Myna offence: Why do these birds sound so annoying?

If you've ever heard a raucous racket cut through the otherwise dulcet birdsong of a New Zealand backyard, there's a good chance the offender was a myna.

This noisy character, typically found hanging around North Island roadsides, is considered a pest because it feeds on fruit and causes damage to crops.

Annual surveys have shown how their populations are on the rise – to the point they today outnumber even our friendly fantail in urban gardens.

Now a just-published study shows New Zealand mynas aren't even playing us the greatest hits their Asian native ranges enjoy, but a dull set-list of harsh screeches and shrieks.

The findings come from Dr Sam Hill, a former Massey University ecologist whose previous focus has been on tui, which, conversely, boast a colourful repertoire of more than 300 tunes.

His latest project stemmed from something that struck him while visiting a Nepalese village four years ago.

"I recorded a myna that sang a hugely complex song, which got me wondering why on Earth the mynas we have here in New Zealand have such ludicrously simple and noisy ones."

He now put this down to a phenomenon called the founder effect.

"Our understanding from previous research in other species was that birds introduced to new areas from their native ranges generally have these founder effects - which lead to genetic bottlenecking, isolation and sometimes inbreeding, and in terms of vocal behaviour, more simple songs."

For his study, just published in Ibis - International Journal of Avian Science, Hill and his colleagues sourced songs from multiple mynas across their native range, which stretched from Kazakhstan and Uzbekistan across to India, Nepal and China.

They also gathered field recordings from mynas in countries they'd been introduced to – New Zealand, Australia, Oman, South Africa, the United Arab Emirates and the United States.

Next, they assessed 75 individual birds across all ranges to compare the complexity of their songs, using a set of measures including length and the number of unique syllables.

"Our results suggested, as predicted, song complexity was higher in the native areas in a 'statistically significant' sense," Hill said.

"This could be a reflection of their reduced genetic diversity - but this needs more investigation."

Nonetheless, it was in line with previous studies of other species such as the Chinese hwamei, whose songs in Taiwan were much simpler than those heard on the mainland.

"Even here in New Zealand, European-introduced chaffinches on the Chatham Islands showed a founder effect."

Hill said the study offered a new platform for examining variations between founder and native populations in the factors that govern song complexity production, such as the volume of HVC, a critical part of the songbird brain that controls song learning and production.

"There is strong evidence that HVC volume correlates positively with song and syllable repertoire size."

However, he added, the HVC volume might also reflect the capacity of an individual for learning a greater number of songs.

"If HVC volume in founder populations is found to be significantly smaller than in native populations, reflecting the lower vocal diversity in these populations, this would help to advance the understanding of the development of song nuclei and its effects on song evolution."