Volume 68 Number n November 1975

Mortality in the Black-headed Gull J. J. M. Flegg and C. J. Cox Details of the dispersal of Black-headed Gulls Lams ridibundus ringed as nestlings in colonies in Britain and Ireland were given in Flegg and Cox (1972). That paper compared the results from an intensive study at colonies on the Rivers Medway and Swale in Kent during 1956-65 with those gathered by the national ringing scheme, now run by the British Trust for Ornithology, during 1908-69. The same comparison forms the basis of this paper, using recoveries from the study in Kent to 1974 and from the national scheme to 1972.

There are intrinsic problems in a study based on the recoveries of a nidifugous species ringed at a variety of ages between hatching and fledging. In the Black-headed Gull, nesting colonies are largely associated with estuarine or freshwater habitats and an unknown and variable proportion of the young die before fledging. There is an inevitable bias in the association of Man with any recovery: those birds dying directly or indirectly at the hand of Man, or from other causes in close proximity to him, are clearly much more likely to be reported than those dying in remote areas, for example well out to sea or at moorland colonies. Biases of this nature cannot be eliminated, and should not be forgotten, but meaningful indications of change in many recovery circumstances can be derived from comparative studies, of area with area, or of one time period with another. Of course, it is quite possible that recoveries are representative of the population as a whole despite their association with Man, and this assumption has been made here. Field observations show that many Black-headed Gulls are positively associated with Man

[Brit. Birds, 68: 437-449, November 1975] 437

438 300.

Mortality in the Black-headed Gull

J A S O N D J F M A M J recovery month

Fig. i. Monthly totals of recoveries of Black-headed Gulls Lams ridibundus (top figure first-year, centre second-year, bottom adults), comparing those from the Kent study during 1956-74 (broken lines, sample sizes on left) with those from Britain and Ireland as a whole during 1969-72 (solid lines,

sample sizes on right)

throughout the year—on farmland, in towns, at sewerage outfalls, on refuse tips and, during the breeding season,, at colonies on industrialised rivers or estuaries or close to recreational activities such as sailing.

Mortality in the Black-headed Gull 439 With a long-lived bird, additional factors assume importance:

for example, the life-span of the ring may be shorter than that of the bird (Harris 1964, Coulson and White 1955). For much of the study in Kent, abrasion-resistant molybdenum/nickel alloy rings were in use, although some may have suffered more corrosion than the lighter alloys previously used. Unlike those of the Kittiwake Rissa tridactyla, the normal life style and colony situation of the Black-headed Gull are not considered to cause excessive ring wear. An examination of returned rings indicates that, even with aluminium alloys, wear is usually minor until about eight years have elapsed and becomes serious only after 14 or 15 years.

W H E N D O T H E Y D I E ?

Fig. 1 compares the months of death of nesdings from the Kent study area with a 1969-72 sample of the national recoveries. The 'year' starts with July—the peak month for fledging—and ends in June. The majority of Black-headed Gulls do not breed in their second year (Flegg and Cox 1972), so first-year, second-year and adult or potential breeding birds are considered separately. Although the number of second-year recoveries is small, and little reliable pattern emerges, the Kent study is closely similar to the sample of national recoveries in the other age groups. The patterns of first-year birds and adults differ greatly. As expected, mortality in the first year is high immediately after fledging, due to a variety of causes (table 2) doubtless often associated with the birds' inexperience. After September, mortality remains surprisingly low, especially in the Kent birds, many of which endured the 1961/62 and 1962/63 severe winters. In this connection, it seems probable that many of the dead Black-headed Gulls reported by Dobinson and Richards (1964) in their assessment of the effects of the 1962/63 winter were of Continental origin.

There is more winter mortality in the adults, perhaps associated with their lesser range of movement at that season (Flegg and Cox 1972), but here the peak is in the breeding months of May-July. Table 1 indicates the proportion of recoveries reported as 'dead in colony', many of these being 'egg-bound' or 'dead on nest'. If this peak represents a disproportionate female mortality in the breeding season, it is interesting to speculate on that of the males: is it uniform throughout the year, or is it also influenced by the rigours of obtaining and holding territory, and later of feeding the young ?

