Cooper et al .: Foraging of Forest Red-tailed Black cockatoos 
Clout (1989) suggested that Allocasuarina feed trees 
used by Glossy Black Cockatoos had a higher ratio of 
seed mass to fruit mass than non-feed trees, which 
indicates reduced handling costs compared to energy 
gain. For Forest Red-tailed Black Cockatoos there was 
also a significant difference in the ratio of total seed mass 
to fruit wet mass. Therefore the cockatoos receive higher 
energy returns for an equivalent effort by selecting trees 
with higher energy yields per fruit. 
Johnstone & Kirkby (1999) found that wild Forest Red¬ 
tailed Black Cockatoos take up to 2.45 minutes to remove, 
husk and eat the seeds from a Marri fruit. When feeding 
from feed trees at this rate, they would obtain 3.6 kj min* 1 , 
compared with 2.70 kj min' 1 from non-feed trees. The 
25% greater feeding rate that Forest Red-tailed Black 
Cockatoos attain by selecting feed rather than non-feed 
trees is considerably higher than the 14% increase 
reported for South Australian Glossy Black Cockatoos 
(Pepper el al. 2000). This increased rate of energy 
acquisition from feed trees would translate into a 
reduced daily feeding time or an increased daily energy 
intake. 
South Australian Glossy Black Cockatoos needed to 
feed for 6 h 24 min day' 1 to meet their predicted field 
energy requirements of 626 kj (Pepper et al. 2000). Forest 
Red-tailed Black Cockatoos have a predicted daily field 
energy requirement of 934 kj (Cooper et al 2002), so 
when feeding from feed trees they would require only 5 
h 11 min to obtain this energy, but would require 6 h 29 
min day' 1 for non-feed trees. The ability of these 
cockatoos to discriminate between Marri with high and 
low seed yields enables them to greatly reduce their 
foraging time and increase their energy intake. 
Decreasing the time required to meet their daily energy 
demands is of particular importance during the breeding 
season when the cockatoos have to meet the additional 
energy requirements of a chick as well as themselves. 
This is particularly important because the ability to 
obtain sufficient food, especially when breeding, is a 
major factor limiting the reproductive success of 
cockatoos (Saunders et al. 1985; Johnstone & Kirkby 1999; 
Pepper et al. 2000). 
Predicting Marri seed yield 
Flow the cockatoos determine which trees have the 
highest seed yield is unclear. Pepper et al. (2000) suggest 
that experience may play an important role in feed tree 
selection for South Australian Glossy Black Cockatoos 
feeding on Allocasuarina. Visual cues may indicate the 
potential energy yield of cones/fruits to experienced 
birds. Pepper et al. (2000) suggested that the external 
morphology of Allocasuarina cones may indicate the 
internal seed content, although Clout (1989) found no 
difference in cone size (mass, length or width) between 
feed and non-feed trees. For Marri, there was no visible 
indication to us in either shape or colour of fruits from 
feed and non-feed trees. Despite this, there were 
measurable differences in fruit length and diameter, and 
seed number, individual mass and total mass, of fruits 
from feed and non-feed trees. However, these differences 
were related to site variability for all fruit and seed 
characteristics, and year to year variability in fruit wet 
mass and diameter. The flowering /fruiting cycle of Marri 
occurs on a three to five year cycle, with flowering and 
fruiting varying greatly from one year to the next 
(Robinson 1960; Mawson 1995; Johnstone & Kirkby 1999). 
A feed tree with a high fruit yield in one year requires at 
Mean Total Seed Dry Mass 
Figure 1. Frequency distribution of seed number (A) and total seed dry mass (g; B) for feed (dark bars; n = 31) and non-feed (light bars; 
n = 31) Marri trees (means for 6 fruits from each tree). 
141 
