Cooperatively breeding noisy miners (Manor ina melanocephala) are well know
n in Australia for their persistent and very vocal group mobbing of heteros
pecifics. Here I investigated the nature of this extraordinary behaviour, i
n particular its role in nest defence, in a colour banded population of noi
sy miners in south-east Queensland, Australia. I focused on two questions.
First, did the intensity of mobbing vary according to factors such as the t
hreat to the nest, or the 'value' of a clutch? Secondly, what role did grou
p mobbing play in the success of a nest? To answer these questions, I exper
imentally manipulated the nest defence behaviour by placing one of three st
uffed models near active noisy miner nests. The response of noisy miners to
intruders was not indiscriminate. However, I found that the number of bird
s that mobbed a model did not simply reflect the potential threat posed. Th
e response of noisy miners to raptors and other potential nest predators ma
y have reflected their rarity as well as the threat posed. The number of mo
bbers did not vary with the age or size of a brood. In this study, the fate
of nests was independent of the number of mobbers or visitors at nests. Fi
nally, up to 80% of mobbers were never seen to make any other type of contr
ibution to a nest, and many could not be related to the brood that they wer
e 'defending'. Hence, for some noisy miner 'helpers' the benefits that they
accrued were probably not wholly dependent on the survival of the broods.
I suggest that, in this gregarious species, mobbing behaviour at the nest m
ay be a display of social status or individual quality. This hypothesis war
rants further investigation.