Duck breathing with chest?

[BornToQuack]

Ok, so I'm completely new to keeping ducks and am still learning what's "normal." Lately it seems like my three Runner Ducks have developed a hollow line/dip in the middle of their chests. They're 7 weeks old, so I'm not sure if this is a natural progression from duckling to adult, or if something is wrong.
Sometimes it looks like the hollow section moves in and out as they breathe. Is that normal?

This evening I also noticed that two of them have a slightly sticky sound to their breathing, but their nares are clear, they're eating/drinking fine, and very active. Could a change in temperature and/or humidity cause this? Lately it's been very hot and dry, but a cold front came in last night and brought cooler temps with moister air.

Anyways, here are pictures of two of my girls so you can see the lines/hollow dips I'm talking about.

 

[chickens really]

That's the breast meat..they appear to look good to me. As long as they are eating and drinking it's nothing to worry about.

 

[Runner duck mom]

My runners have that too. Very pretty ducks

 

[BornToQuack]

My runners have that too. Very pretty ducks

Thanks! I love how they have a silver sheen in certain lighting. They're also growing in some gorgeous blue-green underwing feathers! Can't wait to see what they look like once they're fully grown.

 

[Miss Lydia]

Cleavage you can barely see it on mine they are over 2 yrs old now.

 

[Runner duck mom]

Thanks! I love how they have a silver sheen in certain lighting. They're also growing in some gorgeous blue-green underwing feathers! Can't wait to see what they look like once they're fully grown.

The shiny blue feathers were a nice surprise for me as well. Can't see them very well in pictures but so pretty when the sun hits them

 

[Miss Lydia]

I love the blues but the blks really gleem too.