You got me so intrigued with these photos, that i had to google it up, why can these birds sit on a cactus without pain. And the first obvious answer was - they've learned how to ... =)

Second answer was more detailed. They are so light that the force pushing down on the needles isn't strong and they manage to spread it on multiple needles. And the last thing that helps them is that they have zero or very little nerve endings in their feet and lower legs.

Cool photos, haven't seen Cactus wrens before! :)