CES Data

The CES database contains the capture histories for every individual bird en- countered during CES mist-netting. For example, a typical segment of CES data for a bird would be:

A334831 SEDWA AD 301 1994 0 0 0 1 0 1 0 0 0 1 0 0 A334831 SEDWA AD 301 1995 0 0 1 0 0 0 0 0 0 0 0 0 A334831 SEDWA AD 301 1996 0 0 1 1 1 1 0 0 1 0 0 0 A334831 SEDWA AD 301 1998 0 0 0 1 0 1 0 0 0 0 0 0

From left to right the columns represent the ring ID, species, age, site of capture, year of capture and the 12-visit encounter history in which “1” denotes capture and “0” that the bird was unobserved. Therefore, the Sedge Warbler with ring ID A334831 was caught as an adult at site 301 in 1994, 1995, 1996 and 1998, and not seen thereafter. In 1994, for example, A334831 was caught at visits 4, 6 and 10, and not encountered at any other visit that year.

From this subsection of data it is impossible to ascertain whether mist-netting had been carried out at site 301 during any other year in which bird A334831 was not encountered. For example, CES mist-netting took place in 1997 at site 301, but as A334831 evaded capture that year there is no record for 1997. To construct the full capture history the above data must be augmented by information giving the years each site was operated. In the above example the full capture history for bird A334831, from 1994 to 1998, is therefore:

A334831 SEDWA AD 301 1994 0 0 0 1 0 1 0 0 0 1 0 0 A334831 SEDWA AD 301 1995 0 0 1 0 0 0 0 0 0 0 0 0 A334831 SEDWA AD 301 1996 0 0 1 1 1 1 0 0 1 0 0 0 A334831 SEDWA AD 301 1997 0 0 0 0 0 0 0 0 0 0 0 0 A334831 SEDWA AD 301 1998 0 0 0 1 0 1 0 0 0 0 0 0

The fourth row, corresponding to 1997, now indicates that A334831 was not caught during that year, although mist-netting was conducted and A334831 was clearly alive.

Under the assumption of constant effort any variability between years in the numbers of birds caught can be attributed to changes in the population level and to stochastic variation, and not to varying intensity of capture effort. Thus, for each species monitored under the CES scheme, the total annual catches of adults and juveniles can be used to estimate between-year and long-term changes in species’ abundance and productivity (Peach et al., 1998; Miles et al., 2007; Robinson et al., 2007; Cave et al., 2009a,b). In addition, from annual recaptures of adult birds, adult survival is estimable (Peach et al., 1990; Cave et al., 2009b; Freeman, 2008). Such a monitoring scheme, based on ringing across a number of sites, is perhaps unique in providing this combination of demographic information.

Note that in general juvenile songbirds, in contrast to adults, are not site faith- ful. Due to the high dispersal of juvenile birds from their natal sites, a juvenile ringed under the CES scheme is unlikely to be recaptured again and hence CES data alone will not enable accurate estimation of juvenile survival. Furthermore, any estimates of juvenile survival will be confounded with natal dispersal.

Missing CES Visits

In practice there is considerable deviation from the “constant effort” ideal un- derpinning the CES scheme with some years receiving fewer than the full 12

visits. These visits may be missed entirely due to extremely bad weather, vol- unteer ringers’ personal circumstances etc, or else not achieve the minimum required catch effort. That is, effort spent mist-netting may be unavoidably reduced, for example, by bad weather or net damage necessitating the early cessation of mist-netting at the site. If the standard mist-netting effort, in terms of the total net length and the time the net is open, falls below 80% of the norm at the site (as indicated by the volunteer) the visit is considered to be missed. Any visit where catch effort is at least 80% is deemed to be made. In the CES database a missed visit is denoted by “9”. In the above example say mist-netting had not been carried out at site 301, in 1998, at visits 1 and 12. Further, say the additional information giving the years, and visits within those years, site 301 was operated indicated that CES mist-netting had only taken place in 1997 at several visits (1 to 5). The full capture history for bird A334831, from 1994 to 1998, would then be:

A334831 SEDWA AD 301 1994 0 0 0 1 0 1 0 0 0 1 0 0 A334831 SEDWA AD 301 1995 0 0 1 0 0 0 0 0 0 0 0 0 A334831 SEDWA AD 301 1996 0 0 1 1 1 1 0 0 1 0 0 0 A334831 SEDWA AD 301 1997 0 0 0 0 0 9 9 9 9 9 9 9 A334831 SEDWA AD 301 1998 9 0 0 1 0 1 0 0 0 0 0 9

The data now indicate that A334831 evaded capture at all visits made in 1997 (visits 1 to 5), and for those visits made in 1998 (visits 2 to 11), A334831 was caught only at visits 4 and 6.

The presence of missed visits in the data, and the resulting violation of the constant effort assumption, require particular attention when building models to monitor abundance, productivity and adult survival from CES data. As the standard practices for analysing CES data do not adequately address the problem of missed visits, this is one of the key areas of research considered in this thesis.

Note, no attempt is made to correct data for any reduction in catch effort above the 80% threshold (i.e. for those visits deemed made but in which catch effort was reduced by up to 20%), as this information is difficult to obtain. However, as most frequently it is the end of a visit period that is prematurely curtailed, the time in which numbers of birds captured have typically declined, such reduction in the catch effort is believed to incur only a small proportional cost to catch and re-sighting numbers (Miles et al., 2007).