I am studying a very small population of an endangered songbird, and as part of my research I am studying post-fledging habitat use and (hopefully) post-fledging survival. As such I set out to radio-tag chicks and track them during the post-fledging period.
In 2016 I radio-tagged 4 chicks from 4 nests, and all of the chicks in each nest (tagged and un-tagged) were given unique combinations of colored leg bands so that they could be identified after they left the nest. Due to a poor choice in transmitter attachment method 3 of the transmitters fell off within 48 hours of attachment, and the fourth chick was predated within a day of leaving the nest. In order to determine if the transmitters recovered from the other 3 chicks were due to depredation or early loss I spent a span of 13 days searching for the fledglings that had been tagged with radio-transmitters to be certain if they were alive. I was able to locate 3 of the previously tagged fledglings, and also located one un-tagged sibling during that time. Due to the difficulty in locating fledglings without transmitters and time constraints to collect other data I did not spend more than those 13 days searching for fledglings in 2016. Then in 2017 four of the 2016 chicks returned to breed (two of them had carried transmitters in 2016, and two had not). The two returning 2016 chicks without transmitters were not observed during the short time I looked for fledglings in 2016. The 2016 "season" was in essence only 13 days long.
In 2017, I radio-tagged 6 chicks from 6 nests, and all chicks were given unique combinations of colored leg bands. All transmitters remained attached thanks to a switch in attachment method. One of the radio-tagged chicks was presumed to have been predated after 5 days (too young to move out of study area on own, still dependent on adults, transmitter should not have failed that soon, poorly flighted, poor movement abilities). Radio-tagged chicks were tracked daily, and because the adults split the brood in half and within each half individuals tends to stick relatively close together, I was able to observe an additional 7 chicks (siblings of radio-tagged chicks) occasionally while radio-tracking their siblings. During other portions of my field work I incidentally was also able to observed chicks from two un-tagged/un-banded broods 31 days after they had fledged. I did not search for the halves of broods that did not included radio-tagged fledglings due to the difficulty in locating them. The 2017 "season" was 59 days (from the day the first tagged chick fledged to the day the last tagged chick was observed).
2016
-5 chicks observed over the span of 13 days (4 with tags, 1 without)
- 4 returned in 2017 (two of which were not observed in 2016 after fledging)
2017
-6 chicks were radio-tagged and observed daily over a span of 59 days
-7 siblings were observed occasionally while tracking radio-tagged chicks
-5 unmarked fledglings were observed (with known adults) 31 days after fledging (fledge date known)
I plan on using MARK/RMark to analyze fledgling survival using the nest survival framework. I would like to include all of these data from various sources to be able to increase my sample size. If I only use the 2017 radio-tagged chicks (n=6) then sample size is small and estimated survival is very high due to only one death early in the post-fledging period (not that high survival is bad, but this likely doesn't approximate reality).
2). Is there a good way to estimate survival when 2016 and 2017 "seasons" are so different in length (13 days vs 59 days?
3). Is there a way to incorporate the information from the returning 2016 birds? Since they returned in 2017 I know that they survived the entire year, much longer than the time I spent searching for them during the 2016 breeding season.
Thank you!