Dawn Chorus in My Home 'patch'

 

Listen to Dawn Chorus in my Home 'patch'

 

The singing of birds at dawn is celebrated across the world on International Dawn Chorus Day, the first Sunday in May. Inspired by this event and various scientific reports exploring and building hypotheses on why birds sing at dawn, I studied the dawn chorus in my own Home 'patch' over the entire spring and summer seasons of 2023. Each morning, I listened to the birds from the comfort of my home and noted the species vocalizing in 15-minute intervals before and just after sunrise. A total of forty-nine species were heard vocalizing at dawn - a few only once, some for a few days and twenty species fairly regularly. Although our sample size is small, daily tracking of individuals in the same location and correlating their singing patterns with their nesting activities gives a unique perspective.

Waking up before the birds meant shifting my alarm clock with the time of sunrise. I discovered that some locally resident bird species had their internal clock well-tuned to the sunrise as it gradually shifted by over an hour-and-a-half. Singing began at a preset time before sunrise. Daily changes in light levels with rain, clouds, clear skies, or moonlight had no impact. Nor did the size of the bird (body mass) correlate with the time at which a species began to sing. Both these observations are contrary to recent published reports on dawn chorus in the USA and other parts of the world.

Participation in the dawn chorus picked up with the approaching nesting season. For the species nesting in the Home 'patch', singing ended abruptly when the last brood fledged. From then on, vocalization changed from prolonged singing to occasional short calls. These observations are consistent with proposed hypotheses in the scientific literature of dawn chorus association with finding a mate, territorial defense, and communication with female partners and nestlings.

Dawn chorus data were entered in our AvianActs Vocalization (AAV) database (under development by Hemant Soagni residing in India) for tracking. Combining this AAV database with the existing AvianActs Phenology (AAP) database allowed us to correlate the dawn chorus with migration and nesting activities in the Home 'patch' and get more extensive views of bird behavior. A sneak preview of the findings is given here with American Robin, typically the first species to begin singing at dawn, as an example. A detailed description documenting the participation of all bird species is available here.

About an hour before sunrise, when it is still dark, American Robins began to sing softly. It took a minute or so to get the full volume and cadence of the 'charlie, cheerie, cheery-up' song. Fig. 1 shows the time of the sunrise, start of civil and nautical twilights and the StartTime, the time first robin was heard from March 24^th to September 9^th, all in minutes after 4 AM. Black crosses indicate days on which no data were collected. In early spring, robins began singing near civil twilight. By mid-spring through early summer (May, June, and early July), the StartTime was about an hour before sunrise before the beginning of the civil twilight. By late July, the StartTime moved back up to the commencement of civil twilight. After August 11^th, the vocalization changed to short calls at random times indicated in orange triangles.

Fig. 1 Time of sunrise, civil and nautical twilights and StartTime of American Robin vocalization plotted in minutes after 4 AM for the period March 24^th - through September 9^th. Black crosses (x) indicate days on which no observations were made.

At the time robins began to sing, the light intensity was < 0.1 Lux, too low to be measured accurately with our light meter. The pre-sunrise light intensity increases exponentially with time, so we used the measurements made around 23 and around 8 minutes before sunrise. Fig. 2 shows the StartTime from sunrise vs. light intensity measured at 23 minutes before sunrise. Although the light intensity varied from 1 Lux to 16 Lux, the StartTime remained at 52 +/- 3 minutes before sunrise, within the error in recording. This supports our observation that dawn singing is triggered by the time of sunrise and not by the light intensity level.

Fig. 2 American Robin StartTime from sunrise vs. light intensity . Time for sunrise, shown as orange line, is designated as zero.

There was one pair of robins nesting very close to my house and one or two more in the vicinity. Fig. 3 shows the number of 15-minutes intervals (Quarters) in which at least one robin was heard. The total singing time interval increased from one Quarter to four Quarters (one hour) by April 14^th - just when the first nest building started. The chicks in the third nest of the season built in a vine next to my house fledged around July 10^th. Robins were seen in flight carrying food as late as August 1^st, and singing ended by August 11^th, switching to short calls. The dawn singing, presumably by males appears to be related to breeding, from finding or bonding with a mate and territorial defense to communications with the occupants of the nest.

Fig. 3 Number of Quarters (15-minute intervals) before sunrise American Robins were heard vocalizing from Marh 24^th to September 9^th, 2023.

Carolina Wrens, Chipping Sparrows, and Northern Cardinals also built nests close to the house, each species with its own rhythm. Many other species were nesting nearby and seen carrying nesting material, carrying food, or attending to fledglings. StartTimes of Baltimore Oriole, Blue Jay, Eastern Bluebird, Eastern Wood-pewee, Gray Catbird, House Finch, House Wren, Mourning Dove, Red-bellied Woodpecker, Song Sparrow, Tufted Titmouse and White-breasted Nuthatch are documented in detail. Canada Geese honked in flight, and a Barred Owl hooted at night or at dawn.

Read more on our observations, recording methodology and data analysis documented in greater detail here.