Harris (1962, 1964) found that the peak first-year mortality of Herring Gulls L. argentatus was during August-October and of Lesser Black-backed Gulls L. fusctis during September-November. These later peaks may be partly due to the larger sizes (and thus increased incubation, fledging and dependent periods) of these

440 Mortality in the Black-headed Gull

species and partly also, in the case of the Lesser Black-backed Gull, to its migratory pattern delaying the breeding season and exposing the population to hunting pressure on the Continental coast. Harris also found a slightly later mortality peak among older Herring Gulls, in July-August, than is apparent in adult Black-headed Gulls.

Fig. 2 shows the distribution of recoveries from the Kent study by years of life, including all recoveries reported up to December 1974 of birds ringed during 1956-65 (it has been assumed that the great majority of recoveries from these nestlings will have occurred by then). The high first-year rate is to be expected, but there are

Table 1. Reported causes of deaths of Black-headed Gulls Larus ridibundus as percentages of the total recoveries in each of four periods

All ringed as nestlings in colonies in Britain and Ireland and recovered during 1908-72. "Includes some taken under licence by the Department of Trade and Industry, f Includes some (perhaps as many as half) trapped and subsequently

killed by Man

1908-24 1946-52 1953-68 1968-72

Found dead Shot

Trapped/killed by Man Injured/died later Dead under wires Dead on road Killed by train Killed by aircraft Killed by predator Dead in colony Found oiled Other causes

Found alive

29-1% 37-2%

8.6% 8 . 1 % 2 .1%

0.6% —

i.7°/o 4.4% —

i . 3 %

7-i%t

56.5% 7-2%

0.5% 13-7% 3-6% 1.4% 1.0% 0.2% 1.2% 3-6% 0 .2% 1.0%

9-6%

60.0% 6.0%

0.6% 6.8% 3-9% 2.6% 1.1% 0.3% 2.9% 2.4% 0.7% 1.4%

10.4%

68 .1% 4-4%

1.8%* 3-9% 3- i% 4.6% 0.3% 0.6% 2.2% 3-5% 0.8% 0.8%

5-9%

SAMPLE SIZES 52O 416 I,8ol 649

also indications of adult mortality peaks at five to six years and again at about nine years. It could be suggested that the nine-year peak represents some sort of physiological end-point of reproductive life, with only increasingly exceptional individuals breeding beyond their eleventh or twelfth year. The hump at five and six years is more difficult to explain, but again could be due to a physiological cause, perhaps affecting only one sex. Known females are to be found in colonies, and at least laying eggs, in most year classes up to and beyond ten. Much remains to be discovered from a study of mortality of known males.

Mortality in the Black-headed Gull

% 3Qr7s

441

Fig. a. Percentage distribution, by year of life, of 351 recoveries of Black-headed Gulls LOTUS ridibundus from the

Kent study during 1956-74

HOW DO THEY D I E ? When considering as wide a time span as 1908-72 in table 1, changing human attitudes to, for example, the shooting of non-game species should be remembered. These may account for a little of the altered proportions of birds reported as 'shot' compared with those 'found dead'. It is thought that many finders of ringed birds, especially in highly literate and conservation-conscious countries such as Britain and Ireland, do consider and remark on possible causes of death when reporting the ring. Clearly the most striking change is in the number of birds found shot, which in 1908-24 was the major reported recovery cause. With the cessation of this adverse pressure exerted by man, it is presumed that other, more natural factors assumed the dominant role. Jennings (1955, 1958), Jennings and Soulsby (1956, 1957) and Macdonald (1963) all reported on post-mortem examinations of Black-headed Gulls sent to them by members of the BTO. Of 31 cases, one died due to adverse weather, two to trauma, five to poisons, and five from unknown causes, while diseases claimed 18. Jennings and Soulsby (1958) examined young from a Norfolk colony and suggested that infectious disease and especially parasite burden would have had a considerable adverse effect on their survival. The impact of parasites may not, of course, become fatal until after dispersal from the breeding colony.

