Why do you think the islanders find the voice of the cuckoo bird thrilling?

The lines you're referring to in William Wordsworth's "The Solitary Reaper" can be found below:

A voice so thrilling ne'er was heard 

In spring-time from the Cuckoo-bird, 

Breaking the silence of the seas 

Among the farthest Hebrides. (13-16)

By referring to islanders, I assume you mean inhabitants of the Hebrides, which is a collection of islands off the coast of Scotland. We can't be sure exactly why the song of the cuckoo might be thrilling to these people but, since the cuckoo (like the nightingale) was often used in Romantic poetry because of its beautiful song, we can guess that the song of the bird is thrilling simply because it has a beautiful sound. Moreover, the song can be seen as an example of the pure beauty of nature in general, which would also explain its thrilling aspects.

However, it's important to note that Wordsworth is actually saying that the reaper's song is more thrilling than the cuckoo bird's. As such, though the bird's song might be enjoyable to listen to, the reaper's song is far more beautiful. By making this claim, Wordsworth highlights the transcendent beauty of the reaper's song, as he claims it trumps one of nature's most beautiful qualities: the cuckoo bird's song.