The influence of colour-rings on recovery rates of Herring and Lesser
C. B. Shedden, P. Monaghan, K. Ensor and
N. B. Metcalfe
Ringing and Migration. (1985) 6: 52-54.
Comparison of recovery rates of Herring and Lesser Black-backed Gulls
with and without colour-rings in addition to a metal ring shows that colour-
ringed birds are twice as likely to be recovered and reported to the British
Trust for Ornithology. Implications for studies of mortality rates are
C. B. Shedden, P. Monaghan, K Ensor and N. B. Metcalfe, Zoology Department,
Glasgow University, Clasgow G12 8QQ, UK
Received 18 October 1984; revised and accepted 6 December 1984.
The most commonly used methods of estimating wild bird mortality rates
rely upon ringing recoveries, and are based on "ratio" or "maximum
likelihood" analyses. An underlying assumption of such analyses is that the
probability of a ringed individual of a particular species being reported
when it dies is the same for all age classes, and constant between years
(Anderson et al. 1981, Lakhani and Newton 1983). Differences between age
classes in recovery rates are thus held to reflect differences in mortality
rates. This assumption may not always be valid, since behavioural
differences between age classes may affect the recovery rate. A further
problem may be introduced if the method of marking influences the
probability of recovery. GossCustard et al. (1982) suggested that the
presence of colour-rings may increase the conspicuousness of corpses and
thereby increase the recovery rate, and Harris (1984) reported this to be the
case for adult Puffins Fratercula arctica. We report a similar
enhancement of the recovery rate of colour-ringed Herring and Lesser
Black-backed Gulls Larus argentatus and L. fuscus, and discuss
how this affects the interpretation of ringing recoveries.
This study examines the recovery rates of Herring Gulls caught outside of
the breeding season (between October and March) at refuse tips in the
Strathclyde and Central Regions of Scotland, and Lesser Black-backed
Gulls caught in the same area during the breeding season (between March
and September). All birds caught were ringed on the tarsus with a British
Trust for Ornithology numbered metal ring. A proportion of the birds were
additionally marked with individual combinations of either three or four
Darvic colour-rings, 15-20 mm. in height. All recoveries were received in
the normal manner through the B.T.O. ringing scheme.
To avoid any biases resulting from differential mortality rates between age
classes, only birds ringed as adults are considered here. Different
proportions of the captured gulls were colour-ringed in different years. It
was essential therefore to ensure no further biases were introduced due to
birds ringed by the two methods being eligible for recovery over different
periods of time. This was done by keeping the recovery period constant.
Thus the recoveries of Herring Gulls ringed between October 1978 and
March 1979 have been analysed over the period April 1979 to August 1983
and those for birds ringed between October 1979 and March 1980 have
been analysed for the period April 1980 to August 1984. In each case the
recovery period extended over 53 months. Lesser Black-backed
Gulls ringed between March - September from 1979 to 1981 were
considered for this analysis only if the recovery was made in the 36 month
period after the 30 September in the year of ringing.
To check whether or not there were any differences in mortality rates of
birds marked by the two methods, we examined the rate at which we
recaptured metal and colour-ringed Herring Gulls during the periods
specified above. To ensure an adequate sample size, birds caught throughout
central Scotland were included; catches were made at least once a month
during this time. Since all of the birds were caught at refuse tips, we have
treated males and females separately, to avoid any bias due to differences in
the extent to which they use refuse tips, and thus in the probability of their
being recaptured (Monaghan 1980). Birds were sexed using the head and
bill measurement as specified by Coulson et al. (1983).
Table 1 gives the recovery rates of metal ringed Herring Gulls with and
without colour rings, during the 53 month period specified. The differences
between the proportions recovered is significant (X2 = 9.1, 1 df, p < .01),
with over twice as many of the colour-ringed birds being recovered as
compared with the metal ring only birds. There was no significant
difference in the recapture rates, and presumably therefore mortality rates,
in relation to marking method for either males or females (Table 2).
TABLE l.The proportions of Herring Gulls ringed as adults with metal
rings, and metal plus colour rings. which were recovered in a 53 month
TABLE 2. The proportions of male and female Herring Gulls ringed as adults
with metal rings,and metal plus colour-rings, which were recaptured in a 53 month
period. There were no significant differences in the recapture rates for either
males or females (males X2 = 0.16, 1 df, NS: females X2 = 0.02, 1 df, NS).
Table 3 gives the recovery rates of metal ringed Lesser Black-backed Gulls
with or without colour rings, during the 36 month period specified. Again,
almost twice as many of the colour-ringed birds were recovered as
compared with the metal only birds, though in this case, due to the smaller
sample size, the difference is not statistically significant (X2 = 2.4, 1 d.f.).
These data clearly demonstrate that the presence of colour-rings greatly
enhances the proportion of ringed, dead birds which are reported to the
British ringing scheme. This is presumably because the presence of colour-
rings attracts the attention of the casual observer to the dead bird, and thus
increases the probability of the metal ring being noticed and reported. These
findings have implications for the calculation of mortality rates based on
ringing recoveries. For example, if differing proportions of birds have been
colour-ringed during two time periods, a comparison between the two
periods may lead to the erroneous conclusion that mortality rates have
changed, when it is in fact the recovery rate which has altered.
TABLE 3. The proportions of Lesser Black. backed Gulls ringed as adults
with metal rings - and metal plus colour rings, which were recovered n a 36
No colour rings
Similarly, if the proportion of birds of different age classes which carry
colour-rings is not the same, as could arise due to loss of colour-rings by
older birds or differential colour-ringing at the outset of a study this
could lead to additional errors in the calculation of mortality rates. Thus
care should be taken by ringers to ensure that comparisons of recovery rates
are made only between birds marked in the same manner.
We would like to thank the British Trust for Ornithology for use of
recovery details, District Authorities in central Scotland for access to
rubbish tips, all those who assisted us in catching gulls, and Drs. R. W.
Furness and J. C. Coulson for comments on an earlier draft of the
ANDERSON, D. R., WYWIALOWSKI, A. P. and BURNHAM, K. P. 1981. Tests of the
assumptions underlying life table methods for estimating parameters from
cohort data. Ecology 62: 1121-1124.
COULSON, J. C., THOMAS, C. S., BUTTERFIELD J. E. L. DUNCAN, N., MONAGHAN, P.
and SHEDDEN, C. B. 1983. The use of head and bill to sex live gulls Laridae.
Ibis l25: 549-557.
GOSS-CUSTARD, J. D., Le V. dit DURELL, S. E. A., SITTERS, H P. and SWINFEN, R.
1982. Age structure and survival of a wintering population of Oystercatchers.
Bird Study 29: 83-98.
HARRIS, M. P. 1984. Movements and mortality patterns of North Atlantic Puffins
as shown by ringing. bird Study 31: 131-140.
LAKHANI, K. H. and NEWTON, 1. 1983. Estimating age-specific blrd survival
ratios from ring recoveries - can it be done? J. Anim. Ecol. 52: 83-92.
MONAGHAN, P. (1980). Dominance and dispersal between feeding sites in the
Herrng Gull (Larus argentatus). Anim. Behav. 28:521-527.