A small update on the new Bird Park. Construction of the park started last month and will take about 2 years, which puts completion at mid-2020. It will probably open towards the end of 2020 after the birds move in and settle down. Links below are for a 3D computer render of the park (seems like the final design), and concept art for the park's penguin exhibit.
Mandai Bird Park | VALERIANO ENDAYA CABILES | Archinect
Singapore Zoo Penguin Exhibit
Construction for the other parts of the Mandai project - West Arrival Node, Eco-Link bridge (wildlife crossing) and animal quarantine facility - are underway. Construction has not yet started on the Rainforest Park (and I don't think the plans have even been finalized yet).
Those four enormous walk-thru aviaries look promising.