Capital incomes, such as dividends received from the distribution ETFs in the Birdee portfolios, are taxable incomes. You must therefore pay the tax on the dividends you receive.
In the case of foreign accounts, withholding tax is not deducted at source. You must therefore declare the dividends from your Birdee portfolio in your tax return.
All these data will be communicated in your tax report, which you will automatically receive on your Birdee account during the first quarter of the next year.
Note that in Belgium, from 1 January 2018, the first 627 euros of annual dividends received on all your investments will be exempt from tax. In order to benefit from this measure, you will have to enter all dividends received in your personal tax return.