Rate Of Food Delivery Research Articles

Reproductive parameters in relation to food supply in the whiskered tern (Chlidonias hybrida)

It is generally accepted that breeding terns are sensitive to food supply and that their reproductive effort could be substantially affected by the availability and access to resources. In this study we examined reproductive parameters in the whiskered tern Chlidonias hybrida in relation to food supply during the courtship feeding period (food brought by males to females) over a 2-year period (2004–2005). We also studied whether the condition of the nestlings [body condition index (BCI)] was related to a proxy of the reproductive investment of the adults (the clutch size) during a season of food shortage. Behavioural observations showed a decrease in the intensity of male courtship feeding between years (2004 > 2005), and a strong shift in the relative abundance of the two prey groups (invertebrates/fish; invertebrate prey dropped from 88.0 to 49.3%) brought by males. This change in food delivery rates did not result in a delay in laying, but there was a significant difference in mean clutch size between years (2.71 ± 0.49 eggs in 2004 and 2.05 ± 0.78 eggs in 2005) without any within-year variation in relation to the laying dates. The egg size (volume and length) was related to the year (2004 < 2005), suggesting a trade-off in the quantity and the quality of eggs between the two seasons. We also found no evidence that the investment in a large clutch affected nestling BCI in the course of the food shortage season. Since many pairs (about 60%) interrupted breeding during the incubation stage, we assumed that parents that succeeded in rearing nestlings in these conditions were probably ‘high-quality’ individuals. Our results therefore showed that whiskered terns are sensitive to the varying food conditions they experienced throughout the courtship period. The diversity of prey types could be a key factor in the reproductive investment of this tern species.

Breeding behaviour and parental care in the Griffon Vulture Gyps fulvus on the island of Crete (Greece)

A field study of the breeding behaviour of griffon vultures (Gyps fulvus) was conducted in Crete (Greece) during 1997–2000. The main behavioural patterns examined were aerial displays, nest defence, copulation, nest building, brooding and food provisioning to the chicks. The nesting territory consisted of a semicircle ca 5 m radius around the nest that was defended against conspecifics. Copulation attempts mainly occurred in the nesting ledge with a success rate of 77%. On average every pair accomplished 0.16 ± 0.34 copulations per day (range = 0–4) while 3.3% of the total attempts accounted for extra–pair copulations. Supplying material to the nest started 32 ± 9 days prior to egg laying (range = 17–44) at a rate of 5 ± 3.52 bouts/pair/day (range = 1–14).The mean number of items added to the eyrie was 2 ± 1.3/bout/pair (range = 1–8) with 63.5% of these being soft greenery.The mean frequency of relief at the nest relieves was 0.5 ± 0.64 bouts/pair/day (range = 0–2) while an incubation shift lasted on average 37.4 ± 0.5 hr (range = 25.6–50.2 hr). Nest attendance by breeding birds was continuous during the first 2 months after the hatching of the egg, dropping to 35% of daytime hours before fledging. Adult griffons foraged on average every other day while food delivery rate to the chick was calculated at 1.9±1.09 bouts/pair/day (range = 1–4). The most critical period of the reproductive cycle was May when chicks (6–10 weeks old) grew at their fastest rate and parent birds foraged on alternate days. Differences in behavioural patterns among this insular population and continental ones were minimal.

Hopeless solicitation? Host-absent vocalization in the common cuckoo has no effect on feeding rate of reed warblers

Begging behavior of nestlings can signal both hunger and competitive ability. Studies of begging in evicting avian brood parasites exclude the influence of nestling competition and may provide new insights into the host–parasite conflict and the evolution of signaling. Apart from the begging call, common cuckoo Cuculus canorus nestlings use special vocal displays in the absence of their hosts, termed here host-absent vocalization (HAV). Since these conspicuous calls can increase the risk of predation and require energy, their costs should be balanced by some benefits, such as increased food provisioning. However, there has been no evidence that chicks convey information about their hunger by HAV. We therefore tested experimentally whether cuckoo chicks use HAV as an additional signal to enhance food-delivery rate by their hosts. We used playback of HAV recorded from cuckoo nestlings to determine whether their hosts, reed warblers Acrocephalus scirpaceus, increase their provisioning in response to an apparent increase in HAV. Older chicks spent more time in HAV than younger chicks, suggesting that HAV is not caused by inaccurate discrimination of host arrival stimuli. Negative correlation of HAV with feeding rate and mass gain between the two experiments suggested that hunger was the motivation of HAV. The playback experiment, however, did not prove that HAV affects host provisioning rate. We discuss possible reasons for this result and provide alternative explanations for HAV, such as creating a bond between the hosts and the parasitic young used later in the postfledging care.

