Food Abundance and Territoriality: To Defend or Not to Defend?

In seasonal environments, vertebrates are generally thought to time their reproduction so offspring are raised during the peak of food abundance. The mismatch hypothesis predicts that reproductive success is maximized when animals synchronize their reproduction with the food supply. Understanding the mechanisms influencing the timing of reproduction has taken on new urgency as climate change is altering environmental conditions during reproduction, and there is concern that species will not be able to synchronize their reproduction with changing food supplies. Using data from five sites over 24 years (37 site-years), we tested the assumptions of the mismatch hypothesis in the Tree Swallow (Tachycineta bicolor), a widespread aerial insectivore, whose timing of egg-laying has shifted earlier by nine days since the 1950s. Contrary to the mismatch hypothesis, the start of egg-laying was strongly related to food abundance (flying insect biomass) during the laying period and not to timing of the seasonal peak in food supply. In fact, food abundance generally continued to increase throughout the breeding season, and there was no evidence of selection based on the mistiming of laying with the seasonal peak of food abundance. In contrast, there was selection for laying earlier, because birds that lay earlier generally have larger clutches and fledge more young. Overall, initial reproductive decisions in this insectivore appear to be based on the food supply during egg formation and not the nestling period. Thus, the mismatch hypothesis may not apply in environments with relatively constant or abundant food throughout the breeding season. Although climate change is often associated with earlier reproduction, our results caution that it is not necessarily driven by selection for synchronized reproduction.

1. The relationships among food supply (Field Voles, Microtus agrestis), reproduction and blood parasites was investigated in Tawny Owls, Strix aluco, in Kielder Forest, Northumberland, in 1994 and 1995. Vole populations were significantly lower in 1995 than in 1994. 2. Birds did not lose parasites after initial infection, and the level at which infections were maintained was characteristic of individual birds. 3. In 1994, the number and intensity of parasites was higher in adult owls that had experienced low food supply when they themselves were reared. This indicated that food supplied to chicks in the nest has a long‐term effect on the parasite burden of adults. 4. In addition, there was evidence that parasite burdens of adults were influenced by their current food supply. Birds that suffered a decline in food abundance on their territories between 1994 and 1995 showed an increase in parasite load over the same period. In 1995, there was also a significant negative correlation between the parasite loads of owls and vole abundance on their territories. 5. The best predictor of parasite number of chicks reared in 1995 was the parasite load of their fathers. The parasites chicks developed were not the parasites with which their fathers were heavily infested. This result could be due to inherited immunity. 6. Our results indicated that food resources should be measured when investigating interactions between parasites and their hosts, and that offspring quality as well as quantity might suffer when food abundance is low.

Ungulates can respond to changes in food supply by altering foraging behavior, digestive function, and metabolism. A multifaceted response to an environmental change is considered robust. Short seasons of plant growth make herbivores sensitive to changes in food supply because maintenance and production must be accomplished in less time with fewer options in a more fragile response. Caribou live at high latitudes where short summers constrain their response to changes in food supply. We measured the ability of female caribou to resist and tolerate changes in the quality and quantity of their food supply during winter and summer. Caribou resisted changes in food abundance and quality by changing food intake and physical activity with changes in daily temperature within each season. Peak food intake rose by 134% from winter pregnancy to summer lactation (98 vs. 229 g kg-0.75 d-1), as digestible requirements to maintain the body increased by 85% for energy (1,164 vs. 2,155 kJ kg-0.75 d-1) and by 266% for N (0.79 vs. 2.89 g N kg-0.75 d-1). Caribou required a diet with a digestible content of 12 kJ g-1 and 0.8% N in pregnancy, 18 kJ g-1 and 1.9% N in early lactation, and 11 kJ g-1 and 1.2% N in late lactation, which corresponds with the phenology of the wild diet. Female caribou tolerated restriction of ad lib. food intake to 58% of their energy requirement (680 vs. 1,164 kJ kg-0.75 d-1) during winter pregnancy and to 84% of their energy requirement (1,814 vs. 2,155 kJ kg-0.75 d-1) during summer lactation without a change in stress level, as indicated by fecal corticosterone concentration. Conversely, caribou can respond to increased availability of food with a spare capacity to process digestible energy and N at 123% (2,642 vs. 2,155 kJ kg-0.75 d-1) and 145% (4.20 vs. 2.89 g N kg-0.75 d-1) of those respective requirements during lactation. Robust responses to changes in food supply allow caribou to sustain reproduction, which would buffer demographic response. However, herds may decline when thresholds of behavioral resistance and physiological tolerance are frequently exceeded. Therefore, the challenge for managing declining populations of caribou and other robust species is to identify declines in robustness before their response becomes fragile.

