Nest-box Breeding Research Articles

Age-dependent diet change, parental care and reproductive costin tawny owls Strix aluco

Abstract Tawny owls Strix aluco breeding in nest-boxes were studied in a mixed oak/hornbeam/beech forest located in the Duna-Ipoly National Park 30 km north-west of Budapest, Hungary, during the period 1992–1999. Diet composition, prey mass, breeding performance and body mass of the parents of known age were recorded. Older males had a greater ability to choose alternate prey, delivered a greater mass of prey with a higher feeding frequency and achieved higher productivity than younger males when the availability of the preferred prey declined. The reproductive cost was paid only by young parents. We suggest that the lowest breeding performance, which was observed with young parents in adverse weather conditions, may be due to both the lower ability of these younger birds to exploit alternative prey and to their poor body condition which resulted in them providing fewer resources to their offspring because of their need to provide for their own survival.

Behavioral Domination of Food Delivery by Tree Swallow Nestlings

In altricial birds, scramble competition for favorable feeding positions in the nest may cause younger chicks to starve. We studied nestling survival in a population of Tree Swallows (Tachycineta bicolor) breeding in nestboxes near Seattle, Washington, in a year of bad weather. Only a mean of 0.5 chicks fledged per nest (range 0–2, 22 nests). Nestling mortality appeared to be caused by starvation. Video filming showed that when chicks were old enough (largest chick within the brood had reached a wing length of about 55–60 mm), they spent much time in the nestbox opening, calling loudly. This behavior seemed to be driven by hunger because it occurred mainly in broods where chicks had low body masses. Parents fed the chick in the opening and did not try to push it back into the nest cavity or go around it. We suspect that blocking of the nestbox opening accelerated mortality of other young in the nest. Such behavior by dominant chicks may be more readily achieved in hole nesting birds with narrow entrances to their nest site than by open nesting birds.

Seasonal shifts in sex ratios of fledgling American kestrels (Falco sparverius paulus): The Early Bird Hypothesis

We document a seasonal shift in the sex ratios of broods produced by resident southeastern American kestrels (Falco sparverius paulus) breeding in nest boxes in Florida. Early in the breeding season, most biased broods were biased towards males, whereas later in the season, most biased broods were biased towards females. Computer-simulated broods subjected to sex-biased egg and/or nestling mortality demonstrate that it is possible that differential mortality produced the pattern of bias that we observed. However, these simulations do not exclude the possibility that female kestrels were manipulating the primary sex ratio of the broods. We present evidence that this sex ratio shift is adaptive: for males we detected breeding as yearlings, all had fledged early the previous season. No such relationship between season and the probability of breeding as a yearling was found for females. We propose the Early Bird Hypothesis as the ecological basis for the advantage of fledg ing early in males. We hypothesize that pre-emptive competition among post-fledging, dispersing males for breeding sites confers an advantage to males fledged early in the season. This hypothesis may explain why a non-migratory population of the Eurasian kestrel (F. tinnunculus) and non-migratory American kestrels breeding in Florida (F. s. paulus) exhibit this seasonal shift in sex ratios, whereas migratory American kestrels (F. s. sparverius) breeding in Saskatchewan, Canada, do not. We discuss the relevance of the Early Bird Hypothesis for other animal species.

Parental Energy Expenditure and Offspring Size in the Pied Flycatcher Ficedula hypoleuca

To determine if parental daily energy expenditure (DEE) is an appropriate measure of parental care with a positive effect on offspring fitness, the DEE of pied flycatchers Ficedula hypoleuca at 24 nests was measured with the doubly-labeled water method in a population breeding in nest boxes in a pine plantation in central Spain. DEE was measured when parents were feeding chicks of 12 d of age. In 17 of the nests, DEE of both parents was measured simultaneously. To expand the range of parental efforts observed, clutch size was manipulated by adding or removing one egg after laying and keeping some nests as controls. The significant brood size differences between treatments at hatching disappeared before chicks were 13 d old. All nestlings were weighed and measured (tarsus and wing length) at the age of 13 d. Chicks in enlarged nests had significantly shorter tarsi than chicks in control and reduced nests. The mean DEE was 64.4 kJ/d. There was no relationship between the DEE of mates measured simultaneously. Feeding rates of the labeled birds were not correlated with DEE. Treatment or brood size did not affect parental DEE. Female DEE was positively correlated with mean nestling mass and tarsus length. The effect on tarsus length was also significant when controlling for treatment and brood size. Male DEE was correlated with mean nestling tarsus length only when controlling for treatment. The relationship of parental DEE with tarsus length was linear. Female DEE explained more of the variation in mean tarsus length than did the midparent tarsus length. Parental and especially maternal DEE is a good measure of parental care as it positively affects offspring fitness, and should be considered as a parental performance effect on offspring skeletal size.

