Camphuysen C. J., 2013.
A historical ecology of two closely related gull species (Laridae): multiple adaptations to a man‐made environment.
Ph.D. thesis, Univ. Groningen, Groningen. 421 p.
In this thesis, results are presented of a comparative study of the foraging ecology, breeding biology, foraging distribution, and population dynamics of two sympatric large gulls, the European Herring Gull Larus argentatus (HG) and the Lesser Black-backed Gull Larus fuscus (LBBG), breeding at Texel in the western Wadden Sea. The work aimed at understanding recent, contrasting population trends: a decline in HGs and a continuing increase in LBBGs. This was evaluated using three hypotheses: (1) HGs being outcompeted by a newcomer, the LBBG, (2) LBBGs profiting from an empty niche; population trends unrelated, or (3) overwinter survival is responsible for differences in demographic trends (Chapter 1).
The relaxation of the Victorian exploitation with its ruthless destruction of wildlife is one of the main factors explaining the rapid growth of many populations of (sea-)birds in the 20th century, large gulls included. Only two species of gulls nested in The Netherlands around 1900, while 10 species occur as breeding birds today. Gull breeding populations reached unprecedented levels in the late 20th/early 21st century: HGs peaked in the 1980s, LBBGs around 2005. Multiple causal factors have been at work simultaneously to effectuate these population changes. Four episodes of major change were identified: (1) early measures of conservation in the early 20th century, (2) a resumption of persecution in the 1930s to 1960s, (3) a relaxation of persecution coinciding with a general recognition of the effects of chemical pollution in a period when anthropogenic (unnatural) resources became very large, and (4) population declines or reduced growth in the late 20th century when landfills were closed and when fisheries discards started to decline (App. 2).
Contrary to expectations, the reproductive success of LBBGs was very low, particularly as a result of high levels of chick predation (cannibalism). The breeding success of HGs was consistently higher than that of LBBGs and levels of chick predation were much lower (App. 3). Productivity indices for both species were poor-moderate (<50% of the eggs laid produced fledglings). LBBGs were clearly more marine orientated than HGs and the diet of the former was dominated by discarded fish species. HGs utilised intertidal resources extensively and added fish prey to their diet during chick care (Chapter 2).
Cyclic fluctuations in chick growth were found in both species: similar drops and gains in body mass, indicating alternating periods of low and more favourable provisioning. Body mass increments of chicks were significantly reduced in weekends and enhanced during the week. A weekly rhythm in commercial fishing effort matched these patterns exactly (Chapter 3).
With a supplementary feeding experiment, it was investigated if parents would increase the time devoted to chick defence when the needs for chicks to feed were lowered artificially. To measure nest attendance, adults were instrumented with radio-transmitters. The results were ambiguous: nest attendance of supplemented pairs was enhanced in one study plot, but reduced in another. Although supplemented birds fledged more young than controls, the difference was not significant. Chick predation and fledging success were significantly different between study plots. Autocatalytic interactions and collective, site-specific differences in the ability to defend chicks from attacking adults may explain the results (Chapter 4).
The timing of first returns, prospecting, egg-laying, hatching, chick care, when the breeding areas were abandoned in autumn, and changes therein over the years was examined in LBBGs. The birds were geographically segregated by age throughout their annual cycle and southward movements after the breeding season had a different start in immatures (early), adults (intermediate) and juveniles (late). Juveniles wintered furthest to the south. The timing of spring migration was early in adults, intermediate in immatures, and late in juveniles. There was no evidence that one of the sexes returned earlier and prospecting breeders in the colony were synchronised with Herring Gulls. The seasonality in fisheries effort could not explain the onset of breeding, but an annual rise in effort in June coincided with the earliest phase of chick care. A comparison between older (<2000) and more recent data (≥2000) indicated an overall shortening of the time spent at breeding latitudes by one month (Chapter 5).
