Signs of Spring #11: Field Sparrows and Bluebirds

Photo by D. Sillman

Deborah and I were up at Harrison Hills Park two weeks ago hiking the route between our assigned bluebird boxes and keeping our eyes open for wildflowers and interesting birds. We parked near the cluster of purple martin houses (no martins yet!) just across from the Environmental Learning Center and were immediately greeted by a long, trilling, melodic song coming from the top of one of the many privet bushes/trees that are strewn across the meadow.

I had my larger (and heavier!) binoculars in the trunk of the car (I carry my smaller ones with me when I hike), so I got them out and zoomed in on the privet bush. Our singer was a male field sparrow (Spizella pusilla) (pictured above) singing so hard and loud that his little body shook with the effort!

The field sparrow is one that I never see around my house. They are a true “field” species that is intolerant of human activity and the structure (and hustle and bustle) of urban and suburban habitats. They are year-round residents of our area, but really show themselves off in the few weeks prior to mate selection and nest establishment.

Photo by D. Sillman

In the winter, male and female field sparrows join multi-species flocks that included a mix of other sparrow species (including song sparrows and white throated sparrows). Field sparrows are much less aggressive than these other two types of sparrows and, so, probably spend much of their winter dominated by these more energetic birds. The field sparrow’s response to continually being displaced from overwintering food sources (primarily grass seeds) is frequently to return to sites following predator disturbance more rapidly than its flock mates. The increased risk of a lingering or returning predator is more than offset by a chance at a decent feed!

Male field sparrows break away from these winter flocks in early spring and return to their specific breeding sites with great reliability. In these sites, the males begin to sing to mark their territories and interact with each other to establish their breeding boundaries. Females arrive later, and they tend not to return to the territories of their birth nests or even to the territories in which they might have bred in the previous year. This more random selection of breeding sites by females (couple with the great specificity of site selection by males) very effectively reduces inbreeding within this species.

Females are frequently greeted by the male with physical contact and superficially aggressive behaviors (the male may fly at a female entering his territory and actually knock her to the ground! Maybe there is some pent energy from being so passive and submissive all winter?). Mate selection, though, quickly follows these altercations, and the female (with little help but often with much attention by the male) builds the nest.

Field sparrows can have more than one breeding cycle in a season, and their nest locations are quite different in the spring vs. the summer. In the spring the nests are usually built on the ground amid the growing grasses and weeds. Tree or shrub locations are still not adequately leafed out to provide cover or concealment. Later in the summer, though, the nests are built in the fully leafed out trees and shrubs where they are more protected against nest predators.

In the spring and summer field sparrows take advantage of a wide range of available foods in addition to their nearly continuous diet of seeds. Insects (including flies, bees, beetles, grasshoppers, and caterpillars) and spiders are protein and energy rich food sources consumed by the adult field sparrows and fed almost exclusively to their nestlings.

Field sparrow numbers have declined in recent years primarily due to habitat loss. They are a very welcome member of the Harrison Hills Park avian community, though.

Photo by D. Sillman

This is our second year monitoring the twenty-eight bluebird boxes scattered across Harrison Hills Park. The original placement of the boxes was based more on convenience of access than on bluebird ecology, and one of last year’s goals of our “Cavity Nesting Team” (the name of our eight membered volunteer group) was to explore the possible variables that influence the selection of nesting boxes by bluebirds and also tree swallows (another important “cavity nester”).

We found that, in general, successful bluebird boxes were located about 30 to 40 feet from the edges of the woods (definitely not right on the edge of a field and definitely not in the center of a field). Most bluebird occupied boxes faced south (or southeast), and that bluebirds did not occupy boxes close to other bluebirds. Two bluebird boxes were very close to (within 100 feet) occupied swallow boxes, though. Bluebirds reproduce in staggered waves (three reproductive time events (April/May, May/June, July/August). Not a rigidly timed event). Survival to fledgling in our boxes was 66% overall. Early clutches had a higher survival to fledging (77%) than later clutches (50%).

Successful tree swallow boxes, on the other hand, were located further from the edges of the woods than the successful bluebird nests and were much more in the centers of the meadows. Two swallow nests were located near the Park’s big pond. Two swallow nests were located within 100 feet of active bluebird nests, and tree swallows reproduce in a more rigid time frame. Almost all nesting occurred in a May/June time frame (only one nesting cycle).  Survival of egg to fledgling was 71% in our boxes.

Last Fall the Cavity Nesting Team initiated a plan to put some of our observations and hypotheses into practice. We left all boxes that had tree swallow or bluebird nests in them where they were, and we moved the “no nest” boxes according to the following criteria:

1. To the centers of the fields (to favor swallows)
2. Near the ponds (to favor swallows)
3. To 10 to 20m from the field edges, at least 100m apart, near mowed areas if possible, and put them out in pairs (to increase visibility and influence the birds’ perceptions of habitat quality)(to favor bluebirds)
4. And we tried to orient all boxes to face the south or southeast

In October 201 we moved seven of the nine non-nested boxes into locations that corresponded to these hypothesized “more favorable” locations. We are now watching our boxes!

On our hike around two weeks ago Deborah and spotted our first bluebird eggs! We also opened one of our boxes and found a female bluebird protectively incubating (and maybe laying) eggs! Several of the other boxes have starts of nests, eggs and active bluebird “look outs,” too. Lots of bluebird action at Harrison Hills! I will keep you posted!