A big belly.
Repeated upward whistles (low to high on musical scale). Intermittent brief chittering sound and periods of silence. Mostly the consistent whistle call.
Did not see the bird.
The chittering bird, not the robin
Video: https://youtu.be/PgxNV6WD5Yg?si=iaYC-T7DLqS3Gd8q
Flying from tree to tree and to power lines and to ground calling in sharp sounds for at least 30 minutes. Appears to be juvenile or female present as well, which landed on power line before following the male.
Possibly parents guiding a fledgling? They ended up flying to the ground in a yard with a dog, which supports the possibility that an inexperienced flyer chose the path. (No birds were harmed.)