Comment on the article 'Genotype, obesity and cardiovascular disease--has technical and social advancement outstripped evolution?'.

Individuals allocate resources to the expansion of their foraging area and those resources are no longer available for the traits that determine how well those individuals are able to protect their foraging area against competitors. The resulting trade‐off between foraging area size and the traits associated with the ability to compete for the resources within the foraging area applies to ecological scenarios as different as territorial defence by individuals and colonies, and light competition in plants. Whether the trade‐off affects species performance in competition for resources at the area of overlap between foraging areas depends on the symmetry of resource division. In symmetric competition resources are divided equally between the competitors, while in asymmetric competition the individual with the smallest foraging area, and consequently the greatest competitive ability, gains all the resources. Competition may also be a combination of the symmetric and asymmetric processes. I studied the effects of competitive asymmetry on population dynamics and coexistence of two annual species with different sized foraging areas using an individual‐based spatially explicit simulation model. Symmetric competition favoured the species with the larger foraging area and did not allow coexistence. Competitive asymmetry favoured the species with smaller foraging area and allowed coexistence, which was due to the consequences of losing an asymmetric competition being more severe than losing a symmetric competition. The mechanism of coexistence is the larger foraging area's superiority in low population densities (little competition) and the smaller foraging area's ability to win a large foraging area when competition was intense. Competitive asymmetry and small size of both foraging areas led to population dynamics dominated by long‐term fluctuations of small intensity. Symmetric competition and large size of the foraging areas led to large short‐term fluctuations, which often resulted in the extinction of one or both of the species due to demographic stochasticity.

One of the important issues in the population biology of house mice (Mus musculus L.) is the extent to which social grouping influences gene flow and demographic processes. This species is territorial, at least in moderately large captive colonies (Eibl-Eibesfeldt 1950; Crowcroft 1955, 1973; Anderson 1961; Crowcroft & Rowe 1963; Anderson & Hill 1965; Reimer & Petras 1967). Evidence also suggests that territories are not necessarily individual but held by social groups (Eibl-Eibesfeldt 1950; Young, Strecker & Emlen 1950; Reimer & Petras 1967; Selander 1970; Busser, Zweep & van Oortmerssen 1974). It is still not known, however, whether this arrangement produces a static genetic structure, or it the combination of group cohesion with active dispersal leads to a constantly changing genetic situation. DeFries & McClearn (1972), for example, state that 'stable populations of house mice are comprised of small breeding units, or demes, with little genetic interchange among them'. Moreover, Ehrlich & Raven (1969) use the evidence for micro-geographical differentiation in this species to argue that species are not primarily held together by gene flow as is often assumed. On the other hand, the house mouse is well known as a very successful colonizing or 'weed' species. Social behaviour may also influence birth, death and emigration rates. It is difficult to study social behaviour and geographical structuring simultaneously; the one requires intensive observation on small captive groups (see Archer (1970) for review), whilst the other requires study of feral populations. The study reported here has been an attempt to narrow this gap between laboratory and field studies. My plan was to construct enclosures large enough for the development of spatial substructure, yet small enough to obtain direct evidence of social behaviour. The experimental design differed little from some commensal populations of Mus which live with abundant food and shelter in the midst of large areas of inhospitable habitat. The main difference was that normal dispersal was prevented, but, of course shortrange movements remained possible, and escape behaviour could be monitored. Moreover, enclosures are valuable for studying frustrated dispersal and hence the normal role of dispersal behaviour (Lidicker 1975). Supplies of food, water and shelter were provided in excess of demand, although food and water were dispersed only at fixed locations. I intended to study social and demographic behaviour in the absence of constraints by these basic resources. This arrangement stressed the cohesiveness of social units and provided a test of the potential responses of this species to extreme conditions.