WITHIN-PAIR INTERACTIONS AND PARENTAL BEHAVIOR OF CERULEAN WARBLERS BREEDING IN EASTERN ONTARIO

The Cerulean Warbler (Dendroica cerulea) is currently the focus of considerable management interest; however, our ability to develop effective management strategies is hampered by a dearth of life history and basic behavioral data. Here, we present information on male-female interactions of Cerulean Warblers and parental nest attentiveness that is, to our knowledge, among the first such rigorously collected data for this species. Males feed females during nest building and on the nest during incubation; the relative infrequency of these events suggests that they play more of a role in pair-bond maintenance than they do in enhancing female energetics. Female incubation rhythms were not significantly influenced by temperature, time of day, or egg age. Compared with other Dendroica warblers, we observed relatively infrequent female departures during incubation, perhaps in response to a high risk of nest predation. As the nestlings aged, females spent less time brooding nestlings, presumably to allow for more frequent feeding; however, both males and females exhibited relatively low rates of food delivery compared with other Dendroica warblers. Despite the low rates of food delivery, feeding trips were more frequent at successful nests than unsuccessful nests. Our results suggest that Cerulean Warblers are tightly constrained by the competing pressures of predation risk and sufficient food provisioning for nestlings.

Paying for information: partial loads in central place foragers

Information about food sources can be crucial to the success of a foraging animal. We predict that this will influence foraging decisions by group-living foragers, which may sacrifice short-term foraging efficiency to collect information more frequently. This result emerges from a model of a central-place forager that can potentially receive information on newly available superior food sources at the central place. Such foragers are expected to return early from food sources, even with just partial loads, if information about the presence of sufficiently valuable food sources is likely to become available. Returning with an incomplete load implies that the forager is at that point not achieving the maximum possible food delivery rate. However, such partial loading can be more than compensated for by an earlier exploitation of a superior food source. Our model does not assume cooperative foraging and could thus be used to investigate this effect for any social central-place forager. We illustrate the approach using numerical calculations for honeybees and leafcutter ants, which do forage cooperatively. For these examples, however, our results indicate that reducing load confers minimal benefits in terms of receiving information. Moreover, the hypothesis that foragers reduce load to give information more quickly (rather than to receive it) fits empirical data from social insects better. Thus, we can conclude that in these two cases of social-insect foraging, efficient distribution of information by successful foragers may be more important than efficient collection of information by unsuccessful ones.

Daily routines of body mass gain in birds: 2. An experiment with reduced food availability

Theoretical models predict that small birds should adjust daily patterns of body mass gain in response to environmental and internal factors. In a companion paper, we described a model on daily fattening that allows the analysis of precise changes in the shape of hourly patterns of body mass. In this study, we tested one of the main predictions of the model: the inflexion point of the body mass trajectory should be delayed in response to a decrease in food availability, increasing body mass as soon as possible when food is scarce. This effect might be stronger in subordinate than in dominant birds. We tested both predictions with four pairs of coal tits, Periparus ater, kept in cages where food was delivered at high and low rates. Daytime increase in body mass was 1 g in both treatments. As predicted by the model, the tits increased body mass as soon as possible when food was delivered at a low rate, and the inflexion point of the body mass trajectory was 16.7% delayed compared to the high food delivery rate. However, dominance rank had no significant effect on the shape of daily body mass increase. To our knowledge, this is the first precise estimate of the change in the shape of body mass trajectories. Our findings have important implications for the analysis of daily patterns of body mass and for the design of studies of body mass in small birds.