Conditions such as these are exceedingly unlikely to be noted by die public, but some more easily recognised causes are reported. Contrasting the four time periods in table 1, a fall in the proportion trapped and killed by Man towards the present day (parallel to the drop in numbers shot) is clear. There are rises in the proportions of victims of road and air traffic, which are to be expected in the

442 Mortality in the Black-headed Gull

Found dead

7i-3% 72-7%

Shot or taken

5-7% 9-4%

'Accident'

••9% 3-6%

Wires etc

5-7% 4-5%

Road victim

10.7% 5-o%

Rail victim

0.6% 0.8%

Aircraft Found victim oiled

0.6% 0.3% n % 0.3%

Predators

3-i% 2-5%

SAMPLE SIZES

3'7 359

Table 3. Reported causes of deaths of juvenile Black-headed Gulls Lotus ridibundus as percentages of the total recoveries in the first six months of life contrasted with

those of adults aged at least two years, all during July-December All ringed as nestlings in colonies in Britain and Ireland and recovered during 1967-74

Juveniles Adults

light of our dramatically increased use of these forms of transport. It could be argued that the differences between 'found oiled' and 'dead under wires' are a reflection of finder bias but, with the increasingly rural rather than marine habits of the Black-headed Gull, these may be a fair representation of the situation. The handful of birds killed by falling sods of earth while following the plough supports this view. Nevertheless, the Black-headed Gull does of course still frequent coastal waters and there the increase in oiling is the expected situation today, however sad, though some of the birds in this category may have been cast ashore and fouled with oil after death. The proportions of 'dead in colony' and 'killed by predator' remain consistently quite high and these deaths are reflected in the peak breeding season mortality shown in fig. 1.

Harris (1962) found that approximately 50% of Lesser Black-backed Gulls were 'found dead', some 40% shot or killed by Man (three-quarters of these overseas) and only 3% killed by collision with wires or vehicles. Within Britain and Ireland the proportion of Lesser Black-backs 'shot' fell from 57% in 1910-19 to 6% in 1950-59, while about 60% of overseas recoveries were 'killed', indicating the penalty paid by migration to foreign waters.

MORTALITY Studies of mortality (or survival) based on recoveries are subject to the same biases as investigations of the causes or whereabouts of the deaths concerned. Lack (1951) considered, with stated exceptions, that little serious error would emerge from assuming that the recovered proportion was typical, and produced a simple calculation procedure with minimal assumptions. Haldane (1955) amplified Lack's techniques, but Eberhardt (1972) considered that 'the Ghapman-Robson (Lack) equation for estimating survival from banding recoveries is clearly the best of those studied'. The following calculations are based on Lack's technique, accepting that a small bias will occur as a very small proportion of birds ringed in recent years have yet to be recovered. It has been possible to compare recoveries from well-defined groups of gulleries. In fig. 3, the results

Mortality in the Black-headed Gull 443

«—* southeast •—• Yorkshire *—* Wales

mean

5 6 7 8

YEAR OF LIFE

Fig. 3. 'Extinction curves' showing mortality rates of a nominal 100 Black-headed Gulls Larus ridibundus from various regions of Britain and Ireland. 'Southeast! and 'northwest' refer to England, the latter comparing the years from 1945 (1945-72) with those to 1924 (1908-24)

444 Mortality in the Black-headed Gull are presented as 'extinction curves' starting from a nominal ioo birds and assuming no recruitment. Most of the regions considered conform well with one another, but Yorkshire colonies show a lower rate of mortality during the first four years.

In general, the Black-headed Gull is an expanding species (Gribble 1962 and in press), by some 2 5 % between the censuses of 1938 and 1958 and by over 100% between 1958 and 1973. With an average annual mortality of about 24% over the first five years of reproductive life, the doubling in 15 years recorded by Gribble would be achieved if every 100 pairs of Black-headed Gulls produced 50.5 young each year that subsequently survived to breeding age. As the mean clutch size falls between two and three eggs, and as at least two replacement clutches can be produced, this seems easily achievable. The number of 'successful eggs' per 100 pairs to maintain a steady population over the whole of Britain and Ireland is 48.2, indicating just what a difference results if this figure is increased by less than 5 % .

