ESTIMATING THE CHICK SURVIVAL OF COLOUR RINGED GULLSEURING 2003, EURING Technical Meeting 2003, poster abstract 6.-12.10.2004 Radolfzell
Risto Juvaste & Jari Valkama
We present a method to estimate the chick survival of individually colour ringed gulls. It is based on large scale colour ringing of Herring Gulls (Larus argentatus) (HG) and Lesser Black-backed Gulls (Larus fuscus) (LBBG) in Finland during 1993-2000. Altogether 7 636 chicks of Herring Gulls and 6 045 chicks of Lesser Black-backed Gulls were ringed with individually coded colour rings (cr). By 14 February 2003, there were 42 646 sightings of 4 342 HG individuals (57%) and 21 827 sightings of 1 669 LBBG individuals (27%) in the database of Helsinki Ringing Centre. Because most of the birds were aged during ringing by measuring the wing length, the estimate for survival after ringing can be calculated from classified percentages of total sightings. The method has been tested with HG and LBBG populations in Finland. It was found that due to the high percentage of cr-sightings, the survival of HG chicks can be estimated even in small populations. An example is a cohort of 178 chicks ringed near Lappeenranta (SE Finland), wings 55 mm-335 mm, median 200 mm. Altogether there are 370 sightings of 72 individuals from these birds. By simple analysis of cr-sightings based on linear regression we estimated that about 125 chicks fledged. The survival of chicks during the last 3-4 weeks before fledging was about 50%. By use of population analysis (Popan5/Jolly-Seber/full/birth) of the sighting data (57 sightings/37 gulls) from a nearby dump it was estimated that 104 of the fledged birds (83%) came to the dump. Estimate is however rough. Based on the yearly sightings we estimate that 24 birds (SD=5.2) were still alive in the year 2001. (Popan5/Jolly-Seber/dead only). In a the similar analysis of LBBG-chicks ringed at Finnish lakes during years 1993-2000 (n=3457) there was an unexpected decrease in survival in the group of nearly fledged chicks. The average survival decreased nearly 10% from the group 250-300 mm to 300-380 mm. The percentages of sightings in these groups were 54% and 45% (χ2=5,9 df=1 P=0.015 n=940). The reason for this decrease, which is against the expected trend and found in all year cohorts, will be studied by analysing the sub-cohorts, also using metal ring recoveries.
The whole poster (286 kB) >> Juvaste, R. & Valkama, J. Estimating the chick survival of colour (read) ringed gulls