The figure below shows the number of adults captured each year and within that the proportion of newly banded adults and birds who had returned to Wascana Marsh with a band already on its leg. Of course, because 2010 was our first year of banding, we did not have any previously banded birds to recapture. In each year, returning GRCA's make up 28 to 42% of the total number of adults captured. These are some of the highest return rates we see for all of the species we band at this site, except for perhaps Yellow Warblers.
 |
| Click on image to see a clearer graph |
Interestingly, we can look at what the survival rate and return rate for GRCA's is from other MAPS stations thanks to the Institute of Bird Populations. In our continental zone (North-central) there are 36 different MAPS stations that have banded 3940 GRCA between 1992-2006. 772 GRCA returned in subsequent years. Based on this, the IBP is able to calculate that adult GRCA's in our zone have a 50% chance of surviving and returning to the same areas they bred in the year before. Remember that this is an estimate as to whether the bird has survived and returned to breed in the same area, it is not a 50% chance that we will actually recapture it that year. The IBP website does calculate recapture probabilities which for GRCA's is 48.5% given it has survived and returned to the same area. So ultimately this means that the bird has a 50% chance of survival, then we have a 48% chance of catching it, so we can expect to recapture about 24% of the birds captured in the previous year... You can check out the IBP website and find these stats for many other bird species.
If we look at our recapture rates, in 2011 we recaptured 22% of the adult GRCAs that were banded in 2010, in 2012 we recaptured 46% of the adult GRCAs captured in 2011 and in 2013 we recaptured 19% of the of the GRCAs captured in 2012. Recapture rates in 2011 and 2013 are comparable to the expected recapture rates, but in 2012 the recapture rate was significantly higher.
 |
| Gray Catbird with a band on its leg |
The actual birds we have recaptured over the years have been interesting as well. In 2011, we recaptured 3 individual GRCAs that were originally banded in 2010 (a total of 5 were recaptured this year). These were 2 males and 1 female. Amazingly we recaptured all three of these same birds again in 2011! I think it is so fascinating to think that these birds that weigh 35-40 grams will breed in the Habitat Conservation Area, fly south for the winter hundreds of kilometers away along gulf of Mexico, and then come back to exactly the same area the next spring (sometimes being captured in the same net as the year prior!) and doing that for 3 years in a row!
We had 2 more birds return to Wascana Marsh being banded in 2011 and then recaptured in 2012 and in 2013! Amazing little birds!
 |
| Juvenile Gray Catbird |
Another fascinating component to our returns of Gray Catbirds, has been the juvenile GRCAs who have returned to the HCA. In each year, 2011, 2012, and 2013 we have recaptured a single GRCA that was banded the year before as a juvenile bird (~4% of all juveniles banded; juvenile bird meaning it was hatched that year). All three of these returning juveniles have been males that we are able to determine once they return (as a juvenile we are not able to determine the sex of GRCAs). While this is a very low return rate of juvenile catbirds (we've banded 74), it is interesting that only males have returned. This suggests that females disperse from their natal ground, while males may return to breed where they were raised. Remember too that juveniles often have a significantly lower survival rate then adults. So the 50% survival rate we discussed above does not apply to juveniles in their first year.
The Birds of North America Gray Catbird account has limited information on return rates of juvenile GRCA. It does not give information about sex differences in returns to the natal ground. Lehnen and Rodewald (2009) found a return rate of only 3 juvenile GRCA out of 304 young banded (1.1%), in Ohio, but did not make any note of the sex of the returning birds.
Overall, the Wascana Monitoring Avian Productivity and Survivorship (MAPS) station is providing some really interesting information about Gray Catbirds and many other species! This information will be vital to help conserve Wascana Marsh in the future and promote this space as a very important area for birds!
Literature Cited
2009. Dispersal, interpatch movements, and survival in a shrubland breeding bird community. Journal of Field Ornithology 80(3):242-252.
doi:10.2173/bna.167
No comments:
Post a Comment