The rate of expansion in Hampshire is considerably greater. Cohen and Taverner (1972) described an increase in the number of pairs at Needs Oar , the major colony in that county, from 75 in 1938 to 1,130 in 1957, 6,516 in 1965, 10,500 in 1968 and about 20,000 in 1971, this despite regular collection of eggs until early in May. This rate of expansion characterises several other colonies in that area. Regrettably, the sample of recoveries from Hampshire colonies is small, but shows indications of an unusually high first-year mortality. A 17-fold increase over 14 years would necessitate the production of rather more than 6 0 % 'successful eggs' additional to the norm, assuming the same rates of adult mortality. Whether (locally) this success rate is achieved or mortality rates differ considerably, or whether (unusually) recruitment occurs from outside the area, remains to be determined by an intensive study: as yet there is insufficient evidence one way or the other.

Comparable figures are available for the Kittiwake (Coulson 1974), a similar-sized but marit ime gull. During 1959-69 numbers of nesting Kittiwakes (based on a large sample of colonies) increased by 49%. Coulson and White (1959) calculated adult mortality at about 12.4% and first-year mortality at 2 1 % , though problems of ring wear caused them to use sight records of colour-ringed birds in these calculations. The nest site and chick behaviour of the Kittiwake are better suited than those of the Black-headed Gull to a study of pre-fledging mortality, and Cullen (1957) found an average of 1.18 young fledging from the normal clutch of two eggs. This is probably considerably in excess of the figure for Black-headed Gulls, and certainly higher than the 0.51 or less reported for the Herring Gull (Paynter 1949) and Lesser Black-backed Gull (Paludan

Mortality in the Black-headed Gull 445 1951). If Cullen's figure is representative of all Kittiwake colonies, then there is more than enough productivity to produce the increase documented by Coulson.

Fig. 3 also compares mortality rates from colonies in north-west England of birds ringed during 1908-24 and 1945-72. The pre-1924 curve differs considerably from the post-1945 picture, which is close to the national norm. Lack (1943) showed that before 1940 shooting was a material mortality factor in this area, and that the effects were greatest on first-year birds, possibly biasing the mortality figures. Reference to table 1 gives some indication of the reduction in birds 'shot', and it may be that this plays a major role in the changed pattern. In Lack's analysis, 4 4 % of the recoveries were of birds shot, but table 3 shows clearly that at present first-year birds suffer, if anything, less than adults from shooting.

Estimates of mortality rates in various years of life from regions of Britain and Ireland are given in table 3. 'Year' 1 here is the first six months of life, July-December, while subsequent years are January-December. Beyond ten years (and in some cases earlie'r) sample sizes are too small for percentage mortality figures to be meaningful. The overall mortality is high in the first 'year' of life, the rate falling to a low point in the third, fourth and fifth years before rising again in the sixth and eighth to tenth years, when the rate is about twice that in the first three years of breeding life (few breed at one year old, i.e. in their second year).

For the last four years of the intensive study in Kent , recoveries 'yet to come' were calculated from the results of the first six years,

Table 3.

Year

1 2

3 4 5 6 7 8 9

10

, Annual rates of mortality of Black-headed Gulls Larus ridibundus from various

All ringed as nestlings

SE England

35-o% 25-5% 2 4 . 8 % 18 .5% 2 3 - 5 % 3i-4% 27-8% 34-6% 50 .0% 47-o%

Yorkshire

28 .4% 2 0 . 4 % 16.0% 18 .3% 2 7 - 3 % 15-3% 24 .6% 2 6 . 2 % 2 6 . 3 % 39-3%

regions and periods in Britain and Ireland and recovered during 1908-72

NW England 1908-24

59-9% 34-2% 35-6% 4 0 . 6 % 43-2% 2 8 . 0 % 38.9% 54-5% — —

NW England 1945-72

35-5% 3i-4% 19-2% 19-1% 18.7% 3 4 . 0 % 27-4% 37-5% 2 3 - 3 % 39-i%

Wales

38.3% 4 2 . 9 % 29-2% 26 .4% 3 0 . 7 % 36.9% 1 4 - ' % — — —

Scotland

47-4% 2 8 . 1 % 15-0% 18 .9% 8.4%

14-5% 14-9% 3 0 . 0 % 25-0% 38 .1%'