SummaryThe breeding biology of House Martins nesting in artificial boxes was examined in relation to aerial insect abundance and weather. The insect food supply was continuously monitored with a suction‐trap at 40 ft above ground level. Aerial insect abundance started to rise from the winter level in April and reached a high stable state in June. Insects remained abundant until September; the subsequent decline continued at least until the end of October. The size of first clutches (mean size 3.87) was correlated with the abundance of aphids in spring, though not with temperature. It is suggested that this was an adaptive response because high food levels in May were associated with high levels in the main nesting period. Clutch‐size declined through the season but was not matched by changes in food abundance. Second clutches (mean size 2–95) were most frequent following a high food level in June.No correlation was found between egg‐weight and preceding food conditions, nor with season, clutch‐size or order of laying within clutches. If food was scarce on the day of laying of the first egg, these clutches suffered a suspension of laying of subsequent eggs. Some of the young in these nests showed a reduced growth rate. The duration of incubation (mean 14.6 ± 1.1 days) was not apparently correlated with food abundance, however it must have been potentially extended by extreme shortages because incubation duties were neglected at such times. Infertility was probably the main cause of hatching failure (14.2%).A model was used to predict the date of onset of laying. It was based on the assumption that nestling tissue was produced with the same energetic efficiency as eggs. The model indicated that House Martins could collect enough food to lay eggs earlier than the observed date in most years. The high probability of food shortage before the laying period, however, appeared to discourage laying at this time. The observed onset of laying coincided with the appearance of aphids in the air, probably because they comprised an abundant and stable food source for egg formation. Breeding generally occurred in the period of highest food abundance. Pairs rearing only a single brood each year did so in July when food levels were highest.The growth of first brood nestlings was more closely associated with food levels than any other factor investigated, while in second broods, rainfall was found to have the greatest influence. The total variance in growth explained by environmental factors in the first broods (41%) was greater than in second broods (22%). This relative independence of second broods from adverse environments probably arose from more abundant food and the feeding of some of these broods by the young of first broods. An exceptionally late brood showed an extended nestling period indicating deteriorating conditions later in the breeding season. It is proposed that the spread of laying may reflect differences in the feeding efficiency of nesting House Martins, because the growth of two early first broods was more rapid at given food levels than two late broods. Laying patterns conformed to a distribution based on a progressive threshold mechanism. The more efficient pairs may be able to attain breeding condition and lay at low levels of food abundance, and hence breed earlier in the season.Subsequent layings are facilitated by the rise in food levels in early summer. Mortality within the nest was low (5.8%) and associated with food shortage. Nestling periods (mean 30.6 ± 2.3 days) were not shown to be correlated with food abundance; they were more dependent on brood‐size.Recruitment into House Martin populations is thus widely influenced by food supply, particularly through an influence on clutch‐size, the occurrence of second clutches and nestling mortality.

Data on the foraging microhabitats of British birds are reanalyzed with the aim of understanding how fluctuations in resource abundance affect niche relationships and community structure. At Marley Wood, overlap in the foraging sites of resident bird species increased during the late spring and summer and decreased during the fall and winter. Among bird species coexisting in the pine forests at Thetford Chase, spatial overlap and spatial niche widths were positively correlated with food abundance over a 4-year period. These results suggest that in variable environments similarity in spatial niches is an inverse function of the intensity of competition for food. As food supplies drop, consumer species must apparently occupy increasingly different foraging areas in order to coexist. In less variable environments, however, resource stability may allow finer partitioning of the available foraging space and greater spatial overlap within a given foraging area. The results of this paper also suggest a reinterpretation of MacArthur's study of resource partitioning among warbler species in the boreal forests of New England. Rather than providing an instance of niche segregation in order to avoid intense competition, MacArthur's warblers may actually represent another example of increased spatial similarity when food resources are abundant and competition is reduced.