MATCHING AND CONDITIONED REINFORCEMENT RATE

Attempts to examine the effects of variations in relative conditioned reinforcement rate on choice have been confounded by changes in rates of primary reinforcement or changes in the value of the conditioned reinforcer. To avoid these problems, this experiment used concurrent observing responses to examine sensitivity of choice to relative conditioned reinforcement rate. In the absence of observing responses, unsignaled periods of food delivery on a variable-interval 90-s schedule alternated with extinction on a center key (i.e., a mixed schedule was in effect). Two concurrently available observing responses produced 15-s access to a stimulus differentially associated with the schedule of food delivery (S+). The relative rate of S+ deliveries arranged by independent variable-interval schedules for the two observing responses varied across conditions. The relation between the ratio of observing responses and the ratio of S+ deliveries was well described by the generalized matching law, despite the absence of changes in the rate of food delivery. In addition, the value of the S+ deliveries likely remained constant across conditions because the ratio of S+ to mixed schedule food deliveries remained constant. Assuming that S+ deliveries serve as conditioned reinforcers, these findings are consistent with the functional similarity between primary and conditioned reinforcers suggested by general choice theories based on the concatenated matching law (e.g., contextual choice and hyperbolic value-added models). These findings are inconsistent with delay reduction theory, which has no terms for the effects of rate of conditioned reinforcement in the absence of changes in rate of primary reinforcement.

Using behavioural and state variables to identify proximate causes of population change in a seabird

Changes in animal population size are driven by the interactions between intrinsic processes and extrinsic forces, and identifying the proximate mechanisms behind population change remains a fundamental question in ecology. Here we report on how measuring behavioural and state proxies of food availability among populations experiencing different growth rates can be used to rapidly identify proximate drivers of population trends. In recent decades, the Cape gannet Morus capensis has shown a major distributional shift with historically large colonies in Namibia decreasing rapidly, whilst numbers at South African colonies have increased, suggesting contrasting environmental conditions in the two regions. We compared per capita growth rates of five of the six extant colonies with foraging range (using miniaturised Global Positioning System loggers), foraging work rate, food delivery rates and body condition of breeding adults. We found significant associations between the rate of population change, individual behaviour, energetic gain and body condition that indicate that recent population changes are associated with extrinsic effects. This study shows that behavioural and state data can be used to identify important drivers of population change, and their cost-effectiveness ensures that they are an appealing option for measuring the health of animal populations in numerous situations.

Insectivory versus Piscivory in Black Terns: Implications for Food Provisioning and Growth of Chicks

Abstract The Black Tern (Chlidonias niger) is known for insectivory in their breeding range, but they are piscivorous in winter and feed some fish to chicks. Fish have potentially high value for chick provisioning because of their larger mass, but the relative value of fish and insect diets for chick growth is unknown. In 1999-2000, we documented use of fish and insects for provisioning chicks at four Black Tern colonies in Maine and examined chick growth rates at two colonies (Douglas Pond and Carlton Pond) that differed in fish and insect use. Deliveries of fish and insects to broods were documented using video cameras and observations from blinds, while concurrently measuring chick growth in nest enclosures. Fish use was substantial (>25% of deliveries) at three of four colonies. Fish comprised 29% of items and 56% of metabolizable energy delivered to chicks at Douglas Pond compared to 13% of items and 22% of metabolizeable energy at Carlton Pond. Food delivery rate was inversely related to the proport...

The Process and Causes of Fledging in a Cavity‐Nesting Passerine Bird, the House Wren (Troglodytes aedon)

AbstractLittle is known about the process or causes of fledging or nest‐leaving in passerine birds because researchers can rarely predict when fledging will occur in a given nest. We used continuous videotaping of nests to both document the process of fledging in the house wren, Troglodytes aedon, a small, cavity‐nesting songbird, and test hypotheses as to what might cause fledging to begin. Fledging began any time from 14 to 19 d after hatching commenced. Slower‐developing broods fledged later than faster‐developing broods. Fledging typically began within 5 h of sunrise and over 80% of all nestlings fledged before noon. All nestlings fledged on the same day at 65% of nests and over two consecutive days in most other nests. We found no evidence that fledging was triggered by changes in parental behaviour. Parental rate of food delivery to nestlings did not decline during a 3‐h period leading up to the first fledging, nor was the rate of feeding just prior to the first fledging lower than the rate at the same time the day before. Moreover, parents did not slow the rate of food delivery to nests after part of the brood had fledged. Hatching is asynchronous in our study population which creates a marked age/size hierarchy within broods. At most nests, the first nestling to fledge was the most well‐developed nestling in the brood or nearly so (as measured by feather length). This suggests that fledging typically begins when the most well‐developed nestlings in the brood reach some threshold size. However, at about one‐fifth of nests, the first nestling to fledge was only moderate in size. At these nests, severe competition for food may have caused smaller, less competitive nestlings to fledge first to increase their access to food. We found no strong support for the suggestion that the oldest nestlings delay fledging until their least‐developed nestmate reaches some minimum size, although further experimental work on this question is warranted.

