Overview and future research

The freedom to fly gives long distance migratory birds the opportunity to exploit distant nutrient and energy sources in the course of their annual cycle. In particular, given endogenous sources of energy to sustain periods of flight, it enables or-ganisms to move between islands of abundant food resources across large expanses of wholly unsuitable habitat. At the same time, this very mobility creates patterns of energy and nutrient demands in the form of expensive migratory costs, met from storage accumulated during times of resource abundance. The synthesis presented here is helpful in conceptualising the annual cy-cle of the Greenland White-fronted Goose as a se-quence of discrete periods according to energy/

nutrient acquisition or demand. Typically, this re-lates to periods of storage of energy and/or nu-trients followed by short bursts of use of

accu-mulated stores (generally breeding events or mi-gration episodes). However, there are also peri-ods in the life cycle where extra demands (such as wing moult, or defence of body condition in early to mid-winter) can be met by exogenous supply (Figure 9.4).

It has frequently been asserted that evolution has minimised the overlap of energy- and nutrient-demanding periods of the life cycle of birds (the

“staggered costs” hypothesis coined by Lovvorn

& Barzen 1988). In the Greenland White-fronted Goose, this separation in time and space offers the possibility to specifically identify critical pe-riods in the annual cycle. In this way, it is possi-ble to assess the ability of individuals to reach critical condition thresholds in order to meet each of the specific demands they face in discrete peri-ods within the annual cycle.

From the nature conservation and research stand-point, such an opportunity is fortunate in offer-ing a framework by which to concentrate future study efforts. For each period of accumulation of

Figure 9.4. The annual cycle of an adult breeding female Greenland White-fronted Goose represented as a sequence of discrete calendar events, categorised as a series of periods of energy/nutrient acquisition or de-mand. These generally fall into three categories: (i) Periods of storage of energy and/or nutrients (shown cross-hatched in the bars above). (ii) Use of accumulated stores (generally breeding events or migration episodes, shown as dark bars). (iii) Periods in the life cycle where extra demands can apparently be met by exogenous supply (such as during wing moult, brood rearing or the defence of body condition in early to mid-winter, shown as shaded bars). Note that the period of brood rearing also represents a critical period for the female, during which she must recoup depleted stores and potentially reserves utilised during brood laying and incu-bation.

Winter accumulation of stores Spring migration to Iceland Accumulation of stores in Iceland Spring migration to Greenland Accumulation of stores in Greenland Egg laying Incubation Brood rearing Wing moult Accumulationof stores in Greenland Autumn migration to Iceland Accumulation of stores in Iceland Autumn migration to wintering grounds Winter maintenance

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

stores, the rate of acquisition (and hence condi-tion state by a given time) will be affected by a range of factors acting on the individual. For in-stance, on the wintering grounds, the level to which an individual can maintain suitable food intake rates in relation to maintenance expendi-ture is affected by primary external factors (such as food quality or abundance). This is then modi-fied by secondary factors (such as rates of ruption to feeding patterns through human dis-turbance or intra-specific interference). If we as-sume that the foraging ability of an individual is related to its ability to accumulate stores in an-ticipation of energetic expenditure, in the short term we can use measures of feeding efficiency and/or rates of change in individual condition to contrast the relative costs and benefits of dif-fering foraging situations. Such a comparative approach based on detailed observations of marked individuals offers the opportunity to con-trast, for instance, the ability of individuals of different status to accumulate stores in the pres-ence/absence of disturbance (Madsen 1995). It becomes possible to compare rates of change in body condition based on feeding on different habitat types, or examine differences in birds of different social rank. Viewed over longer time scales, the short term ability to maximise effi-ciency in store accumulation ensures not only the survival of the individual but ultimately the re-cruitment and lifetime reproductive output of the individual. Based on the accumulated life histo-ries of individuals, we can contrast differences in lifetime reproductive output as a fitness measure of the different strategies used by individuals throughout their lives.