A History of Obesity, or How What Was Good Became Ugly and Then Bad

The distribution and density of every species of animal is at least loosely controlled by the distribution and abundance of its food. Particular attention has been paid recently to the distribution of salmonids as affected by selection of the site of the fish's feeding territory (Chapman & Bjornn 1969) and to their numbers as affected by varying abundance of food in different streams (Egglishaw 1967). Evidence that food supply does more than permit or prevent a population of salmonids from inhabiting a stream is as yet equivocal. Mason & Chapman (1965) found that the biomass and numbers ofjuvenile coho salmon (Oncorhynchus kisutch Walbaum) remaining in two stream channels from which they could emigrate were greater in the channel which had a greater food supply. However, since there was no replicate in which the varying rations were switched between channels, the possibility remained that the channel with the greater food supply retained more fish because of a greater number of hiding places, better cover, etc. In comparing two streams, one of which had three times as much food as the other, Egglishaw (1967) found that there were fewer trout in his food-poor stream, but more salmon, so that the total density of salmonids differed little (about 0 45 fish/m2 in the poor compared to 0 55 fish/m2 in the rich stream). Biomass differed even less. The streams were different in other respects besides food, however, which, in this case, may have masked the effect of food supply on density. Le Cren (1965) suggests that while a population of fish may be limited by food in the long run, in some instances other regulatory mechanisms usually reduce the importance of food supply as a regulatory factor. Effects offood supply on behaviour, in contrast to those on population density, have been reasonably well defined. In laboratory experiments with young salmon (Salmo salar L.), provision of food in two daily feedings was followed shortly afterwards by an increase in aggression which subsided gradually 30 min later (Keenleyside & Yamamoto 1962). Symons (1968) found when aggression was measured after immediate effects of feeding had waned, that fish deprived of food for 18-66 h were more aggressive than those receiving an abundance of food. Together these results indicate that, in those natural habitats having a continuous abundant supply of food, young salmon are probably less aggressive than in habitats where they are constantly hungry and meals are sporadic. Chapman (1962) has postulated that aggression of some territorial fish may cause the emigration of others with inferior fighting ability. Presumably an increase in aggression would speed this emigration process. Symons (1968) also suggested that the increase in aggression associated with food-deprivation might, in a natural environment, result in an

Theoretical models on the movement of colonial animals predict that neighbouring colonies may segregate their foraging areas, and many seabird studies have reported the presence of such segregations. However, these studies have often lacked the appropriate null model to test the effect of neighbouring colonies on foraging areas, especially in small colonies or in short‐ranging species. Here, we examined the foraging areas of Adélie Penguins Pygoscelis adeliae from two neighbouring (2 km apart) colonies by using bird‐borne GPS loggers. The field study was conducted at Hukuro Cove colony (104 pairs) and Mizukuguri Cove colony (338 pairs) in Lützow‐Holm Bay, East Antarctica. We obtained GPS tracks for 504 foraging trips from 48 chick‐rearing Adélie Penguins and quantified the degree of overlap in the foraging areas between two colonies. We also produced simulated movement tracks by using correlated random‐walks assuming no inter‐colony competition and quantified the degree of overlap in the simulated foraging areas. Finally, we compared the results from real GPS tracks with those from simulated tracks to examine the effect of neighbouring colonies on Adélie Penguin movement. The results indicate that the degree of overlap was significantly smaller in real tracks than in simulated tracks. In real tracks, the foraging area of the smaller Hukuro Cove colony extended to the other side of the larger Mizukuguri Cove colony, unlike in simulated tracks. Consequently, we suggest that Adélie Penguins from two neighbouring colonies segregated their foraging areas and that the larger colony appeared to affect the foraging area of the smaller colony.

