Nest Visitation Rates Research Articles

Intraspecific Variation in Commuting Distance of Marbled Murrelets (Brachyramphus marmoratus): Ecological and Energetic Consequences of Nesting Further Inland

Radio transmitters were deployed on Marbled Murrelets (Brachyramphus marmoratus) at Desolation Sound, British Columbia, Canada, during the 1998 breeding season to assess individual variation in distance birds nested from foraging areas, and potential energetic and ecological consequences of commuting those distances. Radio-tracking from a helicopter was used to locate nests, and tracking from the air and boats was used to locate murrelets on the water. Twenty-three nests were found, with active incubation at 16, and active chick-rearing at 12. A minimum of 3 nests fledged chicks, 9 were failures, and 11 were unknown. Nests were at an elevation of 806 ± 377 m and a distance of 39.2 ± 23.2 km (range 12–102 km) from locations on the water. Birds spent an estimated 1.2 ± 0.7 h per day commuting to and from nests (range 0.3–3.5 h per day). It was estimated that birds expended 3,883 ± 2,296 kJ (range 1,200–10,144 kJ) over the breeding season when commuting to those nests, which was 5–41% of their estimated field metabolic-rate during the breeding season. There was no relationship between distance to nests and breeding success. Either Marbled Murrelets can accommodate that additional energy expenditure, or reduce commuting costs by modifying their foraging behavior. They may forage closer to nest sites when provisioning chicks, thereby reducing commuting costs with a payload, or alter nest visitation rates in relation to distance they nest from foraging areas. Nests further inland may also confer advantages that compensate for the added commuting, or birds might replenish body reserves at the end of the breeding season.

Parents and helpers compensate for experimental changes in the provisioning effort of others in the Arabian babbler

Biparental investment in birds is implemented through parents making adaptive compensatory adjustments according to the work rate of their collaborator. Predictions for such evolutionarily stable cooperation in parental nestling feeding, and the behavioural mechanisms involved, can also be applied to systems with helpers-at-the-nest. Using the cooperatively breeding Arabian babbler, Turdoides squamiceps, we experimentally increased the provisioning rate of one or two subordinate birds in each group by giving these individuals extra food. Compared with control days before and after, the experimental treatment caused ‘fed’ birds to increase and ‘unfed’ birds to decrease their nest visit rates significantly, with no significant changes in load size. Thus, both parents and helpers in this system showed individual compensatory adjustments in their nestling-feeding effort according to the work rates of collaborators. There were no significant increases in total group visits during experimental treatments; therefore this study did not provide evidence for the predicted incomplete compensatory response. These results for helping-at-the-nest are consistent with empirical studies on parental care, and further suggest a central role for nestling begging in any cooperative provisioning of young in the nest.

EFFECTS OF FOOD ABUNDANCE AND MALE PARENTAL CARE ON REPRODUCTIVE SUCCESS AND MONOGAMY IN TREE SWALLOWS

We examined the roles of food abundance and male parental care in the main- tenance of monogamy in Tree Swallows (Tachycineta bicolor). Mated, male Tree Swallows were removed from territories to simulate the lack of male parental care that would be incurred by secondary females. Removals took place in lakeshore and roadside habitats in central Alberta, Canada. Insect sampling over three breeding seasons indicated that the lakeshore had greater biomass of insects than the roadside during egg laying, but there was no difference during the nestling period. Both our male removal experiment and natural cases of polygyny suggested that loss of male parental care had little effect on reproductive success or survival of females or their offspring. Unaided females increased their per-capita nest-visit rate in two of three years, so the rate of visitation per nestling did not differ significantly between unaided females and both the male and female at control nests. Control females at the lakeshore produced more fledglings than control females at the roadside. This difference in productivity was due to earlier laying and larger clutches at the lakeshore than at the roadside and not to differences in brood reduction or insect abundance during the nestling period. In both habitats, male parental care was relatively unimportant to female reproductive success and, therefore, within a given habitat secondary females did not incur a reproductive cost. All natural cases of polygyny occurred at the lakeshore. We suggest that polygyny occurred at the lakeshore, and not at the roadside, because food abundance was greater during laying at the lakeshore, and this allowed secondary females to lay more eggs and produce more fledglings than females mated to monogamous males at the roadside. Received 11 March 1991, accepted 10 February 1992.