Response persistence under variable-time schedules following immediate and unsignalled delayed reinforcement.

Key pecking of three pigeons was maintained in separate components of a multiple schedule by either immediate reinforcement (i.e., tandem variable-time fixed-interval schedule) or unsignalled delayed reinforcement (i.e., tandem variable-interval fixed-time schedule). The relative rate of food delivery was equal across components, and this absolute rate differed across conditions. Immediate reinforcement always generated higher response rates than did unsignalled delayed reinforcement. Then, variable-time schedules of food delivery replaced the contingencies just described such that food was delivered at the same rate but independently of responding. In most cases, response rates decreased to near-zero levels. In addition, response persistence was not systematically different between multiple-schedule components across pigeons. The implications of the results for the concepts of response strength and the response-reinforcer relation are noted.

Foraging effort in relation to the constraints of reproduction in free‐ranging albatrosses

Summary Theoretical models predict that animals will vary their effort to maximize different currencies such as time and energy when the constraints of reproduction change during breeding, but this has been poorly studied in free‐ranging animals. Foraging effort (energy per unit time) was examined by comparing mass changes, foraging costs and activity‐specific behaviours of Wandering Albatrosses (Diomedea exulans Linnaeus) during the incubation and chick‐brooding stages. In 1998, 38 albatrosses (20 during incubation and 18 during brooding) were injected with doubly labelled water and equipped with satellite transmitters and activity data loggers. During incubation, albatrosses travelled 3·7 times farther and were at sea 3·2 times longer, yet foraging costs were significantly lower than trips made during brooding (incubation 4·52 ± 0·50 SD W kg−1vs brooding 4·98 ± 0·55 SD W kg−1). The rate of daily mass gain decreased significantly with time at sea during incubation whereas the rate of daily mass gain increased significantly with time at sea during brooding. Foraging effort was higher during brooding, suggesting that birds were minimizing time at sea to maximize the rate of food delivery to chicks. In contrast, foraging effort was lower during incubation, suggesting that birds were maximizing time at sea and minimizing the energy costs of foraging. Foraging costs were also different between sexes. However, this was related to body size differences and not to differences in foraging effort as suggested in previous studies.

Biparental care in house sparrows: negotiation or sealed bid?

We explored the responses of monogamous house sparrow parents to deviations in their mates' contributions to nestling provisioning. Following 1-2 days of baseline measurement of parental food delivery rates, we applied small lead fishing weights to the tail feathers of either male or female parents. Weighting had much greater immediate impact on male parental care than on female care, but the handicapping had little long-term effect on either male or female provisioning behavior. When parental performance of handicapped males was most impaired, their mates did not show significant increases in parental care as compensation, nor did females mated to handicapped males reduce their provisioning as their mates recovered from weighting. Similarly, males mated to handicapped females did not respond to their partners' recovery with declines in their own efforts; paradoxically, these males showed a sustained elevation of provisioning throughout the post-treatment interval, despite no significant reduction in provisioning by weighted females. The apparent insensitivity of both males and females to changes in their mates' parental behavior, and the ineffectiveness of current partner behavior at predicting an individual's provisioning effort, fail to conform to assumptions of biparental care models that require facultative responses to partner deviations in effort. Instead, the remarkable consistency of each individual's behavior supports the notion of “sealed bids” and suggests that variation in nestling provisioning is largely attributable to factors that are independent of the mate's current behavior, such as differences in individual quality.