-We examined temporal variation in abundance of understory birds and fruiting plants in young (5-7 yr) and old (25-35 yr) successional habitats and in intact, lowland rain forest at Estacion Biologica La Selva, Costa Rica, between January 1985 and May 1986. Fruit abundance varied seasonally in each habitat but was consistently greatest in the youngest site. Frugivores and nectarivores accounted for four (forest and older successional) or five (younger successional) of the five most frequently captured bird species in each habitat. Capture rates of arboreal frugivores and arboreal frugivore-insectivores were greatest in the youngest site and were not different between older habitats. Temporal variation in capture rates of frugivores resulted from habitat shifts by resident individuals and from arrival and departure of altitudinal and latitudinal migrants. Capture rates of frugivores correlated with fruit abundance in forest and the older successional habitat but not in the youngest site. Capture rates of nectarivores and insectivores varied over time and among habitats, but rates showed no correlation with capture rates of frugivores. The lack of positive correlations in seasonal capture rates among trophic groups and the correlation between frugivores and fruit abundance support the view that temporal and spatial variation in bird abundance in tropical bird communities is at least partially in response to variation in resource abundance. Received 3 November 1989, accepted 28 July 1990. BIRD populations vary in abundance over time and space in both temperate (e.g. Kendeigh 1982, Holmes et al. 1986, many others) and tropical habitats (e.g. Fogden 1972; Karr 1976; Leighton and Leighton 1983; Martin and Karr 1986a; Loiselle 1987a, 1988; Loiselle and Blake 1991; but see Greenberg and Gradwohl 1986). Fluctuations in abundance arise from variation in population processes (Orell and Ojanen 1983, Faaborg et al. 1984, DeSante and Geupel 1987) and from movement of individuals among habitats (Karr and Freemark 1983, Wheelwright 1983, Recher and Holmes 1985, Loiselle et al. 1989, Loiselle and Blake 1991). Such movements may represent random redistribution of individuals (Wiens and Rotenberry 1978, Wiens 1984), but such hypotheses often have been advanced in the absence of data on food abundance. Individual movements may instead reflect responses to changes in microclimatic conditions (Karr and Freemark 1983, Petit 1989) or to temporal and spatial variation in food (e.g. Leighton and Leighton 1983, Wheelwright 1983, 1 Present address: Department of Biology, University of Missouri-St. Louis, 8001 Natural Bridge Road, St. Louis, Missouri 63121 USA. Recher and Holmes 1985, Loiselle and Blake 1991). Most studies lack concurrent data on fluctuations in both bird population levels and food abundance (but see Wheelwright 1983; Stiles 1985a; Levey 1988a, b; Loiselle and Blake 1991). Consequently, a more direct examination of avian responses to variations in food supply is central to resolving the controversial role of food as an influence in organization of species assemblages (Wiens 1984, Martin 1986). To address this controversy, we used data from concurrent studies on temporal and spatial variation in abundance of tropical frugivores and fruit among three habitats in Costa Rica. We focus mainly on frugivores, but for comparison we examined seasonal rhythms of bird groups (nectarivores, insectivores) that rely on different resources. Tropical frugivores are particularly appropriate for the study of the influence of resource availability on consumer populations because their diets can be readily determined (e.g. Wheelwright et al. 1984, Loiselle and Blake 1990) and their resources (fruit) accurately measured (Blake et al. 1990). Fruit-eating birds also are a major component of many tropical communi114 The Auk 108: 114-130. January 1991 This content downloaded from 157.55.39.104 on Sun, 19 Jun 2016 05:47:31 UTC All use subject to http://about.jstor.org/terms January 1991] Birds and Fruits in Lowland Tropics 115 ties (e.g. Stiles 1985b, Loiselle 1988, Karr et al. 1990, Loiselle and Blake 1991), and frugivores disperse seeds from a large proportion of tropical shrubs and trees that produce fruits (e.g. Howe and Smallwood 1982, Stiles 1985b). Thus, it is important both from theoretical and practical standpoints to better understand the interactions between frugivorous birds and fruitproducing plants (Stiles 1985b, c). STUDY AREA AND METHODS

Parental investment and parent–offspring conflicts during the postfledging period in Montagu's harriers

