I guess so, but you might come across some spring passage migrants, and there are still quite some resident birds to see.
Depending on whether your friends are serious bird watchers looking for specific species here, or they are looking for a general perspective of birds in Hong Kong. So, there are other choices of interest you can talk about when going to Maipo, like:
1. Overall perspective of birds statics in Hong Kong, how does this compared with other parts of the world, say China, England, world, ...
2. What habitats we have here in attracting all these birds?
3. The birding seasons, and the birding hot spots. With the help of a map of the colony, you get a long list to show.
4. History of Maipo, what it was and what it is, ...
5. Which organizations are active in bird watching and environmental protection? What activities they organize year round in achieving their mission?
6. Birding watching in China, particularly the recently announced nationwide bird watching campaign.
7 ....
Last but not the least, it is up to your choice to make your friends' visit enjoyable and memorable.
HK