HGs colour-ringed as chicks (n= 3124) in 12 colonies in 1986-88 yielded 86,247 ring-readings on 1358 locations by 868 observers. One-fifth of all sightings originated areas within a radius of 5km from the ringing site. Only 0.8% were reported at over 300km from the natal colony (10.7% at 6-10km, 8.9% at 11-25km, 17.7% at 26-50km, 22.9% at 51-100km, 14.4% at 101-200km, and 4.1% at 201- 300km). Colony-specific differences in travelling distance, dispersal rate, and direction of movements suggested a grouping of colonies in three: (1) eastern Wadden Sea islands (Rottumeroog-Vlieland), with higher dispersal rates and winter movements mostly south-west to south-east, (2) Texel and 4 mainland coast colonies (Callantsoog-Wassenaar), with a shorter mean range and movements mostly to the south, and (3) colonies in the Delta area (Europoort- Saeftinghe) with short range movements and winter dispersal in many directions. Adults reached their greatest mean distances on average one month earlier than immatures, which in turn arrived one month earlier than juveniles. In spring, adults moved on two months ahead of immatures, which in turn moved earlier and closer to the natal home-range than juveniles. Herring Gulls breeding in The Netherlands occupied a mid-position between dispersive and sedentary tendencies (Chapter 6).
In HGs, a mean apparent annual adult survival was calculated of 79% in females and 86% in males. Additive year effects provided highest model support in LBBGs, in which apparent survival for both sexes combined varied between 81% and 100% (mean ≈91%) (Chapter 7). These findings, in combination with estimated return rates of recruits replacing adults that died (App. 6), led to estimated of a balance per annum (BPA) of -0.08 [i.e. population growth rate lambda = 0.92) in LBBGs and -0.23 (lambda = 0.77)in HGs (Chapter 2).
Tracking studies revealed exceptionally long or distant foraging trips in actively breeding LBBGs. The characteristics, frequency, and triggers of unusual trips were evaluated. Exceptionally long and distant trips occurred irregularly but annually, in many individual birds and in all phases of breeding. Additional time for individual maintenance rather than extra effort in chick provisioning was suggested as an explanation. Exceptional trips were relatively rare when the chicks were still young and highly vulnerable (<10d of age). The reproductive success in birds that performed exceptional trips was not compromised (Chapter 8).
Inter-pair dietary specialisations were studied in HGs and related to fecundity parameters. The overall dietary spectrum and levels and incidence of dietary specialisations changed when energetic demands increased during chick care. Prior to hatching, most pairs focussed on bivalve prey, but 25% of the pairs had distinct dietary biases. During chick care, both chick growth and fledging rates were positively correlated with the amount of fish prey provided. Prey spectra diversified overall during chick care and fewer specialists were detected. Particularly low chick growth rates were found in pairs that had a dietary bias towards crustacean prey (Chapter 9).
The GPS tracking data from 34 LBBGs encompassing 2199 foraging trips combined with dietary information and reproductive status showed that there are sexually distinct foraging strategies, that are maintained throughout incubation and chick-care. The marginally larger males travelled farther from the colony than females, spent more time in the North Sea. Males fed mostly on fisheries discards at offshore trawlers with few alternative resources nearby. Females foraged predominantly on land or in the Wadden Sea, where they had multiple foraging options including nearshore shrimpers. Foraging range, trip duration and the proportion of time at sea increased with wing length. Our findings did not support the usual inference that sexual segregation is mediated primarily by differences in competitive strength (i.e. differences in size) as both sexes foraged in competitive environments around fishing vessels. Females, accessed a wider variety of resources and a broad prey spectrum, by exploring a whole suite of foraging opportunities and habitats nearer the colony (Chapter 10).
The distribution and feeding range of HGs and LBBGs were assessed, using ship-based surveys in the southern North Sea. Both species occurred in association with commercial fishing vessels and their distribution is described in relation to distance to the coast and distance to the colonies. The feeding range of LBBGs (95% of all birds within 135 km of the colony) was considerably larger than that of HGs (95% within 54 km), a difference that could not be explained by differences in flight capacities. Neither the vulnerability to robbery indices, nor the feeding success indices of both species supported earlier suggestions that LBBGs have outcompeted HGs at fishing vessels. The feeding range of LBBGs could not solely be explained by a general avoidance of HGs near the coast, nor by fisheries further offshore in comparison with the coastal zone (Chapter 11).
Adult LBBGs were tracked during the breeding season using a high resolution GPS. The study revealed intriguing yet infrequent behavioural patterns indicating birds sat on the sea surface, drifting passively with the tidal current, for several hours: resting at sea rather than in the breeding colony. It is assumed that the drifting birds were resting (asleep), and possibly, a good rest, away from the colony, is essential for individual birds (Chapter 12).
