New Zealand Robin - Ecology, Life History, Behavior

Ecology, Life History, Behavior

Invertebrates, including earthworms, beetles, and other arthropods comprise most of the robins’ diet. They forage for these among the leaf litter covering their habitat’s ground. They have also been observed to eat berries from time to time; however, they do not cache berries. They do cache their invertebrate prey whole or in portions. They do this singly at different sites and do not cache more than once at a particular site. They typically store their prey with 10 meters of where they killed it, even if they are not within their own territory. In the winter their primary cache is earthworms, and during the summer it trends more toward cicadas. They have a very good memory of their storage sites; a male has been observed to find as many as five caches successively before returning the stores to his mate during incubation. New Zealand robins have demonstrated an ability to differentiate and prioritize caches of varying sizes, up to 12 total items, without any training. In fact, these robins have the highest numerical competency of any recorded wild animal, which they can use to return to the biggest caches first, followed by less rewarding ones. After storage, a robin will typically return to hunting, unless a particularly large prey was captured and stored, in which case it might preen or rest for a while. Usually robins will not return to a certain cache if it has been more than four days since the kill; the rare exception is an earthworm, which the robin might retrieve after several days. Because of its high metabolic rate, a robin might spend as much as 90% of its day foraging or storing during the winter.

New Zealand robins are genetically monogamous, and extra-pair paternity is rare. As they do typically remain monogamous, are non-migratory, and remain on their breeding territories year-round, a male and female on one territory will compete for food resources during non-breeding times. Since they use slightly different methods for foraging, some competition is reduced. However, they can and will steal from their mate’s cache if given the opportunity. Males do dominate females, and thus will aggressively exclude females from food, such that females will only access food sources when unattended by the males. During breeding season, as the female is solely responsible for nest building and incubation, the male will bring the female and the hatchlings food. Therefore, males will cache less than females at this time but more than females during non-breeding (females’ cache amounts remain mostly stable year-round).

For reproduction, robins have some fairly interesting characteristics which help them, to some extent, recover from predation. Robins become sexually mature between six and 12 months. They begin their breeding season fairly early in August or September, which is aided by their food stores (otherwise they would have to spend too much time hunting for food this early in in the spring). They build open-cup nests in tree-forks or inside tree cavities. Incubation lasts 18 days, and chicks leave the nest after 21 days. Five to 25 days after fledglings leave the nest, the pair may breed again. A modal clutch size is two eggs with the expected tradeoffs between size and number of eggs observed (greater the size, the fewer the eggs). Because females remain with the nest and hatchlings for so long, both she and her young are susceptible to predation. If the female survives, however, she and her mate can reproduce more quickly (3–10 days) than after a normal clutch leaves the nest, making it a little bit easier to recover from the predation.

A male’s call during non-breeding season is a downscale, which means that it is a series of loud notes with descending frequency which start in rapid succession and finish slowly. During breeding season, the males resort to full song to court the females. Full song is a loud, clear, and continuous song which can last for more than 30 minutes with very brief pauses. Full song is typically sung from the canopy or an emergent tree. Singing is also seemingly performed as part of a strategic regulation of body mass. While robins sing during the morning chorus as well as throughout the day, they seem to increase their singing according to food availability. When they have had more food, they will increase energetic behaviors such as singing in order to arrive at a strategic mass by dusk. Thus, even though two robins might begin the day with different weights, they will end the day with similar weights. A side effect of this behavior might be the showcasing of a particular male’s ability to sequester resources: the longer he sings, the more capable he is of finding food, and thus a better candidate for a mate.

Other than foraging for invertebrates on the forest floor, a New Zealand robin might also spend its time vegetation gleaning (4.5%), hawking (0.3%) or fly-catching (0.1%) in order to find food. Outside of collecting food resources, a robin also engages in a few body maintenance activities during the day. It spends about 0.5% of its time bathing, 0.04% anting (using ants or millipedes, which it sometimes eats afterwards), and some adults sun bathe, though this behavior does not seem to be essential as juveniles do not engage in it at all. A New Zealand robin is also relatively long-lived, as it can survive up to 14 years where few or no predators exist.

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