On-going studies have already demonstrated the ability to detect differences in energy accumula-tion rates between Greenland White-fronted Geese using different grass swards during spring staging at the same site in Iceland (Nyegaard et al. 2001). From observations of collared individu-als, it is known that different individuals exploit different sward types, many showing consistent patterns between years (chapter 4). This (not un-expectedly) appears to influence the rate of change in abdominal profile scores of individual geese exploiting different sward types (MS18 and unpublished data). The accumulation of more individual life histories with details of habitat use, patterns of store acquisition and condition on departure from Iceland will enable the assessment of the fitness consequences from such foraging behaviour in the fullness of time. These linkages

between different elements in the life cycle are essential if we are to obtain a deeper understand-ing of how individuals perform in terms of sur-vival and reproduction measures with regard to the environment they exploit.

There are thus 3 measures available to assess in-dividual performance: the balance of food intake rate versus use over short periods, the rate of ac-cumulation of stores for completion of demand-ing episodes in the life cycle and ultimately the survival and reproductive output of the indi-vidual. It is possible to combine specific detailed investigation of these elements with the longer term historical resighting data, which provide records of how an individual has performed throughout its life. For geese ringed as goslings in their first winter, these records include which areas and habitats they exploited at different times of the year, when they separated from parents, how often a bird has changed wintering site, how often it returned to wintering areas with young and how long it lived. What is interesting is to see how individual decision-making can affect feeding efficiency, condition and, ultimately, fit-ness. Although Greenland White-fronted Geese are highly site loyal, birds do change wintering sites (MS9). In chapter 4, we saw how individual birds tend to specialise on a particular grass sward during staging in Iceland in spring, but some birds do show the ability to change from less nutritious swards to more profitable ones (Figure 4.9).

Hence, individual decision-making enables modi-fications to feeding efficiency, condition and fit-ness, and it is the consequences of these decisions which offer insight into how individuals behave and how this contributes to overall population behaviour (Figure 9.5). Combinations of histori-cal data and new investigations enable use of these measures to assess factors affecting indi-vidual breeding success and survival and an at-tempt is made to set out the major research objec-tives in Appendix 2.

The priorities for the immediate future are to con-tinue to monitor the patterns in numbers and dis-tribution which is only possible on the wintering areas (see Appendix 2 for details). The individual marking programme at Wexford must continue if we are to be in a position to interpret the changes in numbers based upon the count information.

This programme should be extended to more in-dividual marking and monitoring at other sites to construct the basis for comparative studies dis-cussed in greater depth below. The basic

ration-ale for all research to date has been driven by nature conservation objectives, and although there are many curiosity driven research objec-tives that could be included as well, the key con-servation questions are as follows: (i) What fac-tors affect changes in abundance at wintering sites? (ii) What factors limit successful recruitment into the breeding class? (iii) How will the effects of predicted global climate change affect the population (iv) How will the Canada Goose population of West Greenland affect the White-fronted Goose population?

Armed with a means of measuring condition, it becomes possible to reformulate these questions in the context of the direct effects of food quality and factors affecting feeding rates (as a result of climate change, inter- or intra-specific competi-tion or human disturbance). Such an approach can offer conservation management solutions on the wintering grounds (for example where interven-tion management can improve food quality or restriction on human activity can reduce distur-bance to feeding patterns). Using measures of condition on the pre-breeding spring staging ar-eas, it becomes possible to measure and contrast density-dependent effects amongst potentially breeding females in the prelude to clutch initia-tion and investigate the role of nutrient limita-tion and effects of competilimita-tion at this time.

