The Manx shearwater (Puffinus puffinus) is a medium-sized shearwater bird in the seabird family Procellariidae. Manx shearwaters are long-lived seabirds that breed on islands and coastal cliffs across the North Atlantic. These birds undertake lengthy annual migrations, traveling between their northern hemisphere breeding grounds and southern hemisphere overwintering areas.
Manx shearwaters return to their breeding colonies in late April or May. Upon arrival, paired birds will reaffirm their bonds through billing and preening behaviors. Manx shearwaters nest in burrows dug into soil or sand cliffs, with both the male and female sharing incubation duties. However, before eggs can be laid and incubated, the birds must construct or refurbish an existing burrow.
The incubation period refers to the amount of time required for Manx shearwater eggs to hatch after being laid. This is an important reproductive milestone that is influenced by multiple factors. To better understand the incubation period of Manx shearwaters, this article will cover:
Manx Shearwater Breeding Biology
– Nest site selection and burrow building
– Egg laying and clutch size
– Shared incubation by parents
– Hatching of chicks
Length of Incubation Period
– Average incubation period
– Range of incubation period
– Comparison to related shearwater species
Factors Affecting Incubation Period Length
– Parental incubation habits
– Egg size
– Environmental conditions
– Temperature
– Humidity
Importance of Incubation Period for Chick Survival
– Synchronizing hatching with food availability
– Predator avoidance
– Facilitating sibling competition
Conclusions
Manx Shearwater Breeding Biology
Manx shearwaters are colonial seabirds, nesting in large aggregations on offshore islands and coastal cliffs. Their breeding biology is well-adapted to the constraints of an entirely pelagic existence. Manx shearwaters do not build above-ground nest structures. Instead, they nest in burrows dug into soil, sand, or turf. These underground nest sites provide protection from predators and shelter from harsh weather.
The timing of breeding activities is highly synchronized, with most birds arriving back at the colony within a few days. Shortly after arrival at the breeding site, paired birds will re-affirm their bonds through billing and preening. They will also interact with neighbors to re-establish social hierarchies.
Manx shearwaters are monogamous, typically maintaining long-term pair bonds that may persist for many seasons. However, divorce and re-pairing with new mates is not uncommon. In most cases, both members of a pair will occupy the same burrow from previous years. If a bird returns to find its former mate absent, it may attempt to take over the burrow or excavate a new one nearby.
Burrow excavation is a key component of site preparation. Shearwaters are not well-adapted for digging, so they often have to renovate old burrows rather than dig entirely new ones. This work is undertaken by both sexes and may take 2-3 weeks to complete. Burrows may extend 1-3 meters into the soil or sand. The chamber at the end is expanded into a nesting cavity. Some sparse nesting material such as dried grasses may be incorporated.
Egg laying starts in late May to early June for most Manx shearwater colonies. The female lays a single large white egg each season. If the first egg is lost early in incubation, a replacement clutch may be produced. Incubation does not start until the egg is laid, so replacement eggs hatch at the normal time. The average clutch size is one egg.
Both the male and female Manx shearwater share in incubating the egg. Incubation shifts span 3-13 days and are closely synchronized between the parents. The egg cannot be left unattended for long due to risk of chilling, overheating, or predation. If one parent fails to return from a foraging trip, the mate may continue incubating until it returns or the egg fails.
After an incubation period of around 54 days, the chick hatches out fully covered in downy feathers. The parents will brood the newly hatched chick for several days until it is able to thermoregulate on its own. They will also provide some partial feeds of oil-rich stomach oil to supplement the chick’s internal yolk sac. Full provisioning with whole food loads starts about a week after hatching.
Length of Incubation Period
The incubation period is defined as the time elapsed between egg laying and hatching. For Manx shearwaters, the average incubation period is approximately 54 days. However, there is variation around this mean. Reported incubation periods range from 50 to 59 days across various studies and colonies.
How does this compare to other shearwater species? The Manx shearwater’s incubation period is slightly longer than some congeners, such as the streaked shearwater at 52 days, but shorter than others, like the short-tailed shearwater at 59 days. The related great shearwater falls in the middle with an average incubation period of 56 days.
In general, the incubation period tends to be longer for larger shearwater species that lay single-egg clutches, while smaller species that lay two eggs have shorter incubation. However, even within the same species, incubation length can vary geographically across different colonies. Local environmental conditions play a role in determining incubation period length.
Factors Affecting Incubation Period Length
Several key factors can influence the duration of incubation in Manx shearwaters:
Parental Incubation Habits
How diligently the parents share incubation duties affects developmental rates. More attentive incubation provides more consistent temperature and moisture. If parents take longer or more frequent breaks from incubation, eggs take longer to hatch. Poor body condition can compel parents to spend more time away foraging.
Egg Size
Larger eggs require longer incubation. Within a shearwater population, egg size can vary according to the age and health of the female parent. Older, more experienced breeders tend to lay larger eggs that take slightly longer to hatch than average-sized eggs.
Environmental Conditions
Temperature
The rate of embryonic development is directly related to temperature. Manx shearwater eggs develop optimally at temperatures between 32-34°C. Temperatures below 30°C will lengthen incubation. Exposure to temperatures above 36°C can be lethal. Cool coastal conditions can delay hatching.
Humidity
Adequate humidity is essential for normal incubation. The developing embryo requires uptake of water vapor through the porous eggshell. In cool, clammy conditions, eggs may hatch a few days later than normal. Excessive moisture can also be detrimental.
Importance of Incubation Period for Chick Survival
The precise timing of hatching after an incubation period of about 54 days is crucial for maximizing survivability of Manx shearwater chicks. Here’s why the incubation period matters:
Synchronizing Hatching with Food Availability
Manx shearwaters rely on seasonal pulses of marine prey. Chicks hatching too early or late could miss the peak availability of fish like sandeels, reducing survival odds. The average incubation period aligns hatching with maximal foraging conditions.
Predator Avoidance
Nesting in dense colonies provides some protection from predators. But synchronized hatching means chicks are only vulnerable for brief periods rather than longer if hatching is irregular. This swamping strategy overwhelms predators.
Facilitating Sibling Competition
Manx shearwater eggs hatch asynchronously at intervals of 2-4 days. The early hatching chick typically outcompetes its later-hatching sibling for food and resources. Shorter incubation gives the first chick a competitive head start.
Staggered hatching also provides insurance in case the first chick dies. The parents can then divert investment to the remaining chick. An adapted incubation period helps promote this key life history strategy.
Conclusions
– The incubation period of Manx shearwaters averages around 54 days but can range from 50 to 59 days.
– Manx shearwater incubation takes slightly longer than some related shearwater species but less than others. Local conditions fine-tune incubation duration.
– Several factors influence incubation length including parental care quality, egg size, temperature, and humidity. More attentive parents and larger eggs result in longer incubation.
– The adapted incubation period aligns hatching with optimal foraging periods and facilitates competitive hierarchies between siblings. Precise timing increases reproductive success.
In summary, the incubation period is a critical reproductive phase that ensures Manx shearwater chicks hatch under favorable conditions. After patiently incubating their single egg for 50-59 days, Manx shearwater parents are rewarded with a fluffy chick, their sole offspring for that year. The incubation period synchronizes hatching with environmental conditions for maximizing survival of the next generation.