Because the designation of marine protected areas (MPAs) for seabirds should bear relevance to the ecological importance of these regions, simple presence/absence information of the occurrence at sea is not sufficiently accurate and could even be misleading. Large amounts of data have been collected to assess distribution patterns of seabirds around the world. Ship-based surveys and sensor data (tracking studies) from instruments attached to individual birds were used to evaluate recently developed protocols to facilitate spatio-temporal quantification of space use and identification of behaviour in the context of MPA designations. Limitations of traditional techniques and exciting possibilities of new protocols and the latest generations of electronic devices attached to seabirds are presented. The advanced approaches in data collection and spatial analysis is expected to enhance our understanding of offshore seabird distribution and their activities, which can guide the designation of marine protected areas (Chapter 13).
The occurrence of mammalian prey in the diet of both gulls was investigated to quantify the predation on mammals, comparing gulls breeding in coastal and inland colonies. Coastal nesting specialised birds and a majority of individuals in an inland colony were found to commonly feed on mammalian prey. Most prey were obtained on inland fields, perhaps during ploughing or similar activities of farmers, some may have been captured within the colonies, and some will have been the result of scavenging at roadsides. Since gulls breeding along the coast increasingly suffer from shortages of discarded marine fish during chick- rearing, inland breeding may become more frequent and gulls are expected to increasingly focus on alternative foraging habitats and prey species, mammals included (Chapter 14).
All relevant parameters describing the breeding success are summarised in App 3. Assuming that fluctuations in resources during breeding would affect fecundity, the fact that reproductive success varied independently between the two species is interpreted as an indication that key resources were different. In fact none of the key parameters expressing breeding success, perhaps with the exception of chick depredation, were somehow correlated between the two gulls (Chapter 15).
Evidence is provided for resource partitioning between and within the two species: LBBGs were numerically dominating in continental mainland foraging sites, within the colony, inland at Texel, and offshore on the North Sea. Male LBBGs utilised offshore resources, females foraged closer to the colony. HGs numerically dominated over LBBGs in all intertidal areas, including the deeper gullies (subtidal areas) of the Wadden Sea. At sea, with increasing distance to the North Sea shoreline HGs were increasingly outnumbered by LBBGs. If the key habitats are seen as resources, the numerical abundance of each species could seen as a proxy for resource use. The species’ exploitation strategies are apparently discretely segregated rather than opportunistic. Each of the key habitats (or resources), are shared with different assemblages of competitors targeting the same or similar prey (guilds). Within these guilds dominance hierarchies become established and in most of the scenarios, HGs and LBBGs are among the largest (most powerful) species. The enormous variety of prey items require highly different skills and in each of these resources more specialised and efficient competitors are encountered (Chapter 15).
The at-sea studies gave an important insight in the abundance and distribution at sea. Foraging behaviour and prey choices were studied in a competitive setting: both species were confronted with the same prey in experiments with different numbers of competitors, around the North Sea and through the year. HGs were the more successful foragers at trawlers (competitive strength). The abundant HGs at sea in winter are probably wintering birds from Nordic colonies rather than local residents. In summer, LBBGs become numerically dominant at some distance away from the coast. It is doubtful of HGs ever foraged at distant trawlers, and fish prey delivered to the chicks today originated from different fishing vessels than that of most LBBGs (App 7, Chapter 15).
It is obvious that the breeding populations of both species are currently not flourishing. Even in HGs, where the reproductive success is often reasonably high, the low survival of adults, immatures and fledglings and the low recruitment rates lead to a negative BPA. The near-absence of even only prospecting “recruits” and nesting birds in immature plumage at Texel suggests that these colonies are currently “ageing” (Chapter 15).
The recent population trends of HGs and LBBGs breeding in the Western Wadden Sea fluctuate more or less independently, even though the birds do compete for certain resources (no support for hypothesis #1). HGs currently have more favourable breeding results, but relatively poorer winter survival and recruitment rates. It is concluded that winter mortality may have increased as a result from reduced access to open landfill areas and intensified intra-specific competition within their wintering range (supporting hypothesis #3) (Chapter 15).
LBBGs have profited from a fishing fleet modernisation in the 1960s: a type of fisheries that has generates vast amounts of discards, but that peaked in the late 1980s and early 1990s. This must have been “vacant niche” (supporting hypothesis #2). The problems that LBBGs currently face at Texel (low breeding success, high levels of cannibalism), are signs of regular food shortages during breeding. Recent reductions in fishing effort will have led to reductions in the amount of discards produced at sea; a key resource for this species. Further declines in discards as a resource can be foreseen because the European Commission proposed a complete ban on discarding, to be effectuated sometime between now and 2019 (Chapter 15).
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