an actual whistle.

Depending on how the call is made, it can translate to anything from "I'd like to give her one!" (explicitly sexual) to "You're pretty!" (fairly harmless approval). It can therefore be seen as derogatory or demeaning in some contexts but I would interpret it here as more like "hooray, the pretty girl is in Griffindor!" which Lavender probably took positively.