The high frequency of extra-pair paternity in tree swallows is not an artifact of nestboxes

A common criticism of nestbox studies is one of creating artificial nesting conditions and breeding behavior different from what would be seen under natural conditions. We assessed the frequency of extra-pair paternity (percentage of broods with at least one extra-pair young) in 25 families of tree swallows (Tachycineta bicolor) nesting in natural cavities and compared it to that in a nestbox population. We found that 84% of females nesting in natural cavities obtained fertilizations from extra-pair males. These extra-pair males fathered 69% of all nestlings. Studies of tree swallows breeding in nestboxes have shown that 50–87% of broods contained extra-pair young, with extra-pair males fathering 38–53% of all the young. In broods with extra-pair paternity, natural cavities contained a significantly greater proportion of extra-pair young than did nestboxes. Despite differences in nesting habitat and female age structure, the frequency of extra-pair paternity did not differ significantly between the natural-cavity and nestbox populations. Therefore, the presence of extra-pair paternity in tree swallows is not an artifact of nestboxes or of artificial nesting conditions.

Prevalence and Intensity of Haematozoan Infection in a Population of Great Tits Parus major from Gotland, Sweden

The prevalence and intensity of haematozoan infections were studied in a nest box breeding population of Great Tits on the island of Gotland in the Baltic Sea during 1990 to 1992. Adult birds were captured during the nestling period and blood smears taken. Ten taxa of haematozoa belonging to seven genera were identified. Haemoproteus majoris dominated with an overall prevalence of around 76%. There were year and age class differences in both prevalence and mean intensity of infection. Annual differences were probably due to variation in vector abundance and activity in the previous year. Older birds (>2 years old) were more often infected, probably as a result of their longer time of exposure to vectors, but had a lower mean intensity of infection than young (1 year old) birds indicating an acquired immunity to parasites. Our results suggest that the frequency of blood parasites in avian populations may primarily be an effect of the abundance and behaviour of suitable vectors, and that birds may have little scope for behavioural adaptations to avoid being parasitized.

The Frequency and Timing of Laying Gaps

The frequency of laying gaps was studied in a nestbox breeding population of Blue Tits Parus caeruleus during the breeding season of 1990. In as many as 27% of the 55 clutches studied, laying was interrupted for at least one day. It is concluded that the interruption of egg laying depends on food availability since the frequency of laying gaps decreased in clutches laid by females experimentally provided with extra food. Most gaps occurred after the laying of the first or second egg, indicating that each female started egg laying on the verge of what is possible for uninterrupted laying. The occurrence of laying gaps was not connected with certain dates, female age, clutch size or mean egg mass of the clutch, instead each female seemed to individually optimize the start of egg laying in relation to the risk of interrupted laying. To some extent the female is able to save energy for egg production because an egg laid after a gap was heavier than corresponding eggs laid in clutches without gaps. Furthermore, eggs laid immediatelly before a gap were lighter than equivalent eggs in other clutches, indicating that females, to some extent, are willing to trade egg quality against uninterrupted laying.

A comparison of the breeding ecology of a secondary cavity nesting bird, the Tree Swallow (Tachycineta bicolor), in nest boxes and natural cavities

Several studies concerned with mate choice, reproductive performance, and life history strategies have been conducted with secondary cavity nesting birds breeding in nest boxes. Although the need for comparative studies has been recognized, populations breeding in nest boxes often have not been compared with those breeding in natural cavities. We compared the ecology of Tree Swallows breeding in nest boxes and natural cavities to determine if nest box populations of Tree Swallows are accurate models of natural populations. Two nest site characteristics, nest site dispersion and cavity height, were similar for birds in both nesting environments. Greater cavity entrance area at natural cavities resulted in increased interspecific competition in natural populations, involving larger competitors, more species, and a greater abundance of each species. Clutch size was smaller in natural cavities compared with nest boxes, likely because floor area was smaller in natural cavities. Fledging success did not differ between populations. Disproportionately more after-second-year females bred in nest boxes, and more second-year females bred in natural cavities, as estimated by a model of Tree Swallow survivorship. Tree Swallows settle at nest boxes before natural cavities in our study area, perhaps as a result of the greater potential for reproductive success and reduced interspecific competition in the nest boxes as opposed to natural cavities. For some aspects of the ecology of secondary cavity nesters, nest boxes do not provide an accurate representation of natural populations. Therefore, evolutionary interpretations of nest box studies should be compared with observations of birds in natural environments.

