Great Black-backed Gull (Larus marinus), small clutch.
Clutch size refers to the number of eggs laid in a single brood by a nesting pair of birds. 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
avian reproduction and evolution. David Lack and R.E. Moreau were among the first to investigate the effect of latitude on the number of eggs per nest. Since Lack's first paper in the mid-1940s there has been extensive research on the pattern of increasing clutch size with increasing latitude. 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
.
Food limitation and nest predation hypotheses
David Lack observed a direct relationship between latitude and avian clutch size.
field studies have provided little support for either of these hypotheses.[5]
It is clear that Lack's Food Limitation Hypothesis and the Nest Predation Hypothesis are plausible explanations for explaining the latitudinal variation in avian clutch size. However, further analysis is required as field studies have provided little support for these hypotheses.
Skutch's Hypothesis
Skutch's Hypothesis is similar to the Nest Predation Hypothesis as it states that higher nest predation decreases the rate at which birds can deliver food to their offspring and thus limits clutch size.
subtropical Argentina where they monitored 1,331 nests. They found that clutches were larger in Arizona (4.61 eggs/nest) than in Argentina (2.58 eggs/nest) and that Skutch's Hypothesis explained the variation in clutch size within each, North and South America, but did not explain the latitudinal difference in clutch size. In Argentina, the number of offspring was half that of Arizona, yet parents were bringing food to their young at greater rates in Argentina.Therefore, clutch size in southern areas cannot be explained by predation rates or food delivery. The authors of the study suggested analyzing parental mortality rates in southern and northern climates in addition to considering Skutch's theory.[5]
Therefore, Skutch's Hypothesis - by itself - is not an accurate predictor of the latitudinal clutch size trend and evidence for it remain equivocal.
Ashmole's Hypothesis
Red-shafted Northern Flicker: female (left), male (right).
The latitudinal variation in clutch size is influenced by the food abundance per unit area of habitat.
Colaptes auratus) across a wide range of localities in North America.[10] The study found that as the localities become more resource abundant in the winter, clutch size significantly declines.[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.[10] Avian clutch size should be proportional to breeding season resource productivity per breeding pair of birds.[10] This relationship has been found in a series of studies from Alaska and Costa Rica.[10]
According to Ashmole's Hypothesis, the clutch size of resident birds is proportional to the level of
resident birds that survived to lay larger eggs.[11] A study performed supported this hypothesis. 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] India had the largest proportion of migrants while Australia had the smallest.[11] The study found that clutch size was significantly higher in India than in the other two regions (Australia, Southern Africa).[11] Furthermore, the study tested whether there exists a negative relationship between clutch size and the length of the breeding season (i.e. the larger the clutch size the shorter the breeding season).[11] The length of the reproductive season was greater in Australia than in India (with South Africa having a breeding season lower than Australia but higher than India).[11] The researchers attributed the differences in the clutch size of resident birds to the larger proportion of Palearctic migrants wintering in India.[11] The differences that the study found in terms of the length of breeding seasons were accounted for by differences in the climatic characteristics between the regions.[11]
Environmental seasonality
As latitude increases, clutch size and seasonality also increase. The clutch size of birds occupying environments with low seasonal variations are smaller than those of birds residing in habitats that depict greater seasonality.[12][13] Highly seasonal environments force birds to survive periods of low temperatures and reduced food availability during the non-reproductive season which causes increases in parental mortality.[12][13] As well, the risk of adult mortality is increased in seasonal environments due to the need for avian species to migrate.[12][13] The combination of increased parental mortality during the non-breeding season and food abundance during the reproductive season has favoured the evolution of larger clutches in seasonal environments.[12] Producing more eggs during the reproductive season increases the individual fitness during the periods of reduced food availability.[12] The greater the fluctuation in resource availability, the greater the tendency for avian species to increase their clutch size.[12] Since seasonality increases with latitude, avian clutch size increases in environments that are closer to the poles.[12] In some cases, seasonality is also affected by longitude. Eastern Europe is more seasonal than Western Europe.[2][14] Given the effects of seasonality on clutch size, birds in Eastern Europe tend to have larger clutch sizes than birds in Western Europe.[2][14] However, the only avian species that show this longitudinal pattern are the ones that also show the latitudinal trend.[2]
Day length
Pied Flycatcher Birdomente chicks in Finland.
As
nocturnal owls should show the opposite trend. Namely, their clutch size should decrease as day length increases. A field study was performed to examine nesting provisioning rates of Tengmalm's Owls in two populations at different latitudes (Finland 63 degrees N and Czech Republic 50 degrees N) and thus different day lengths.[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.[16] Furthermore, owls at northern latitudes were constrained by the yearly fluctuation of resources.[16] Both of these factors are likely to influence the reproductive investment in owl species.[16] However, other studies have not shown that photoperiod is a factor in determining nocturnal avian species clutch size.[17]
Therefore, further studies regarding clutch size in nocturnal species are needed as it still remains unclear how latitude is linked with parental investment.
Egg-viability hypothesis
Temperature is a possible factor that could explain the pattern in latitude and seasonal trends in clutch size.
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.[18]
Nest type
Woodpeckers are capable of excavating their own cavity nests.
A strong intrinsic determinant of clutch size is nest type.[12] Open nesters tend to be exposed to higher rates of nest predation in comparison to cavity nesters (e.g. woodpeckers).[8] Thus, open nesters tend to have smaller clutch sizes.[12] Half-open nests have clutch sizes that lie in between open nesters and cavity nesters.[20] There are two major hypotheses that attempt to explain variation in clutch size among cavity-nesting bird species.[20] The nest site limitation hypothesis states that weak excavators invest more energy in each breeding attempt and thus lay larger clutches because their nesting opportunities are more limited.[20] The other competing hypothesis is that clutch size among cavity – nesting birds could be determined by diet.[20] The clutch size of strong excavators may be larger because they are able to specialize on a more seasonally stable food source.[20] Annual stability of food resources tends to have a larger impact on the variation of clutch size in excavators.[20] Limitation of nest sites has a much more limited impact but is still relevant in determining clutch size variation among excavators.[20]
Exceptions
Western Jackdaw in London, England.
Some avian species do not conform to the general clutch-latitude relationship where clutch size and latitude are proportional to one another (i.e. as latitude increases, clutch size increases). A field study was performed on a single-brooded hole nesting bird by examining 228 Jackdaw (
Corvus monedula) nests.[21] The data were collected between 1979 and 1983 in Hoya de Guadix (a region that is north of the province Granada in Spain). The study found that the Jackdaw was an exception to the general clutch size-latitude trend.[21] In fact, they found that increases in latitude decreased the clutch size of this bird species.[21] However, the accuracy of this study can be questioned as the results were observed in one small region (i.e. Hoya de Guadix). Another study looked at seven widespread single-brooded British birds at numerous localities across Britain.[22] The study found that seasonal clutch size declines were not related to latitudinal gradients.[22] However, the researchers found strong seasonal declines in clutch size.[22] Moreover, of the three environmental variables that they analyzed (daylight, plant productivity and seasonality), they found that daylight had the most influence on clutch size.[22] The study found little evidence for the direct proportionality between clutch size and latitude.[22]
^ abcdefghijTov, Y.; Geffen (2002). "Examining Ashmole's hypothesis: are life-history parameters of resident passerines related to the proportion of migrants?". Evolutionary Ecology Research. 4: 673–685.
