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Silvereyes: tiny Tasmanian travellers


 
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Julie Funnell completed her doctoral studies at UTS researching one of the smallest of our migratory birds – Silvereyes: so called because of the ring of white shiny feathers around their eyes     
Silvereyes are small (10 to 12 grams) native Australian birds that come from a family of that is spread virtually across the world. Julie’s research compared local Sydney birds with those that migrate annually from Tasmania. The work, funded by an ARC large was an important study because so little research has been done on Australian migratory birds and especially laboratory studies.

 
The overall aim was to show that the Tasmanian migratory birds have an innate program that prompts them to migrate: that tells them when to migrate, where to migrate, when to put on fat, when to shed their feathers and to eat foods that will help put on fat. These factors have been demonstrated in northern hemisphere migratory birds but never in Australian birds.

Silvereyes are one of the most extensively banded birds in Australia, making it possible to track the migratory paths of individual birds.

 
Only some populations of Silvereyes are migratory, so that their behaviour and physiology could be compared with a nonmigratory local population. Thry are more migratory the further south they live, reflecting the more extreme winter climate in the colder Tasmanian regions. They leave Tasmania around March/April; arriving on the mainland in May.  They gradually travel up the coast even as far as Brisbane, then return to Tasmania from September to November for the breeding season. While they are gregarious during migration, with flocks of many hundreds of birds and mixing in well with the local Sydney birds, during the breeding season, they separate out and don't mix until the offspring have fledged.
Even when the migratory birds were held in Sydney for 18 months, they still showed the same patterns of wanting to complete the migratory cycle in spite of the availability of food and better climate conditions. Birds held in captivity for 18 months did not lay eggs however this may relate to being kept individually in cages since Silver Eyes are particularly life long monogamous and may require the presence of their partner to trigger egg laying.

 
There were five main outcomes from the studies.
    
 1. Orientation behaviour: testing was to determine in what direction the birds would prefer to escape. The migratory Tasmanian Silvereyes showed strong orientation preferences, so that in autumn, they showed a northerly direction and in spring a southerly direction.  This is in accordance with their northerly migration and return southerly for return to Tasmania.
The local nonmigratory birds showed no preferences.
Right: This is an example of an orientation graph. It is similar to a compass, with North, South, East and West indicated.

 
The dark triangles are the mean direction that an individual bird was heading and the arrow represents the mean direction of all the birds.
This graph shows the orientation of the Tasmanian birds during their first autumn migration period in captivity.
They had a significant NNE direction, which is consistent with the direction they fly from Tasmania to reach the mainland.


 
2. Seasonal activity: The migratory birds showed heightened activity during their migratory period and levels were higher than in nonmigratory birds.

3. Weight and fat stores: The migratory birds showed greater fat deposition and body weight than mainland birds. Tasmanian birds are about ten percent heavier than the local Sydney birds. Also the migratory birds put on fat to assist in their flights and would expect migratory birds to have larger fat deposits during the migratory season. This was the case and the nonmigratory locals had very low fat stores at any time of the year while the migratory birds put on large fat deposits during the migratory season.

4. Food consumption: both subspecies ate the same amount of nectar however the migratory birds consumed more meal worms than did the mainland birds and this may contribute to their ability to put on fat. Using insects (rather than fruit) to put on fat is different from northern hemisphere populations.

 
Graph shows the median number of mealworms consumed by each subspecies over a 12 month period.

The Tasmanian Silvereyes ate substantially more mealworms, more frequently than the mainland birds.

 
5. Moult: migratory birds moulted earlier and faster than mainland birds so that they were able to finish the moult before migration occurs.
Silvereyes continued to go through their annual moult cycle in captivity at the same time as the wild birds.

 
Above: graph shows the moult of each bird of each subspecies (each line represents the total time taken for moult by an individual bird).
All photos and figures from Julie Funnell.

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