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Chimney Swifts arrived a few days later than usual in 1998. Our first sighting was mid-morning on April 1st with 6 birds flying overhead. Droppings indicated that both the North and South Towers were occupied overnight on April 2nd. On April 3rd, one bird was seen roosting in the North Tower while two individuals were observed in the South Tower. The Castle was first occupied on April 4th. By April 12th, there was a flock of 30 swifts roosting in this concrete tower. A pair of swifts returned to the Prism Tower on April 28th, and the Garden Mini-tower was occupied on May 2nd.
On April 21st, the first nest stick was attached in the South Tower. Initial construction was observed in the Castle on April 25th and in the North Tower the following day. By May 11th, nest construction was underway in the Garden Tower and the Prism.
Both the North and South Tower pairs produced 6 eggs, as did the Garden. The Castle had 5 eggs and the Prism had 3. All 12 eggs hatched in the Twin Towers (North and South). Four of 5 hatched in the Castle and all three in the Prism. None of the Garden Tower eggs hatched - probably due to the extreme heat.
Three banded adult swifts were captured on June 14th in the South Tower. Two were identified as the breeding pair which successfully double-brooded in 1997. The pair showed obvious swelling of the saliva glands, indicating that they were a breeding pair again in 1998. The third bird was the single offspring of the pair's second 1997 brood - referred to as "The Slug" in Volume 3. The North Tower family was captured on June 16th. Five banded adults were roosting at the time. The pair which successfully bred in 1997 were present - both with active saliva glands. The other banded adult swifts included one banded as a nestling in the North Tower in 1993 (now 5 years old), a hand-reared return from our facility which was released in 1997 and ... "The Slug". We were surprised to find this bird switching roost towers. In order to be able to visually monitor The Slug's activities, we marked a single head feather with white typewriter correction fluid. In subsequent weeks, we observed this bird feeding the nestlings in both the North and South Towers.
The progress of the nestlings in the Prism were monitored visually and auditorily. However, with no access door in this experimental structure (a serious design flaw for bird banders), we were unable to band them.
By July 4th, 17 new swifts had fledged from the 5 structures on the Mansfield Dam Station. On July 6th, the first of 3 eggs of a second brood was laid in the South Tower. All three hatched and The Slug was observed feeding this brood also.
A Canyon Wren was observed investigating the inside of the South Tower on August 5th. The bird appeared to be hunting for live food in the corners and grooves of the tower. We were attentive to this intruder since Canyon Wrens have been observed piercing the eggs of other cavity-nesting birds within their territory. The wren seemed curious about the unguarded nestling swifts and approached to within 12" of the nest. However, it showed no aggression toward the baby swifts.
The 3 South Tower nestlings all fledged on August 31st.
A roosting flock began to build in the North Tower shortly after the babies fledged in June. The size of the flock fluctuated between 39 and 102 birds until it reached a high of 133 individuals on September 9th. The flock began to diminish rather than increase from that point. Based on our experience, this is unusual for a fall roosting flock of Chimney Swifts. On September 23rd, we discovered that a Ringtail had taken up residence in a space just below the floor of the tower. By October 7th, there were no more swifts roosting in the North Tower. We speculate that the activity of the Ringtail caused the swifts to abandon the site.
A flock of 40 to 50 swifts roosted in the Castle with the nesting pair and their young from early May until late August. The flock continued to fluctuate in numbers until October 21st when the last 7 swifts departed. Chimney Swifts were observed on the station for 203 days in 1998.
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