Begging signals more than just short-term need: cryptic effects of brood size in the pied flycatcher ( Ficedula hypoleuca )

The begging of nestling birds is known to reliably signal short-term nutritional need, which is used by parents to adjust rates of food delivery and patterns of food distribution within broods. To test whether begging signals reflect more than just short-term feeding history, we experimentally created 18 "small" (4-nestling) and 18 "large" (8-nestling) broods in the pied flycatcher (Ficedula hypoleuca). Compared to small broods, large broods were provisioned by parents at a greater rate, but at a lower visit rate per nestling and with no obvious differences in load mass per visit. However, lower rates of food mass delivery per nestling in large broods did not result in any measurable reduction in nestling growth (i.e. "long-term need") or in any increase in the begging effort per individual nestling whilst in the nest. Mid-way through the nestling period we also used hand-feeding laboratory trials to assess in more detail individual begging behaviour and digestive performance of the three mid-ranking nestlings from each brood. More food items were required at the start of each trial to satiate nestlings from large broods, but despite this initial control for "short-term need", nestlings from large broods went on to beg at consistently higher rates and at different acoustic frequencies. Large brood nestlings also produced smaller faecal sacs, which were quantitatively different in content but did not differ in frequency. We suggest that different nutritional histories can produce cryptic changes in nestling digestive function, and that these can lead to important differences in begging signals despite controlling for short term need.

PARENTAL PROVISIONING PATTERNS OF WEDGE-TAILED SHEARWATERS AND THEIR RELATION TO CHICK BODY CONDITION

Abstract I studied the provisioning patterns of a subtropical, northern-hemisphere procellariiform, the Wedge-tailed Shearwater (Puffinus pacificus) and measured the responses of parents to chick body condition. Foraging trip length, meal size, delivery rate, and chick body condition were measured during nesting periods in 1996 and 1997. Wedge-tailed Shearwaters adopted a unimodal foraging strategy with foraging trips that averaged 1.5 to 2 days duration and averaged about 40–50 g of food per trip. The rate of food delivery (g of food per day spent foraging) decreased with increasing trip duration, such that one-day trips resulted in the greatest rate of food delivery to the nest in both 1996 and 1997. Chicks left in poorer body condition were more likely to receive visits from one or both adults the following evening (1996 and 1997) and received larger meals (1996) than those left in better condition. Although foraging strategies may differ from subpolar and temperate shearwater species, rates of food de...

FEEDING OF HOUSE WREN NESTLINGS AFFLICTED BY HEMATOPHAGOUS ECTOPARASITES: A TEST OF THE PARENTAL COMPENSATION HYPOTHESIS

Abstract This study asked whether parent House Wrens (Troglodytes aedon) in a Wyoming population increased rates of food delivery to broods parasitized by hematophagous blow fly larvae and mites. We observed no significant difference in rates at which pairs fed nestlings at nests with naturally heavy infestations of fly larvae (6–19 larvae per nestling) and nests in which nestling exposure to larvae was experimentally eliminated or severely reduced (0–2 larvae per nestling). The apparent failure of parents to compensate nestlings for resources lost to parasites may, in part, explain the reduced rates of mass gain by parasitized nestlings that we observed and the presence of reduced hemoglobin levels in parasitized nestlings documented in a companion study. Parasitized nestlings may be too weak or anemic to intensify begging activity which would alert parents to their increased need for food. Alimentacion de Polluelos de Troglodytes aedon Afectada por Ectoparasitos Hematofagos: Una Prueba de la Hipotesis d...

Inter-annual variation in the diet, provisioning and growth of Cassin's auklet at Triangle Island, British Columbia: responses to variation in ocean climate

We studied parental provisioning and chick growth rates of Cassin's auklet Ptychoram- phus aleuticus at Triangle Island, British Columbia, Canada, from 1996 to 1999. Auklet reproductive performance and ocean climate conditions during these years were highly variable, and reflected in the diet composition. Chick growth was maximal when the diet was predominated by copepods, in particular Neocalanus cristatus. Provisioning and growth were high in 1999, intermediate in 1997 and poor in 1998. Exceptional was 1996, when growth was low but provisioning rates were high. Pro- visioning and growth were depressed late in 1997 and throughout 1998 when larval rockfish Sebastes spp. (5200 cal g -1 ) replaced N. cristatus (6236 cal g -1 ) in the nestling diet. Zooplankton sur- veys indicated that N. cristatus was substantially more abundant in May 1999 than in May 1998, and during 1999 the auklets foraged in areas with the highest concentrations of copepods. Through impacts on prey availability, variation in ocean climate affects the reproductive performance of Cassin's auklet. Performance tends to be favorable in years when spring is late and cold, and poor when spring is early and warm. Equations for predicting food delivery rates from 24 h mass changes of chicks were highly year-specific, precluding their application in other years or at other sites where Cassin's auklets breed. Between-year differences were also found in relationships between adult provisioning and chick growth. These were strongly positively related in 1999, positively related in 1996 and 1997, but unrelated in 1998; differences attributed to the magnitude of temporal variation in the nestling diet. Finally, we detected annual differences in parental response to chick needs. In 1999, parents delivered more food to chicks in poor condition and less to those in better condition, responses not observed in 1998. Different responses between years may have reflected variation in the availability of prey.