Survival Rate in the Great Tit Parus major in Relation to Sex, Age, and Immigration Status

SUMMARY (1) Survival rates of Great Tits were studied using data collected in Wytham Wood, Oxfordshire. The breeding population was split into four groups by sex and status (born in nestboxes = residents, not known to be born in nestboxes = immigrants). Survival rates were only measured for breeding adults, the large majority of which were 1 year old when first found breeding. (2) Survival rates were estimated using a recent modification of Cormack's (1964) method. Relationships between these estimates and several environmental variables were investigated. (3) The comparison of two different estimates of capture rate showed that some birds seem to miss 1 year of breeding in nestboxes more often than others; these are more likely to be immigrants. (4) Age, sex, and immigration status of the birds have a strong influence on the survival rate. (5) Male survival rate is negatively related to the density of blue tits, while that of females is not. We suggested that this may be related to the cost of defending a territory. (6) Female survival rate is negatively related to the beech crop production of the previous year (BCC). BCC is probably an indirect measure of the number of resident males, so that the relationship reflects competition between males and females. (7) Survival rates of immigrant males and resident females are affected by different variables from those affecting resident males and immigrant females. Although it is difficult to explain these differences satisfactorily, they may be related to sex differences in dispersal behaviour. (8) The survival rates of the four groups seem to differ irrespective of the age of the birds. In any year females may survive better or worse than males and independently immigrants may survive better or worse than residents. These differences in survival between the four groups are not consistent in direction, although sometimes strongly significant.

Selection of nest sites by a hole‐nesting duck, the Goldeneye Bucephala clangula

Nest site preferences were examined for a population of Goldeneye Ducks breeding in nest boxes in Värmland, central Sweden. Some nest boxes were occupied more often than others even if females returning to the same nest box were excluded from the analysis. Nest boxes located higher up trees were occupied more often than those close to the ground and some spatial ‘cluster groups’ of boxes were occupied more often than others. Otherwise nest site prefernces were not related to any measured physical attributes of the boxes. Prefernces for nest boxes seemed to be based mainly on a tendency for females to select those that had been occupied by other females in the preceding year, especially if they had bred successfully. As a result of this, the occupancy of nest boxes was not random over years but rather progressed in a series of runs; a period of consecutive years in which a box was occupied was followed by a period of years in which it was empty.There were reproductive consequences for these prefernces in that females occupying preferred boxes were less likely to lose their clutch to a predator. These females also bred earlier in the year and produced larger clutches and broods than females breeding in other boxes.

Breeding and Natal Dispersal of the Goldeneye, Bucephala clangula

SUMMARY (1) Breeding and natal dispersal were examined for female goldeneye breeding in nest boxes in Sweden. Females changing nest box between years dispersed a median distance of 0 75 km to the new nest box (or, to the tenth nearest nest box). Individuals (N = 17) which returned to the natal area to breed, nested in the vicinity (median = 0 73 km) of the box in which they had hatched. (2) Neither the proportion of females changing nest box, nor the distances moved between nest boxes, varied among years. (3) Forty-two percent of females reoccupied the same nest box in successive years. Females which failed to breed successfully were less likely to return to the same nest box than females which bred successfully. (4) Females which moved to another nest box were less likely to hatch their broods successfully than females which returned to the same nest box. When they moved they produced smaller clutches and broods, and bred later in the season than when they returned to the same nest box. (5) Among females which moved, those which had failed to breed successfully moved further to another nest box than those which had bred successfully. In the case of females which lost clutches to predators, moving away from the vicinity of the previous nest box may have been advantageous, since boxes in which clutches were preyed-upon tended to be preyed-upon in the following year and also tended to occur close together. (6) The greater the distance from the previous nest box at which females bred, the less likely they were to start egglaying early in the season. (7) Females whose previous nest box was occupied by another female moved further to new nest boxes than females whose previous nest box was left empty.