In the vicinity of the Pribilof Islands in the Bering Sea, abundance of food available to surface-foraging seabirds was greater during the chick-rearing period in 1988 than in 1987, whereas abundance of food available to pursuit-diving seabirds was greater in 1987. Here we examine how breeding success and resource allocation of surface-foraging black-legged kittiwakes Rissa tridactyla (BLKI) and pursuit-diving thick-billed murres Uria lomvia (TBMU) varied with the fluctuations in their food supply. We also examine a difference in resource allocation among parents raising chicks at the large colony on St. George Island and those at the nearby small colony on St. Paul Island. We studied breeding success (BS), field metabolic rates (FMR, assessed by using doubly labeled water), foraging distribution, and nest attendance of parents and growth rate (GR) of chicks. The BS of BLKIs was lower in 1987 (a season of less abundant food for kittiwakes) than in 1988 (a season of more abundant food), and parents had higher FMRs in 1987 than in 1988. At-sea distributions and nest attendance suggested that in 1987 BLKIs foraged farther from the colonies, which could have resulted in the higher FMR of the parents. GR of BLKI chicks did not vary between 1987 and 1988. The BS of TBMUs was not significantly different between 1987 (a season of more abundant food for TBMUs) and 1988 (a season of less abundant food). Parent TBMUs had similar FMRs between the seasons. Densities of foraging TBMUs were higher within 20 km around colonies in 1987 than in 1988. Although the total time parent TBMUs spent foraging did not vary inter-seasonally, they performed more foraging trips of a shorter duration in 1987 than in 1988, and the GR of TBMU chicks was higher in 1987 than in 1988. Inter-colony comparisons do not suggest that parents reproducing at the large colony work harder to raise their young compared to parents breeding at the small colony. In 1987 parent BLKIs failed in raising young at the large colony, whereas one-third of BLKIs fledged their chicks at the small colony. In 1988, however, RS and FMRs of parent BLKIs were not significantly different between the colonies. Also, TBMUs at the large colony had higher BS than those at the small colony in both 1987 and 1988. Furthermore, in both years parent TBMUs feeding young at the small colony foraged farther from the colony and had significantly higher FMRs than at the large colony. These results suggest that fluctuations in food supply affect resource allocation in seabirds. However, a decrease in food abundance is likely to cause an increase in energy expenditures of parent BLKIs, whereas growth rates of their chicks are less affected. For the TBMUs, food shortages are likely to cause a decrease in growth of the chicks, but not an increase in energy expenditures of the parents.

Summary Social organization and dispersal of red squirrels (Sciurus vulgaris L.) differ between sexes, and intrasexual competition is intense. Therefore, we predicted that demographic parameters should be gender‐specific: that is density‐dependent factors will be more strongly related to density of the same sex than to density of the opposite sex. We studied the relative importance of within‐ and between‐sex density‐dependent factors and of density‐independent factors (habitat type, food abundance, winter temperature) on different demographic parameters, in two populations in northern Belgium. Spring density of males was positively correlated with tree‐seed abundance in the previous year, but this was not the case for females. None of the population parameters we measured differed between habitats, indicating that the same density‐dependent and density‐independent mechanisms prevailed in coniferous and deciduous habitat. Within each sex, we found several demographic parameters that were dependent on the densities of the same sex; however, none of these parameters was found to be dependent on the density of the opposite sex. Reproductive rate increased with food abundance and decreased with female density. Adult survival of females decreased with female density in autumn–winter, while survival of adult males in spring–summer increased with the size of the previous year's seed crop. Immigration rate of males was higher in spring than in autumn, and spring immigration increased with food abundance. Male recruitment rate, in both seasons, increased with food abundance, but was male density dependent. However, spring–summer loss rates also increased when food supplies were good, suggesting that despite high food availability, emigration of juvenile and subadult males increased when intrasexual competition was intense. Recruitment rate of females decreased with increasing female density. After a good seed crop, more subadult females dispersed, but their settlement success (recruitment) was lower at high female density. Seed crop size positively affected red squirrel densities through increased reproduction, immigration and adult survival of males, but density‐dependent reproduction and within‐sex density‐dependent recruitment of locally born juveniles and dispersing subadults limit the fluctuations in numbers and regulate densities in winter–early spring, as well as in summer.