Biparental care: short-term manipulation of partner contribution and brood size in the starling, Sturnus vulgaris

© 1990 International Society for Behavioral Ecology In species with biparental care, the evolutionarily stable investment strategy depends, among other factors, on the reproductive value of the brood and on the contribution of one's partner. A shortfall in work rate by one parent should be partially compensated for by its partner, while both parents should invest more effort when there are more young. We simultaneously tested these predictions by attaching weights to one member of pairs of European starlings (Sturnus vulgaris), thus reducing one partner's nest visitation rate, and by daily manipulation of brood sizes (between three and seven chicks). Both sexes compensated to an equivalent degree for their partner's lower work rate, while at the same time adjusting flexibly to the daily brood size changes by increasing visit rates to larger broods. However, this ability to compensate was limited in the larger brood sizes, perhaps due to an upper limit on provisioning rate. Weighted birds and their partners showed differences in prey types delivered, relative to controls, taking a lower proportion of leatherjackets. With regard to the proximate cues affecting parental provisioning rate, begging noise levels (automatically recorded by computer from microphones in the nest-box) increased with brood size, but average length of begging bout did not. As expected, visit rates per chick decreased as brood size increased, and this was reflected in lower weight gains per day by chicks in larger broods. These results agree in general with games theory models but reveal important departures from our previous work with respect to (1) parents reaching an apparent ceiling to nest visitation rate at high brood sizes and (2) the ability to track these brood demands with short-term adjustments. [Behav Ecol 1990; 1:116-124]

Manipulation of sex differences in parental care

In a species with biparental care two parents cooperate to provide the appropriate amount of care for the young. Recent theoretical treatments consider the evolutionarily stable investment strategy. Under most conditions, the parental investment of the two partners should be negatively correlated, with the shortfall of one partner being partially compensated for by the other. Previous experimental manipulations of biparental care have involved removal of one partner, yet the response of a widowed bird may differ from that of a mated bird whose partner is doing less than its fair share of parental care. We present the first data involving subtle manipulations of sex differences in parental care where both partners continue to care for the young. This study involves pairs in a nestbox colony of european starlings (Sturnus vulgaris L.) with all brood sizes manipulated to five chicks. Pairs were randomly assigned to three groups: (i) male parental care reduced; (ii) female parental care reduce; and (iii) control pairs. Parental care was manipulated by attaching small weights to the base of a bird's tail feathers. Regardless of sex, nest visitation rate was reduced in the weighted birds with an incomplete compensatory increase by their unweighted partner. Additional parental duties were also considered, including shifts in prey type delivered to the nest, in both weighted birds and their partners. The shift in diet and the overall lower total visitation rate in experimental nests contributed to slower chick growth and lower chick weights than in control nests. The data accord with models suggesting that equality of invesment in biparental species is evolutionarily stable, but reveal new dimensions of parental response that need to be taken into account in theoretical treatments.

Influence of age and territory quality on the reproductive behaviour of male Yellow Warblers

Individual male Yellow Warblers (Dendroica petechia) exhibit differences in their level of parental investment, which correlate with the amount of brown streaking on the breast. We tested two alternative hypotheses to explain these patterns. The age effect hypothesis proposes that the variation in plumage and parental behaviour is an effect of differences in male age. This was tested by analyzing the nest visit rates and plumage scores of banded males present in the study in more than 1 year. The age effect hypothesis is rejected because there was no evidence of significant within-male variation due to increasing age, nor of any consistent directional change in either parental effort or plumage pattern between years. The territory quality hypothesis proposes that the quality of territory occupied has an important proximate influence on male parental effort. By observing the same territories over different years we were able to compare nest visit rates of different males with different plumage scores while holding the effect of territory quality constant. Analysis of variance revealed that male plumage rank had a highly significant effect on nest visit rate, while the effect of territory occupied was not significant. Rejection of these two alternative hypotheses provides further support for the hypothesis that phenotypically different males are using alternative and irreversible lifetime reproductive strategies.