Ireland

41-2% 2 0 . 8 % 2 2 . 3 % 35-o% 34-7% 4 4 . 2 % 36.8% 5°-o% — —

Total Britain/ Ireland 1945-72

38.3% 2 7 - 5 % 2 1 - 5 % 2 1 . 8 % 2 2 . 8 % 2 8 . 0 % 2 6 . 5 % 3 6 . 1 % 33-8% 44-3%

SAMPLES 458 253 434 425 136 230 221 1,855

446 Mortality in the Black-headed Gull

for which it could be said with reasonable certainty that all but the most exceptionally aged birds had been recovered by 1974. Table 4 thus gives an indication of the productivity over the ten-year period. Apart from 1956, when the study was single-handed, ringing effort was directed at marking all available young and was probably reasonably uniform, so that totals of nestiings ringed should be indicative of those produced. Unfortunately, no means existed of assessing the proportion of nestlings of ringable age that actually fledged. Over the ten-year period productivity varied considerably, with peaks in 1956, 1957 and 1965. Recovery rates also varied markedly from year to year, and the lower figures (e.g. 1959) probably reflected a high pre-fledging mortality. With the strong colony-faithfulness shown by Black-headed Gulls (Flegg and Cox 1972), it can be postulated that this irregular productivity, coupled with a long breeding life-span, was adequate to support a breeding population of about 2,000 pairs in 1955, doubling by 1961 (Humphreys 1963) and staying at approximately that level until at least 1966 (Humphreys 1967), while a census in 1972 showed a further increase to nearly 7,000 pairs in all (Harrison et al. 1973). Thus it can be argued that the highly productive years, 1956 and 1957, contributed to the rise recorded in the 1961 census; and that, after some relatively lean years, the most successful season of all, 1965, may have contributed largely to the striking rise reported in the 1972 census. Table 5 also shows mean life-span calculated from the ages in months of freshly-dead birds. Inexperience will take greatest toll before six months, and 21 months is the normal age of first breeding.

Table 4. Results to 1974 of ringing Black-headed Gulls Lams ridibundus in the Kent study area during 1956-65

Totals in brackets include calculated future recoveries

Year

TOTALS MEANS

R I N G E D T o t a l

R E C O V E R E D T o t a l R a t e

MEAN T O T A L L I F E - S P A N E X P E C T A T I O N ( M O N T H S ) AT

(a) Fledging (b) 6 months (c) 21 months

1956 ■957 1958 1959 i960 1961 1962 1963 1964 1965

49° • ,857

4 ' 7 890 538 755 695 279 422

3,240

3° 104

14 13 18 26

( 2 1 ) (9)

(17) (99)

6 . 1 % 5-6% 3-4% 1.5% 3-4% 3-4% 3-o% 3-2% 4-o% 3 - i %

34 41 44 37 63 46 41 5° 3 ' 35

5 ' 53 47 52 7i 59 64 63 5° 59

61 68 53 62

103 84 89 63 64 74

9.583 (351) — 3.7% 42.1 ± 9 . 4 56.9 ± 7 . 6 72.1 ± 15.4

Mortality in the Black-headed Gull 447 Table 5. , Annual variations in percentage recovery rates to 1967 of Black-headed Gulls

