Backyard bird-feeding isn't so helpful for one of our popular tiny natives, the grey warbler, new research warns.

A study by University of Auckland researchers, published today, looked at the effects of common bird feeding practices on particular species of birds and whether supplementary feeding of bread and seeds favoured some species over others.

Perhaps unsurprisingly, the two introduced species that particularly benefitted the most were the common sparrow and the spotted dove, whose abundance at feeding sites were respectively 2.4 and 3.6 times higher.

But the native grey warbler, voted New Zealanders' Bird of the Year in 2007, significantly decreased in abundance at feeding sites, with numbers dropping by more than half.

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The findings on the diminutive grey warbler, one of the most commonly-heard songs in New Zealand's forest, was concerning, said study co-author and PhD candidate Josie Galbraith.

"They typically forage on insects in the tree canopy but their ability to forage efficiently may be being affected by the disruption of higher densities of other birds at feeding sites," she said.

"There is some evidence their numbers are declining anyway, so this study does add to that concern."

The researchers, whose work has been published in major journal the Proceedings of the National Academy of Sciences, monitored 23 North Shore gardens over 18 months, with feeding at 11 sites and no feeding at 12 sites.

In all, 33 species, a total of 18,228 birds, were recorded, and the most commonly-observed species were sparrows, spotted doves, blackbirds, silvereye and myna.

They found the abundance of spotted doves in particular increased rapidly within two months of the start of feeding, suggesting the birds were moving to feeding sites from surrounding areas.

"This work certainly suggests bird-feeding favours introduced birds such as spotted doves over native birds, which mostly eat insects, nectar and fruit," said co-author Dr Margaret Stanley.

The findings come after the same research team found that more than 5 million loaves of bread per year are fed to birds, with an estimated $12.3 million spent annually on bird food.

But the study, the first of its kind, also found food put out for birds favours introduced species, such as blackbirds and starlings, over endemic species, with just 17 per cent of householders providing food - for example sugar water - for natives such as tui.

It also found that bird-feeding hygiene habits are relatively poor, with just 8.6 per cent of people cleaning bird-feeding tables and containers appropriately.