The Story of Mr. Bisbing, Part II: Fatherhood
This is part two out of five of our story about a great egret that we tagged at the Outer Banks in Spring 2013. We followed his movements for about 8 months and reconstructed his story with his GPS and ACC data (see part I).
The exact position of the lines in the ACC graph are the result of totally different behaviors, giving us clues about important events in Mr. Bisbing’s life. While the motion of the bird is recorded by changes in lines on an ACC graph, the relative position of the lines to each other show the position of the bird relative to gravity. Leading up to May 19th, Mr. Bisbing would occasionally rest at night in a horizontal position, motionless, with the blue line above the green line, different from his roosting position, which was green line over blue, over red (Fig. 1).
So we concluded that this horizontal posture was actually Mr. Bisbing incubating eggs, sitting all the way down on his nest to keep them warm. This was exciting news! Our tagged bird was going to have babies!
We observed this incubation position only until May 19th, presumably the hatching date. Prior to this date, he incubated every other day or so, sometimes up to 52 hours in a row (May 13th)! Egrets take turns incubating eggs, and we could observe that Mr. Bisbing took turns with his partner because he roosted at places other than the nest.
During these first 3 weeks, from April 21st until May 19th, Mr. Bisbing foraged in two different areas. After waking up and doing his morning grooming, he flew either north, towards Swan and Johnson Islands and spent the day there foraging in the surrounding wetlands, or he flew south to forage in the wetlands west of Corolla. Once in a while he made his way along the coastline, probably searching for new foraging grounds, but always returning to his two favorite wetlands where he spent most of the daytime (see Fig.2).
Then, during the night of May 17th, something exciting seemed to happen. The GPS data show that Mr. Bisbing stayed on the nest for almost 60 hours in a row, until 6:20a.m. on May 19th. The ACC lines tell us little more details about what happened during this time (Fig. 1).
On May 17th Mr. Bisbing flew back to his nest in the evening and settled down in the incubating position. He stood up once in the middle of the night (the switch from blue over green to green over blue), probably to stretch his legs and neck. During the next day he stayed in the incubating position, but moved around quite a bit, more than during other incubating periods– was he nervous? Something seemed to bother him – was there something going on with his eggs?
But he calmed down again in the late afternoon, the lines are flat again. During the following night he switches into the roosting position, still at the nest site. Maybe he took a turn in incubating with his partner. The next morning, he incubated for another 3 hours – the last time that we observed this behavior. When he stood up again he started moving around but stayed close to his nest as the GPS data tell us. Great Egret young usually hatch one at a time, so we assume that those days were probably the time when Mr. Bisbing’s chicks were hatching.
After May 19th, Mr. Bisbing changed his daily schedule. Before that day his routine was rather irregular, including long periods of rest in different places, long periods of incubating and foraging in many different areas. After May 19th, Mr. Bisbing chose the wetlands west of Corolla as his absolutely favorite foraging site. He went there in more than 90 % of the time during the next two months and he flew north only twice, towards Johnson and Swan Islands (see fig. 3).
We assume that the wetlands west of Corolla became the new preferred foraging area, because it’s only half the distance compared to Swan and Johnson Islands. By choosing the closer food source, Mr. Bisbing could save time and energy, which he urgently needed to feed his offspring.
Surprisingly he left the nest all day and came back only in the evenings to help feed the chicks. Typically he took off early in the mornings, around 5:30 a.m. and came back before dusk, between 6 and 8 p.m.. This behavior is likely caused by the remote location of Monkey Island and long commuting time to get there. Long flights cost time and energy, and as Mr. Bisbing was raising his chicks he needed to budget these factors.
Another interesting fact was that he slept on Monkey Island, but at a roost away from the nest, for 12 nights in a row. Furthermore, on three occasions he didn’t come back to Monkey Island for 48 hours. For a new dad, he certainly was avoiding the nursery. However, after these initial days of fatherhood, he slept at the nest in his roosting posture except for three nights. It’s not quite clear to us, why he behaved this way. Maybe his wife took good care of the chicks during the night, so that he could rest and get ready for the next day’s foraging trip.
Other studies have found that great egrets feed their offspring at the nest for 20-30 days, depending on the local food supply. Egret chicks start clambering from the nest after about three weeks and fly away from the colony after 1-2 months. We cannot tell from our data, when Mr. Bisbing’s chicks left the nest. But we do know, exactly when he left the colony – [July 17th, 5:05am] – never to return. Find out where he went in our next blog in a few days.