Starling foraging success in relation to agricultural land-use

Changes in agricultural land-use have been suggested to contribute to the decline of several bird species through negative effects on their food supply during breeding. One important change in land-Use has been loss of pastures, especially permanent pastures. In this study we investigated how different forms of agricultural land-use affected foraging success or a declining bird species, the European starling Sturnus vulgaris. We let caged starlings forage in different forms of agricultural fields and determined time spent foraging and foraging success, The starlings' activity level (time spent actively foraging) as well as the number of prey caught per time unit was strongly related to the abundance of prey in soil samples. Also the body mass change during the experiment was positively related to activity level and prey capture rate, We found consistent differences in foraging variables between habitats. In spring sown grain starlings were least active and found fewer prey items at a lower rate than in any other habitat. The other three habitats differed less, but in general mowed hay fields appeared slightly more valuable than the cultivated and natural pastures. We did not find any differences between natural and cultivated pastures in foraging variables. Thus, starling foraging success is higher in grass-covered fields than in cultivated fields, but the management of the grass-covered fields mattered less. The results are consistent with starlings having higher population densities and breeding success in areas with higher availability of pasture. We suggest that the physical structure of the habitat (sward height) and Moisture may be additional variables that need to be taken into account to explain starling breeding density and success in the agricultural landscape

On starlings and farming: population decline, foraging strategies, cost of reproduction and breeding success

The European Starling Sturnus vulgaris as many other farmland bird species has been declining during the last decades. The cause of these declines is generally believed to be changes with in agricultural methods affecting breeding and survival of birds. For starlings in Southern Sweden the amount of pasture in the landscape surrounding breeding colonies is the key factor. We found a positive relationship between the amount of pasture and breeding success as well as breeding densities. One probable explanation for this is a positive relationship between the amount pasture and food availability. To test this we took soil samples which showed a higher abundance in pasture and other permanently grass grown areas than in cereal fields, starlings also have higher foraging efficiency on grass covered areas. In detailed studies with radio-telemetry we showed that pasture is the preferred foraging habitat, but when the amount of pasture decreases starlings also use margin habitats. In landscapes with low amount of pasture starlings flies longer distances per feeding trip and also spend more of the total foraging time flying. To reduce flying cost the combine long trips to pasture, with shorter to alternative habitats when pasture is scarce. The reason for this is that starlings find higher amounts of their preferred prey, caterpillars, in pasture than in other kinds habitats. When starlings do not forage in pasture they fly longer distances searching for caterpillars. Caterpillars seem to be high quality food for nestlings and we also found a positive effect on nestlings tarsus length of the proportion of caterpillars in the diet. We could not show any effects on male starling humoral immune response of a brood size manipulation. Neither did the amount of pasture affect immune capacity, but both the landscape category and brood size affected breeding success

Landscape composition affects habitat use and foraging flight distances in breeding European starlings

The recent decline of the European starling in northern Europe has been associated with changes in agricultural land-use. We investigated how agricultural land-use affected starling habitat use and home-range size in a heterogeneous landscape in southern Sweden. Breeding starlings primarily foraged in pastures, a habitat with a high availability of potential food for nestlings. When the availability of pasture close to the nest was low, starlings increased their use of other permanently grass-covered habitats such as field margins and ditches. They also spent a larger proportion of their daytime flying because they visited more distant foraging areas. The further from the nest parents foraged, the higher was the probability that they foraged on a pasture. Feeding frequency of nestlings was negatively related to the distance from the nest parents foraged, but no consequences on nestling survival or growth were found. A lower breeding density at low local availability of pasture may compensate for lack of good foraging habitat. It is suggested that recent changes in the agricultural landscape may have increased the mismatch between the availability of good foraging areas and nest-sites. This might have contributed to the recent population decline of the starling in northern Europe by affecting both foraging habitat quality and flight costs paid by parents during breeding

The effect of egg size and habitat on starling nestling growth and survival

In spite of the fact that hatchling size and energy reserves in birds are affected by egg size, many studies have failed to find an effect of egg size on offspring fitness. One possibility is that this is because they have been performed in areas with high food availability and that effects of egg size on offspring fitness are most apparent in areas of low food availability. To investigate this, egg size, offspring mass and survival of European starlings (Sturnus vulgaris) were measured in an agricultural landscape with a low but variable amount of pasture, the preferred foraging habitat of parent starlings. Offspring mass was related to egg size, but egg size explained a declining proportion of the variation in nestling mean mass as nestlings grew older. Offspring survival during the early, but not during the late nestling period was related to egg size. Throughout the nestling period, survival was related to the mass of the nestlings. It is suggested that the effect of egg size on offspring survival is through the effect of egg size on offspring mass, this effect declining as offspring grow older. Offspring survival during the early part of the nestling period was related to egg size when availability of pasture was low, but not when it was high. However, the interaction was not significant. Selection for larger egg size is discussed in relation to the structuring of starling populations into sources and sinks