Lams ridibundus ringed in the Kent study area during 1956-65 The letters under the annual columns grade 'productivity', measured by the number of young

ringed, as good (G), moderate (M) or poor (P). No young were ringed in 1966-67

1st year Adult All ages

1956

37%

37%

G

1957

25%

24%

G

1958

7% «3% 19%

P

1959

3 1 % 2 1 % 22%

M

i960

"% 19% 16%

P

1961

2 3 % 15% 18%

M

1962

38% 25% 28%

M

19% 1964

2 2 % 4 1 % 26% 18% 28% 2 1 %

P P

1965

43% 30% 36%

G

1966 1967

17% 16% 17% 16%

MEANS

27.8 ± 12.4% 20.0 ± 5.4% 23-5 ± 7-3%

If productivity in a relatively long-lived bird can be as irregular as this, what of mortality ? Does this remain more or less uniform year by year, or are there striking variations? Table 5 examines this possibility, considering for each year the percentage of recoveries of first-year, adult, and all ages, reported from those 'at risk'. Particularly in first-year birds there are considerable differences, but the extremes tend to be associated with years when recovery numbers are either very low or very high. If the various years are graded for their 'productivity' (measured by the number of young ringed) as good, moderate or poor, then there are some indications of an association between the good years and high rates of mortality. This may be related to the fact that peak adult mortality tends to occur during the breeding season, and that in a highly productive year the stresses of raising young to fledging may be much greater than if eggs or small young are lost. As a counter to this, it could equally well be argued that the drain on resources of attempting second or third broods after the failure of the first would be greater than that involved in a successful rearing of the first brood. Without more detailed knowledge of the energetics of these situations, this intriguing question cannot be resolved.

A C K N O W L E D G E M E N T S We are indebted to many friends, especially David Musson and the Mid-Kent Ringing Group, for assistance with the detailed study in Kent. Catching young Black-headed Gulls can be both dangerous and exhausting, and our gratitude must be extended to all those ringers who, in Britain and Ireland as a whole over a period of 65 years, have marked the nestlings on which this comparison depends. Robert Morgan and C. J . Mead extended considerable practical help with the analysis, and discussions with Frank Gribble, John Taverner, Norman Pullen and Martin Davies have clarified both our thoughts and the text, for which we are particularly grateful. Finally, we are indebted to the BTO Ringing and Migration Committee for access to the recoveries.

S U M M A R Y Nearly 3,400 recoveries of Black-headed Gulls Larus ridibundus ringed as nestlings at colonies in Britain and Ireland are available for analysis of causes of death and rates of mortality. For first-year birds, mortality is at its peak in July and August; otherwise it is uniformly low in the first year, with little indication of hard weather

448 Mortality in the Black-headed Gull deaths in the winter months. For adults, mortality is greatest during May-August, presumably associated with the rigours of territory-holding and reproduction; it is low in the remaining months of the year, with a slight suggestion of a winter peak in January and February. If Kent recoveries are examined by year of life, there are indications of peaks in the fifth and sixth years, and again at about the ninth year. After the tenth year, recoveries are considerably fewer, the oldest being two in their sixteenth year.

Pre-fledging mortality in the Black-headed Gull is very difficult to assess, but the indications are that it is often considerable. Changing human attitudes are reflected in the sharp decrease in the numbers reported shot from 37% in 1908-24 to just over 4% in 1969-72. Since 1924, birds reported as 'found dead' have predominated, and currendy nearly 70% of recoveries fall in this category. Among other reported causes, there has been a four-fold increase in victims of collisions with overhead wires and various vehicles since 1908. An indication of the largely terrestrial habits of the Black-headed Gull can be gathered from the fact that more than five times as many were reported as road victims as were found oiled on the coastline.

Mortality rates can be estimated from six regions of Britain and Ireland, and are broadly similar. For the whole area, mean first-year mortality is 38.3%, second-year 27.5%, and average annual adult mortality about 24%. In north-west England there has been a considerable change from the 1908-24 pattern (where first-year mortality reached nearly 60% if possible bias is ignored) to that of the present-day (35.5%). With mortality rates of this nature, a doubling of the breeding population of Black-headed Gulls in 15 years is apparently not difficult to achieve, requiring only two or three extra eggs per 100 pairs (over and above maintenance levels) to succeed as far as breeding. In the Kent study area, the few and irregularly spaced years of highly successful breeding could be associated with stepped increases in the breeding population that followed about two years later.