Variation in breeding success in Wilson's storm petrels: influence of environmental factors

The climatic and sea ice conditions of the Southern Ocean are highly variable within and between years, and a better understanding of the influence of climatic conditions on the various parts of the marine food chain is needed. This paper summarizes data on breeding success of a krill-dependent predator, the Wilson's storm petrel Oceanites oceanicus, in a colony on King George Island, South Shetland Islands, Antarctica, over four breeding seasons from 1995/1996 to 1999/2000. The seasons differed greatly in environmental conditions and in the resulting food availability which was reflected in inter- and intra-annual variations in feeding frequency. The breeding success was consequently variable. Starvation, the main cause of chick mortality in three out of four breeding seasons was associated with reduced sea ice cover in winter, which is known to depress food availability in the following breeding season. Food delivery rates also depended on the prevailing winds, which drive the transport of zooplankton-rich water masses. Snowstorms caused additional chick mortality in two out of four seasons when nest burrows became blocked. Implications for the use of Wilson's storm petrels for monitoring of krill are considered.

Contrasting Brood Sizes in Common and Arctic Terns: The Roles of Food Provisioning Rates and Parental Brooding

AbstractArctic Terns (Sterna paradisaea) and Common Terns (S. hirundo) are similar in many aspects of their breeding ecology, but Common Terns generally lay three eggs per clutch whereas Arctic Terns lay two. In our study, Common Terns had a higher rate of food delivery and energy supply to the nest and higher nest attendance, indicating that they made trips of shorter average duration. This suggests that the number of chicks raised by these two species was primarily limited by the rate at which parents could supply food. However, estimated daily metabolizable energy intake of chicks was about 30% higher in Common Terns than in Arctic Terns. Common Tern chicks apparently spent a higher proportion of daily energy intake on maintenance of body temperature. It remains unknown whether this difference was because Common Tern parents could not brood three chicks as effectively as Arctic Terns brooded two or because the energy requirements for heat production in the third-hatched Common Tern chick were particularly high. If brooding did play a less important role in the energy budgets of Common Terns, the number of chicks that Arctic Terns could raise may have been limited not only by the rate at which parents could supply food to the nest but also by the requirements of chicks for brooding. We suggest that more detailed studies on the role of brooding constraints in limiting brood size in these species are required to clarify this matter.

Association of Low Reproductive Rates and High Contaminant Levels in Bald Eagles on Green Bay, Lake Michigan

Bald eagles ( Haliaeetus leucocephalus) nesting on the shores of Green Bay, Lake Michigan, had reproductive rates significantly lower than those of neighboring eagles nesting in inland Wisconsin (0.55 vs. 1.1 young per occupied territory). This study investigated effects of two factors which have depressed eagle reproductive rates at other locations: exposure to organochlorine contaminants and low food availability. Levels of dichloro-diphenyl-dichloroethylene (DDE) and total polychlorinated biphenyls (PCBs) in addled eggs and eaglet blood samples from Lake Michigan and inland Wisconsin reference sites were measured. An index to food availability, the food delivery rates by parent eagles to nestlings, and other behavioral indices that have been associated with food availability, were measured and compared to inland reference data. Mean contaminant concentrations in addled eggs from Green Bay were 8.3 μg/g wet weight DDE and 31.3 μg/g wet weight total PCBs (1987 to 1996, n = 9). Mean concentrations in nestling blood plasma were 53 μg/kg wet weight DDE and 207 μg/kg wet weight total PCBs (1987 to 1995, n = 8). Indices of food availability were generally normal in comparison to inland reference data. Mean food delivery rate to the nestlings was 2.55 items per nestling per day (n = 7 nests). None of the behavioral measures differed significantly from inland reference values, suggesting that prey was adequate. It was concluded that organochlorine contaminants caused all or most of the depression in reproductive rates of Green Bay bald eagles.