The effect of pasture on starling (Sturnus vulgaris) breeding success and population density in a heterogenous agricultrual landscape in Southern Sweden

Populations of European starlings have declined over large parts of northern and western Europe during the last 20 years. It has been suggested that reduced availability of pasture in the agricultural landscape was one reason for this decline. To investigate this, breeding colonies for starlings were established in southern Sweden, in agricultural landscapes with different availabilities of pasture. Agricultural land-use was classified in a circular area of 500 m around each colony. Production of young per nest was positively related to the availability of pasture close to the breeding colony, not because the availability of pasture affected clutch size or hatchability of eggs, but because nestling survival was higher in colonies surrounded by pasture. The growth rate of an offspring feather character, but not offspring final mass or tarsus length, tended to be related to the availability of pasture. When availability of pasture was low, the variability of breeding success between years was high. Breeding density was also positively related to the availability of pasture near colonies. The combined trends in reproductive success and breeding density in this study support the contention that the reduction of pasture in modern agricultural landscape may be one reason why starling population size has declined

Paternal care in the European starling, Sturnus vulgaris: nestling provision

The extent to which male birds in polygynous species with biparental care assist in nestling feeding often varies considerably between nests of different mating status. Both how much polygynous males assist and how they divide their effort between nests may have a profound effect on the evolution of mating systems. In this study we investigated how males in the facultatively polygynous European starling Sturnus vulgaris invested in their different nests. The amount of male assistance affected the quality of the offspring. Polygynous males invested as much as monogamous males, but divided their effort asymmetrically between nests, predominantly feeding nestlings of first-mated (primary) females. Although females partly compensated for loss of male assistance, total feeding frequency was lower at primary females' nests than at monogamous females nests. Secondary females received even less assistance with nestling rearing, and the extent to which males assisted decreased with the length of the interval between the hatching of the primary and secondary clutches. These results are contrasted with those from a Belgian populations of starlings with a much more protracted breeding season and thus greater opportunities for males to attract additional mates during the nestling rearing period. The results show that both the ''defence of male parental investment model'' and the ''asynchronous settlement model'' have explanatory power, but that their validity depends on the potential length of the breeding season

Polygynous male starlings allocate parental effort according to relative hatching date

In many polygynous bird species, males allocate most of their parental effort to their primary females’ broods. There are several hypotheses that may explain this: the relative reproductive value of the brood, the energetic demand of the brood, the genetic quality of the female and the certainty of fatherhood may all be higher for the primary females’ broods. Since these parameters may covary in nature, experiments are necessary to determine their importance. Bigynous male European starlings,Sturnus vulgarispredominantly incubated the eggs and fed the nestlings of their primary females. In an experiment we altered the order in which the clutches hatched by exchanging the primary and secondary females’ eggs before hatching. Even though experimental males mostly incubated their primary females’ clutches, they predominantly fed the nestlings of the secondary female. Experimental males fed secondary females’ nestlings significantly more than control males did. In fact, experimental males invested in their secondary females’ broods to the same extent as control males invested in their primary females’ broods. This result demonstrates that males use relative brood age to decide how to allocate their parental effort between their broods which has important implications for the evolution of the starling mating system

Paternal care in the European starling, Sturnus vulgaris: incubation

In polygynous passerines, males of some species provide food for their nestlings, but male incubation seems to be rare. In the European starling both the mating system and the extent to which males help with incubation vary. This enabled the relationship between mating system and male incubation to be investigated. The extent to which males provided care to a particular nest depended on mating status: monogamous males incubated more than polygynous males did in any of their females' nests. On average, bigynous males incubated as much in their two nests as monogamous males did in their single nest, but polygynous males who invested in only one of their nests incubated less than monogamous males. Females partly compensated for the variation in male incubation, but still nests of polygynously mated females were attended less. Polygynous males invested more in the nests of the first females mated with (primary female) than in the nests of later settling females. Secondary females received less help the later they laid their eggs in relation to the mate's primary female. Bigynous males that incubated in both their nests invested more in the nests of their primary females the earlier these eggs were laid in relation to those of the secondary females. Male help with incubation may affect a female's fitness, both because increased attentiveness resulted in shorter incubation times and because females receiving less help may pay a higher cost in terms of energy expenditure. Hence, there will be a conflict between polygynously mated females over paternal incubation