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Natural History of the Roseate Tern
Population Size and Distribution
Roseate terns are concentrated in two main breeding populations in the North American region. One population breeds along the southern coast of Florida to the Bahamas, and the other breeds along the southeastern Canadian coast through New England. There are also small outlying colonies of roseate terns in Belize and Nicaragua (Nisbet 1992). Birds from the Caribbean population differ morphologically from the Northern Atlantic population- Caribbean roseate terns are smaller and have more red pigment in their bills than their Northeast Atlantic counterparts. No data was available as to what effects (if any) these differences have on interbreeding between the two populations. Information about the roseate terns’ South American wintering area is scarce- most data is defined only by ringing and band recoveries (Nisbet 1992). This is troublesome because it is theorized that the wintering areas could be where most mortality occurs (Mostello 2007).
For over 100 years the size of the Northeastern Atlantic roseate tern population has oscillated, numbering anywhere from 2000 to 8500 breeding pairs (Mostello 2007). In 1900 the population had been reduced to 2000 breeding pairs spread out over four main colony sites- Falkner Island, Connecticut, Bird Island, Massachusetts, Great Gull Island, New York, and Cedar Beach, New York (Nisbet 1992, Nisbet2 1992). These sites still serve as the main breeding grounds for the majority of Northeast Atlantic roseate terns today. Efforts to conserve the colonies began in the 1890’s and by the 1930’s the population had increased to approximately 8500 pairs (Nisbet 1992). However, numbers began to decline in the mid to late 1930’s, and continued to do so for the subsequent decades, reaching a low of 2600 pairs in 1976 (Heinemann 1992, Nisbet 1992). In 2000 the population reached a peak of approximately 4300 breeding pairs but has been declining ever since, despite conservation efforts. The entire Northeastern population is currently estimated at approximately 3300 breeding pairs (Mostello 2007).
Longevity of roseate terns is difficult to determine due to band loss in conjunction with a lack of information regarding their wintering grounds, but it is estimated that the average life span is approximately 16 years (Gochfield et el 1998, Nisbet2 1992). Roseate tern eggs hatch in two and a half to three weeks from the laying date (Gochfield et el 1998, Spendelow & Kuter 2004). The average clutch size varies from colony to colony and from year to year, but across all sites it is highly unusual for a male-female roseate tern pair to lay more than two eggs per breeding season. Older females are more likely to fledge more than one chick (Shealer 1995).Chick survival relies heavily on the order in which the eggs hatch. Second-hatched chicks, or “B” chicks, are at a competitive disadvantage to their older sibling (the “A” chick) because the A chick is already larger and stronger from being fed and cared for in the days before the second egg hatches (Schealer 1995). As a result, survival rates of A chicks are much higher than those of B chicks, and the B chicks also experience a slower overall growth rate (Szcys et el 2001, Schealer 1995). A strategy to reduce the laying interval, and hence, hatching interval, would likely provide B chicks with a higher chance of survival (Shealer 1995).
Chicks fledge at 22 to 28 days of age (Nisbet2 1992). However, it is thought that juvenile roseate terns are dependent on their parents for at least six weeks after fledging, and chicks may continue to depend on their parents until arrival at their wintering grounds (Spendelow et el 2005, Nisbet2 1992). The days and weeks following the breeding season are a very important period in the life history of many seabirds because it is during this time that the young are learning to forage for themselves and are transitioning to independence (Trull et el 1999).
Roseate terns begin breeding around three years of age and virtually all birds breed by five years of age (Spendelow2 et el 2005). The female’s physiological maturity and the development of the capacity to produce eggs are probably the major factors limiting the age at which roseates are able to start breeding (Spendelow2 et el 2005). Roseates exhibit delayed maturity, meaning that they do not return to the nesting site until several years after fledging. This, in conjunction with the lack of information regarding their wintering grounds, makes adolescent survival rates difficult to determine (Nichols et el 1990). Nichols et el estimates that the probability of a fledgling surviving the three year interval before it begins to breed is approximately 40.2 percent, whereas the estimated annual survival rate of mature, breeding adults is 73.8 percent (Nichols et el 1990). Roseates will usually return to the original site at which they hatched in order to breed, so preserving known breeding sites is important for tern conservation (Nichols et el 1990, Nisbet2 1992). Around mid-September, before migrating back to their wintering grounds, roseate terns will move to staging areas. These are areas in which the birds rest in flocks during daylight hours and roost at night (Trull et el 1999). Conserving both known breeding areas as well as staging areas is critical to roseate tern recovery.
Feeding and Diet
Roseate terns are specialized plunge divers that feed primarily on small, schooling marine fish, and occasionally crustaceans. Roseates forage over shallow water, sand bars and around inlets (Gochfield et el 1998). Roseate terns also shoal feed (feed where a very shallow sandbar is adjacent to deep water) because tidal currents sweep prey to the surface. When prey moves into shallow water (less than two meters in depth) it becomes available to the terns because the fish cannot stay below the plunge line for the birds (Heinemann 1992).
