Avian clutch size
The numbers laid by a particular species in a given location are usually well defined by evolutionary trade-offs with many factors involved, including resource availability and energetic constraints.
Several patterns of variation have been noted and the relationship between latitude and clutch size has been a topic of interest in avian reproduction and evolution.
The proximate and ultimate causes for this pattern have been a subject of intense debate involving the development of ideas on group, individual, and gene-centric views of selection.
[1] Comparable bird species near the equator laid approximately half as many eggs as those that resided in northern temperate habitats.
[2] Nature favours clutch sizes that correspond to the average maximum number of offspring that the parent can sustain given a limited food supply.
[7] In another study, researchers analyzed if the rates of food delivery and nest predation explain the variation in clutch size observed among species between and within South and North America.
The authors of the study suggested analyzing parental mortality rates in southern and northern climates in addition to considering Skutch's theory.
[8] However, if food resources were abundant during the breeding season, and everything else stayed constant, raising a larger clutch would be possible.
[9] Since at higher latitudes, there is a large increase in resource productivity during the spring and summer in comparison to the equatorial tropical regions, localities near the poles should theoretically have larger clutch sizes.
[9] According to Ashmole's hypothesis, there should be uniformity of clutch size within a region since the seasonality of resource production should have the same effect on all the bird species in that particular locality.
[8] Most importantly, under Ashmole's hypothesis, average avian clutch size should decrease as resource productivity increases during the non-breeding season.
[10] The study tabulated the sizes of 411 clutches of Northern Flickers (Colaptes auratus) across a wide range of localities in North America.
[10] As predicted by Ashmole's hypothesis, the study also found that Colaptes auratus clutch size is unaffected by the absolute resource productivity during the breeding season.
[11] The resultant lower competition during the reproductive season enables the resident birds that survived to lay larger eggs.
In the study, they compared the evolutionary history of birds inhabiting Australia, Southern Africa and India because these regions have different proportions of migrants.
[11] The researchers attributed the differences in the clutch size of resident birds to the larger proportion of Palearctic migrants wintering in India.
A study that looked at the geographical trends in breeding parameters of Pied Flycatcher (Ficedula hypoleuca) in the Western Palearctic found that the latitudinal clutch size variation depends on the duration of the working day and the energy requirement of the chicks.
[15] However, the energy requirement of the brood and parents could generate the latitudinal clutch size pattern in Ficedula hypoleuca as long as temperature and working day effects were considered.
[15] If photoperiod were a factor in the determination of clutch size in every avian species then nocturnal owls should show the opposite trend.
[16] The study suggested that nocturnal owl species clutch size at higher latitudes were constrained by short nights during the breeding season which limited the number of offspring they could raise.
[17] Therefore, further studies regarding clutch size in nocturnal species are needed as it still remains unclear how latitude is linked with parental investment.
[19] The other possibility under high temperatures is that female birds will initiate incubation earlier in the laying sequence and consequently this will result in asynchronous hatching which would lead to a reduced brood size.
[18] This earlier incubation may also shorten the nesting cycle by decreasing the time between clutch completion and egg hatching.