Such empirical relationships are vital for our un-derstanding of small-scale population processes and individual behaviours. However, there re-mains a need to generate large-scale predictions about the effects of, for instance, macro changes in land use on the wintering grounds, or the ef-fects of climate change throughout the entire geo-graphical range. From the point of view of con-tributing to predictive models, such investiga-tions also provide basic data regarding the be-haviour of individuals in response to local goose densities or their position in dominance hierar-chies. When does a goose of potential breeding age pair and how is this decision condition me-diated? What conditions make an established pair emigrate from a poor quality winter site to an-other site? What are the fitness consequences of changing site for low, medium or high ranking birds at wintering sites of different quality?

Perhaps most important, the measure of the ca-pacity of individuals to make adjustments to their annual cycle which potentially improve fitness measures gives the potential to assess the flibility of the population and its capacity to ex-ploit novel opportunities. This element is impor-tant. In the past, it has been difficult to predict the patterns of development in the abundance of wild goose populations. From the low levels of abundance in the 1930s, protection measures put

NUTRIENT &

ENERGY ACCUMULATION

RATES

CONDITION

ability to meet current and future nutrient

and energy needs

FITNESS CONSEQUENCES

RESEARCH 1. Determining factors affecting food availability to an individual, influenced by site based disturbance, range quality, local density,

position in dominance hierarchy, etc.

OBJECTIVE A. Predicting effects of

change likely to come from climate, land-use policy and conservation

management change

RESEARCH 2. Monitor the processes of individual decision-making which

affect nutrient acquisition, e.g.

emigration from poor quality sites, pairing and departure from family

unit

OBJECTIVE B. Establish the behavioural flexibility of the individual to change its ability to acquire adequate stores to attain condition thresholds at critical

points in the life cycle

Individual decision-making

RESEARCH 3. Measure individual condition

at all stages of the life cycle, especially during transition states (when individuals go from phases of accumulation

to expenditure)

OBJECTIVE C. Establish the effects of individual behaviour on the

accumulation of stores for critical periods of use in the

annual cycle

RESEARCH 4. Determine reproductive and survival consequences for the individual of nutrient/energy accumulation and scale up to

population processes

OBJECTIVE D. Predict future individual behaviour and potential future

population trajectories

Figure 9.5. Schematic representation of the effects of nutrient accumulation rates (mediated by individual be-haviour) on body condition and fitness in Greenland White-fronted Geese, showing associated research ques-tions and objectives associated with each level.

in place from the late 1940s onwards in the United Kingdom ensured the increase in goose numbers to the present day. However, the numbers of sev-eral populations stabilised in the 1970s and 1980s, generally thought associated with density de-pendence (e.g. Figure 1 in Pettifor et al. 2000).

Largely unseen from the perspective of the win-tering grounds, Pink-footed Geese nesting in Ice-land and Barnacle Geese in Svalbard expanded to new colonies, and showed renewed periods of increase that could not be predicted on the basis of population-based models constructed using demographic data from previous years.

It is often extremely difficult to determine the strengths of density dependence in empirical studies (e.g. Pollard et al. 1987). Historical popu-lation data are likely to be collected over a very narrow range of population sizes and environ-mental conditions, unlikely to offer the basis for robust predictions for the future (see discussion in Pettifor et al. 2000). For this reason, it has been argued that models predicting the response of a population to environmental change need to be based upon the aggregative total of individual behaviours (Goss-Custard 1985, Goss-Custard &

Durell 1990). In this way, models can be devel-oped to predict effects of change in the environ-ment on a population based on the cumulative sum of individual responses under novel circum-stances. Such models have been developed us-ing game theory to explore how individuals of varying competitive ability can exploit a patchy and variable food supply. The classic models have been built based upon maximising individual fit-ness in Oystercatcher populations, by Goss-Cus-tard and co-workers at individual site Cus-tard et al. 1995a,b) and at population levels (Goss-Custard et al. 1995c,d). Such models need to be large scale and encompass the entire annual cy-cle, as exemplified by the application of Pettifor et al (2000) to other goose populations. The appli-cation of such models to the Greenland White-fronted Goose would identify the key model pa-rameters required and could prove extremely important to our understanding of future poten-tial change.