Roseate terns have a very restricted dietary breadth. Prior to mid-June 95 percent of the roseate tern diet consists of American Sandlance (Ammodytes americanus). After mid-June one of five species makes up the bulk of their diet. These species are Atlantic, blue-backed, and round herring (Clupea harengus, Alosa aestivalis, and Etrumeus teres) as well as mackerel (Scomber scombrus) and bluefish (Pomatomus saltatrix) (Heinemann 1992). The roseate terns’ strong dependence on a single prey item is closely tied to an almost equally strong dependence on only a few particular feeding sites (Heinemann 1992).
If there is low food availability in the spring, this may act as a constraint on early breeding (Burger et el 1996). The availability of specific fish and sand eels is especially important in determining when breeding begins (Gochfield et el 1998). Most roseate terns will lay when the food supply is increasing and raise the young while the food supply is decreasing. Productivity is markedly limited by the parents’ ability to bring food to the chicks (Burger et el 1996). Both sexes are responsible for feeding the young, however males have a higher prey delivery rate than females (Spendelow et el 2005, Shealer 1995). Males also exhibit mate feeding, and females that are fed the most food at the nesting sites expend less energy traveling to foraging areas, which can be up to 30 kilometers away, as at the Falkner Island colony site, and thus have more energy to spend on egg formation. Mate feeding rates are highest around the four-day interval surrounding the laying of the first egg (Shealer 1995). Males are mostly responsible for chick feeding in the few days following hatching, and it is for this reason that male performance may be more important than female performance in determining how many roseate tern chicks are reared to fledging . Older males seemed to be more proficient at delivering food to the nests- provisioning rates reached a peak at approximately 11 years of age (Shealer 1995).
Breeding and Nesting
The breeding season spans from late April to early July with peak laying occurring from May 24 to May 29 (Gochfield et el 1998). High quality birds- those birds most likely to produce viable young- tend to breed earlier in the season when resources are most abundant. This may explain why eggs laid earlier in the season have a higher hatching and fledging success (Szcys et el 2001). Late nesters are always less successful than those birds that nest earlier in the season (Gochfield et el 1998). Hatching success, however, tends to vary strongly with season, breeding site, and year, and depends on food supply, egg size, parental performance, and predation rates (Gochfield et el 1998, Burger et el 1996).
Roseate terns will utilize a variety of nesting habitats. Traditional nests are initially just a scrape in the ground, and additional nesting materials accumulate during incubation (Gochfield et el 1998). Roseate terns will also nest among wrack and debris washed up along the shore as well as in salt marshes (Gochfeld & Burger 1987, Spendelow et el 2001). Roseate terns have been shown to successfully use materials such as baskets, buckets, cans, boxes, tires, rubble and driftwood for nesting, and nests built in these habitats provide more protection for the young than nests out in the open (Gochfeld & Burger 1987). Well protected nests, such as those under boards, by logs, or in burrows, had the highest reproductive success rates when compared with other nesting habitats (Spendelow 1982). When nesting in tires, roseate tern chicks will hide near the nest site until fledging, but when not nesting in tires, chicks will usually leave the immediate nesting area within one week of hatching, leaving the chicks more vulnerable to predation and aggressive behavior from other terns (Spendelow 1991). Early nesters seem to prefer tires to other nesting sites, and the pairs nesting in tires have higher productivity rates than those nesting elsewhere (Spendelow 1991). Natural materials such as large rocks can also be used to create hidden and protected nesting sites (Spendelow 1982). The optimum nesting habitat for roseate terns appears to be comprised of approximately 30 to 60 percent cover with grass or other plants that are approximately 30 to 50 centimeters tall (Gochfeld & Burger 1987).
If first time breeders fail, they are more likely to move to another colony site for future nesting attempts. Older, more established pairs may be more tolerant to reproductive failure, but will eventually move on to another site as well (Spendelow 2002). There is little evidence of roseate terns laying a second time if their first brood fails (Gochfield et el 1998). Because the parents invest such a significant amount of time on the first clutch laid, there is rarely time for a second nesting attempt within the same breeding season (failed clutches were usually incubated for 35 to 45 days before the nests were abandoned by the parents) (Burger et el 1996).
Roseate terns are usually monogamous with both parents caring for the young (Gochfield et el 1998). If one of the parents caring for a fledgling dies, the fledgling usually dies as well (Spendelow2 et el 2005).
Nesting Among Common Terns
Roseate terns nest exclusively in discrete subgroups among common terns (Burger et el 1996, Nisbet et el 1992). This behavior has both positive and negative conservation implications. When creating breeding habitat for roseate terns, it is imperative that biologists also keep in mind the nesting and breeding needs of common terns, as the roseates will not establish a new breeding colony without it first being occupied by common terns (Leahy & Camp2006). Common terns benefit the less aggressive roseate terns by warding off seagulls, one of the main predators of tern chicks (Nisbet 1992, Leahy & Camp 2006). However, the aggressive behavior of common terns can also prove harmful to roseates because they must compete with commons for nesting space, food, and other resources (Spendelow et el 1982). Also, both roseate and common terns are known to engage physically with chicks that enter their nesting territory, which sometimes results in chick death (Schew et el 1989).
There are few public records of hybridization between roseate and common terns (Zingo et el 1994). It is believed that the principal barrier to hybridization between the two species stems from sexual displays and behaviors, as well as differences in nesting habitat (Zingo et el 1994).
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