Total lifespan expectations calculated from the intensive study in Kent range from 42.1 ± 9-4 months at fledging to 56.9 ± 7.6 months at six months (after the heavy post-fledging mortality) and 72.1 ± 15.4 months at 21 months (the age of first breeding).

REFERENCES COHEN, E., and TAVERNER, J . 1972. A Revised List of Hampshire and Isle of Wight

Birds. Oxford. COULSON J . C. 1974. 'Kittiwake'. In CRAMP, S., BOURNE, W. R. P., and SAUNDERS,

D., The Seabirds of Britain and Ireland. London. and WHITE, E. 1959. 'The post-fledging mortality of the Kittiwake'. Bird

Study, 6: 97-102. CULLEN, E. 1957. 'Adaptations of the Kittiwake to cliff-nesting'. Ibis, 99: 275-302. DOBINSON, H. M., and RICHARDS, A. J . 1964. 'The effects of the severe winter of

1962/63 on birds in Britain'. Brit. Birds, 57: 373-434. EBERHARDT, L. L. 1972. 'Some problems in estimating survival from banding data'.

In Population Ecology of Migratory Birds, U.S.D.I. Bureau of Sport Fisheries and Wildlife Report, 2: 153-171.

FLEGG, J . J . M., and Cox, C. J . 1972. 'Movement of Black-headed Gulls from colonies in England and Wales'. Bird Study, 19: 228-240.

GRIBBLE, F. C. 1962. 'Census of Black-headed Gull colonies in England and Wales, 1958'. Bird Study, 9: 56-71.

HALDANE, J . B. S. 1955. 'The calculation of mortality rates from ringing data'. Int. Orn. Congr., 11: 454-458.

HARRIS, M. P. 1962. 'Migration of the British Lesser Black-backed Gull as shown by ringing data'. Bird Study, 9: 174-182.

1964. 'Recoveries of ringed Herring Gulls'. Bird Study, 11: 183-191.

Mortality in the Black-headed Gull 449 HARRISON, J., HUMPHREYS, J . N., and GRAVES, G. 1973. 'Breeding birds of the

Medway estuary'. Kent Bird Rep., 1972: suppl. HUMPHREYS, J . N. 1963. 'Report of the Medway islands census 1961'. Kent Bird

Rep., 1961:42-51. 1967. 'The third census of birds nesting on the Medway and Swale islands'.

Kent Bird Rep., 1966: 79-98. JENNINGS, A. R. 1955. 'Diseases in wild birds'. Bird Study, 2: 69-72.

1958. 'Diseases of wild birds, fifth report'. Bird Study, 6: 19-22. and SOULSBY, E . J . L. 1956. 'Diseases of wild birds, third report'. Bird Study,

3: 270-272. 1957- 'Diseases of wild birds, fourth report'. Bird Study, 4: 216-220. 1958. 'Disease in a colony of Black-headed Gulls Larus ridibundus\ Ibis,

100: 305-312. LACK, D. 1943. 'The age of some more British birds'. Brit. Birds, 36: 214-221.

1951- 'Population ecology in birds: a review'. Int. Orn. Congr., 10: 409-448. MACDONALD, J . W. 1963. 'Mortality in wild birds'. Bird Study, 10: 91-108. PALUDAN, K. 1951. 'Contributions to the breeding biology of Larus argentatus and

Larus fuscus'. Vidensk. Medd. Dansk Naturh. Form., 114: 1-128. PAYNTER, R. A. 1949. 'Clutch-size and the egg and chick mortality of Kent Island

Herring Gulls'. Ecology, 30: 146-166.

Dr J. J. M. Flegg, British Trust for Ornithology, Beech Grove, Tring, Hertfordshire HP23 5NR C. J. Cox, Kake Cottage, Kake Street, Waltham, Canterbury, Kent

Mortality in the Black-headed GullWhen do they die?How do they die?MortalityAcknowledgementsSummaryReferences