A Migration of Hummingbirds through Santa Clara County, California

During the first part of May, 1933, a wave of migrating hummingbirds passed through southern Santa Clara County. This was at the time locust trees were in bloom and nearly all the birds were feeding in these trees. A few were found along the Uvas Creek but not in great numbers.

The migration was first discovered by a fellow collector, H. R. Eschenberg, on May 3, when he collected some birds and brought them to the writer. On the next day phone calls from different parts of town brought word that many hummmgbirds were being noted wherever locust trees were to be found.

Our observations were carried on mostly along an avenue of locust trees just west of the city limits of Gilroy. The trees on one side of the road had been topped several years before and had grown up in thick bushy form. This was where most of the birds were taken, as the trees on the other side were almost too high for .22 caliber shot cartridges. From ten to twenty birds were in sight at one time and a walk down the avenue showed them to be present in hundreds.

The flight was at its height from May 3 to 6, inclusive. On the 7th the birds were beginning to thin out rapidly, and by the 9th nearly all were gone. Four species were noted, and we could soon identify each at a glance after collecting a few individuals. However, 8x glasses were used to good advantage.

Rufous Hummingbirds (Selashorus rufus) formed the bulk of the migration. At a conservative estimate the females outnumbered the males twenty to one. (Possibly male birds preceded this main flight.) Females are feeding continuously, were found to be excessively fat, and stomach contents showed masses of minute insects. The males were darting back and forth, driving the females in every direction. Dissection showed the gonads of the males to be very much enlarged while the females showed practically no sexual activity. Nine birds were preserved: two males and two females on May 3; two males and one female on May 4; one male and one female on May 5. All the females showed notches on next to middle rectrices.

The Black-chinned Hummingbird (Archilochus arexandri) was seen every day. Although not numerous, a few birds were always present. These birds nest sparingly in this locality. One male and one female were taken on May 5; one male, on May 6, and one female, on May 9. Another male was shot but lost in high vegetation.

A male of the Calliope Hummingbird (Stellula calliope) was one of the first birds brought in by Eschenberg on May 3, thus establishing the first record for Santa Clara County. These little fellows were easily distinguished from their larger relatives, and at least a dozen were seen each day. They kept to the centers of the thick bushy trees and had to be hunted out or were een passing from the center of one tree to the next. Eight skins were made: one male was taken on May 3; two males on May 4; two males and two females on May 6, and one male on May 7.

Anna Hummingbird (Calypte anna) may have been migrating also, as a few birds were seen; but since the species is a common breeder in this locality, they may have been merely feeding with the rest. However, two birds were collected to verify our identification. One male was taken on May 6, and one female on May 7.

W. E. Unglish

Gilroy, California, August 4, 1933