The Influence of Predation Risk on Central Place Foraging Variables in the American Robin, Turdus migratorius

A central tenet of optimal foraging theory is that organisms should maximize net energy intake. There has been a recognition lately, however, that predation risk may influence foraging decisions. Two foraging parameters are examined: bout duration and mean load of prey per bout, in American robins (Turdus migratorius) in relation to predation risk by American kestrels. Foraging adult robins nesting near an American Kestrel nest had increased mean bout duration and mean load of prey per bout compared to pairs without apparent predation risk. Central place foraging theory attempts to predict the optimal solutions to different foraging situations in cases where animals return to a central location to consume, cache, or deliver prey items (Andersson, 1981; Orians and Pearson, 1979; Schoener, 1979). For organisms that return more than one prey item to the central location (multiple-prey loaders), optimization criteria can be applied to predict load size and bout duration as functions of varying distance from the fixed point. The interpretation of data collected to test optimality models (Tinbergen, 1981) typically has relied on two assumptions: (1) the currency maximized is the rate of energy delivery to the nest; and (2) the primary constraint on energy maximization is the foraging capacity of the parents (the loading effect). Fagerstrom et al. (1983) outline constraints on energy maximization for central place foragers, and develop alternatives to the strategy of energy maximization. Organisms might lower nest visitation rates to minimally acceptable levels when predation risk is perceived (visit-minimization strategy). Under the threat of predation the rate of energy delivery should correspond to the minimum requirements of dependent offspring, resulting in 'Present address: Department of Plant Pathology and Ecology, The Connecticut Agricultural Station, P.O. Box 1106, New Haven, Connecticut 06504, USA. This content downloaded from 157.55.39.238 on Sat, 02 Jul 2016 06:02:54 UTC All use subject to http://about.jstor.org/terms 156 TRANSACTIONS OF THE KANSAS ACADEMY OF SCIENCE lower nest visitation rates and larger load sizes per foraging trip (bout). Since nest predation is a major source of mortality in many birds (Lack, 1954; Perrins, 1977), this visit-minimization strategy may be an important alternative to energy-maximization foraging in many birds. Variation in bout duration and load size in a multiple prey loader, the American Robin, is interpreted in terms of the visit-minimization strategy. Of three adult pairs studied, the territory of one pair was only 50 meters from a woodlot containing an American Kestrel (Falco sparverius) nest. Since kestrels are known to prey upon young robins (Bent, 1938, p. 113), it was predicted that this pair would exhibit longer bout durations and larger load sizes per bout relative to the other pairs where perceived predation risk was

Sexual selection and variation in reproductive strategy in male yellow warblers (Dendroica petechia)

Male yellow warblers (Dendroica petechia) exhibit extensive variation in the amount and conspicuousness of the sexually distinctive brown streaking on the breast. We investigated this intraspecific variation in degree of sexual dichromatism to see if male plumage rank (essentially the amount of brown streaking) is correlated with the amount of reproductive effort allocated to parental investment, as sexual selection theory would predict. As predicted, the average rate of feeding visits to the nest by males was negatively correlated with the plumage rank of the male in 1982 (Fig. 1) and 1983 (Fig. 2), and varied little between years or among study areas. Within one study area, the higher nest visit rate of dull (low plumage rank) males was correlated with higher nestling growth rates. But in the other two study areas, where bright (high plumage rank) males predominated, nestlings of bright males tended to have the highest growth rates despite minimal nest visit rates. Thus, overall there was no correlation between male plumage rank and nestling growth rate, a potential index of relative reproductive success, in either year (Figs. 3 and 4). This overall equal nest success likely helps to maintain the behavioral and plumage polymorphism. Since we could find no evidence that the high nestling growth rates of bright males were due to an adjustment of female parental effort to compensate for low male parental effort, there must be some other benefit of being bright which contributes to nest success. To address this, we used the relationship between parental feeding rates and nestling growth rates for each nest as an index of relative territory quality. This analysis suggests that bright males generally occupy the best territories available, possibly because they are more aggressive in obtaining and defending them than dull males. Dull males appear to be relegated to poorer territories, where an increase in male parental contribution would be necessary to achieve high nestling growth rates. We propose that different males are using alternative but evolutionarily stable reproductive strategies, in a way that is consistent with the predictions of sexual selection theory. Allocation of limited reproductive effort to parental investment decreases as sexual dichromatism increases, which probably reflects an increased investment in intermale competition for the best territories.